WEBVTT

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Prof: We've been going
through the generations of

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organic chemistry,
from Lavoisier to Berzelius and

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Gay-Lussac,
to Dumas, Liebig and Woehler,

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talking about their discovery,
or invention really,

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of valence;
in particular the tetravalence

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and the self-linking of carbon.

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is most noted for is the
structure of benzene,

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and we started looking at that
a little bit last time with his

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sausage diagrams for drawing a
structure.

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And this time we'll go on in
more detail with that idea in

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the determination of actual
position in molecules,

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the positions of atoms in
molecules.

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so the next generation:
Wilhelm Koerner,

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who was both in Germany and in
Italy,

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as you'll see,
and Jacobus Henricus van't

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Hoff,
from the Netherlands,

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and also Albert Ladenburg,
from Germany,

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and we'll even have a little
message of your own --

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<<technical
adjustments>>.

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Okay, so we -- Koerner,
van't Hoff, Ladenburg and

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E.P. Kohler,
who is your

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great-great-grandfather
academically.

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Okay, so benzene and molecular
structure.

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This is the simplest of the
so-called aromatic compounds.

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Obviously the name comes from
the smell,

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that it has an aroma,
but it was generalized from

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benzene which --
benzene was discovered in 1825

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by Faraday who isolated what he
called bicarburet of

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hydrogen,
which was C_2H,

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according to his formula,
but actually it's C_6H_6,

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because he had the carbon
doubled and it's six times what

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he thought it was.

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And he got it from gas oil,
which is a byproduct.

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Quite a bit,
you get a gallon -- or they did

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at least then;
I don't know what happens

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nowadays -- for making 1000
cubic feet of the oil,

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that was an illuminating oil;
illuminating gas, pardon me.

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So in 1833 Mitscherlich named
the compound benzine,

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instead of bicarburet of
hydrogen.

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And the origin of the name is
interesting.

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It came from gum benzoin,
via benzoic acid.

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So you knocked the CO_2,
somehow, off of benzoic acid,

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and you get what Mitscherlich
called benzine.

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Now gum benzoin itself has an
interesting etymology,

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because it came originally from
luban jawi,

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which I'm told is Arabic,
meaning frankincense --

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that's luban -- and
jawi is from java.

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So it came from the southeast
Asia.

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But when the French saw this
name, they assumed that

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lu was like le,
la, les,

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and so on, the article.

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So they knocked it off.

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So it became ban jawi,
which doesn't mean anything in

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Arabic, but that then became
benzoin and then benzine.

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Okay?

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The French, in 1841,
named the alcohol version

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and we'll see why shortly.

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We call it phenol,
and we call C_6H_5 phenyl,

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and abbreviate it Ph for
phenyl, or Φ,

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the Greek letter,
for phenyl.

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And it comes from
phainein,

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which means to bring light,
because of its association with

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illuminating gas.

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benzene is hexagonal,
even though he wrote it with

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that sausage thing originally.

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Now why?

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On what basis did he think it
was hexagonal?

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Well this is the paper where he
proposed it's hexagonal,

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and I'll show you all the
evidence he gave in his

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arguments for its being a
hexagon.

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First, "All aromatic
compounds"

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(so,
for example,

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benzaldehyde as well as benzene
and so on) "even the

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simplest are significantly
richer in carbon than analogous

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compounds from the class of
fatty substances."

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Okay, so it's rich in carbon;
that's the first evidence that

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it's a hexagon.

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The second is that
"Aromatic compounds,

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as for fatty compounds,
there are numerous homologous

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substances;
that is, those who composition

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differ by an integral number of
CH_2 groups."

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So if you replace a hydrogen by
a methyl group,

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you add CH_2,
right?

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Or if you put a CH_2 into a C-C
bond, it becomes a little bit

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longer, a homolog.

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So there are homologous
substances in the aromatic

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series.

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The third line of evidence was,
"The simplest aromatic

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compounds contain at least six
carbon atoms."

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Okay?

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And the fourth line of evidence
is,

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"All reaction products
from aromatic substances show a

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certain family similarity,
constituting the group of

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'aromatic' compounds.

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More vigorous reaction can
remove part of the carbon,

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but the major product contains
at least six carbon atoms,

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and decomposition stops at this
point,

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unless there is complete
destruction of the organic

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group."

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For example, burning it.

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And that's it.

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Those are the four lines of
evidence.

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So would you conclude from that
that benzene is a hexagon?

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It's quite a stretch.

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So was this just a wild guess,
based on the idea that carbon

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should be tetravalent?

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And even in a sense that
doesn't help much because

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there's a question of what you
do with that last valence of

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carbon, right?

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So he did have some support for
this, but he didn't cite it in

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the paper.

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He didn't publish it until
later.

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It had to do with counting
isomers.

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So, remember this was the
formula he drew in 1865 of

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chlorobenzene.

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But in the next year,
when he started trying actually

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to say that it was a hexagon,
he drew a hexagon -- he didn't

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try to use his sausages at this
point --

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and he contrasted it with what
you get with a triangle of

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carbons,
with a carbon in the middle of

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each edge.

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Okay, so he labels the
vertices, the carbon atoms,

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a, b, c, d,
e, f, and counts isomers.

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And this is his table of that
count.

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So monobromobenzene,
there's only one form.

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Right?

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Obviously if it were a chain of
six carbons, you could have it

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on the end, or the carbon that
was next to the end,

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or the carbon next to that.

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Right? So there'd be isomers.

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But, in fact,
only one was known,

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monobromo.

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Now how about dibromobenzene?

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It turns out that there are
three modifications,

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he says: ab,
ac, and ad.

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How about ae?

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ae would be next nearest
neighbors, like ac,

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and af would be like
ab.

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Okay, so three modifications.

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That's consistent with its
being a hexagon.

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Now how about tribromobenzene?

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There are three;
drei modificationen.

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Okay, abc.

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So all successive, right?

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Or skip one.

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How about if you skip two?

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How about abe;

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should we have that one?

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How about abe?

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Kate, what do you say?

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Should we count abe as
another one?

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Steve, what do you say?

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Student:  No.

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Prof: Why not?

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Because I asked?

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Student:  Well because
on the thing it says abc,

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abd, and ace.

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Prof: Yes,
but I wonder if you agree.

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Maybe he's wrong.

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There's precedent for people
being wrong.

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Sam?

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Student:  Isn't --
abd is the same as

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abe.

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Prof: Right,
because if you count the other

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way, bae is the same as
abd.

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Right?

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You go two in a row and then
skip one, just go the other way

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around.

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Okay?

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Because it shouldn't make any
difference which way you go

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around.

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Okay?

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Or there would be ace -- right?

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-- where you skip every time.

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But there's only one way of
doing that.

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Okay, so three modifications.

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And if you go to four,
five, six, it's the same thing,

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except hydrogens are what you
count instead of bromines.

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Okay?

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So that was consistent with
experimental observation of how

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many isomers there were of these
mono-, di-, and tri-substituted

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benzenes.

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Yes?

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Student:  What was he
using to observe these

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modifications?

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What technique --
Prof: So how would he

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know they were different,
that there were three different

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substances?

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Suppose he had them.

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He had to prepare them of
course.

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Maybe they were not preparable
-- right?

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-- in which case you'd have a
count that was too low.

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But how would you know you had
three different things?

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How could you tell things were
different?

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Yes?

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Student:  Boiling point.

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Prof: Boiling point;
melting point;

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color maybe,
although these aren't colored;

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density.

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Okay?

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Some physical property you have
to measure.

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Right?

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The analysis obviously is going
to be the same,

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because they're isomers.

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And of course,
if you're not very good at

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measuring things,
you might get a melting point

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between two samples that differs
by two degrees.

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You guys are experienced in lab.

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Does that mean they're
different?

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What do you say Wilson?

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Your experience is that it
doesn't tell they're different?

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Okay.

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So it's not trivial.

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You have to be a good
experimentalist.

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You have to be able to prepare
them and you have to be able to

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tell when things are the same
and when they're different.

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But if you do,
then you can count isomers.

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But you have to bear this in
the back of your mind that you

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might be making a mistake
because of your experimental

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observations.

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Okay, now how about if it were
a triangle, right?

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Then if you had a
monobromobenzene,

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you could have the bromo either
at a or at b;

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although it's conceivable you
couldn't prepare one of them,

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or one of them would be
reactive for some reason.

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Okay?

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And if you had two,
you could have ab,

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or ac,
or bd,

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or ad.

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There'd be four modifications
instead of three,

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assuming you could prepare them
all.

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Right?

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And then you can go on to the
tribromo and see there would be

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six.

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Okay, so by counting isomers,
you could get information about

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the structure.

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Okay?

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In fact, the paper that I
xeroxed this from had written

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in,
in old brown ink,

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the letters from the bottom
u,

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k, e, k, e, l,
going around the ring,

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for a, b, c, d, e, f.

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And written next to these
things was names for the

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compounds;
elukek and so on;

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ekeluk,
ekulek,

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for the order of these things.

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Do you see what's funny?

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Prof: Right.

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No, ekulek,
they said, was Arabic for

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hexagon;
that's what was written in the

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book.

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So you've got humorists in all
ages.

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Okay, now let's try applying
the idea of isomer count to see

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if we can distinguish,
among Dewar's models of benzene

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that I showed you.

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So first there's the one top
and center, which is what we

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okay, single bond,
double bond,

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single bond,
double bond.

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How many isomers?

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Okay, how many monosubstituted
isomers, if you put just one

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bromine in?

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And if that were the structure,
a faithful representation of

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the arrangement,
how many different ones could

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you get?

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Just one.

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Okay all the hydrogens are,
in that sense,

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equivalent.

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Okay, how about if they're di,
how many could you get?

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There's one,
two, three, four,

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five.

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Right?

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So are they all different?

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Kate, what do you say?

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Student:  The purple and
the red are the same -- .

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Prof: You say the purple
and the red are the same.

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Anything else?

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Student:  The two blue
ones.

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Prof: The two blues are
the same.

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Now does everybody agree with
that?

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Sherwin?

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Student:  With the
purple you've got the red ones

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across a double bond.

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Prof: Ah ha!

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the red one is across a square,
and the purple one is across

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just a single bond.

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Right?

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So those are different,
if this model is faithful in

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that respect.

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Okay?

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So, but we should take that one
out, because that one goes

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across both a square and a
single bond.

14:20.080 --> 14:22.060
<<computer tone
sounds>>

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Ah, that's somebody else;
okay.

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Okay, now so there'd be four
disubstituted isomers if it were

14:31.928 --> 14:32.248
Right?

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Now how about if it were this
structure?

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How many monos?

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If you replace just one
hydrogen, how many different

14:40.702 --> 14:41.472
hydrogens?

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Elizabeth?

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Student:  Six.

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Prof: Six.

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They're all different.

14:45.320 --> 14:46.310
Right?

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So there'll be six monos.

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How many di's?

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Well let's count them.

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So we'll start from the top
left there.

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There would be those sets of
di's, the ones connected by

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those arrows.

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And then we could start from
the top --

14:57.500 --> 15:00.530
pardon me, five of those --
then we could start from that

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one and there'd be four,
and three, and two additional

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ones,
and one, from that,

15:05.830 --> 15:07.950
that hasn't been counted
before.

15:07.950 --> 15:12.210
So we'd have fifteen
disubstituted isomers.

15:12.210 --> 15:13.210
Right?

15:13.210 --> 15:14.450
Now how about these?

15:14.450 --> 15:18.830
Notice for these two that in
both cases it's AA,

15:18.831 --> 15:19.671
BB, AA.

15:19.668 --> 15:22.238
In the bottom one it's turned
on its side, but the same

15:22.241 --> 15:23.911
relationship,
the symmetry of it.

15:23.909 --> 15:24.969
Right?

15:24.970 --> 15:27.430
Everybody see that?

15:27.428 --> 15:30.038
Okay, so these will have the
same number of isomers,

15:30.042 --> 15:32.712
even though their structures
are obviously different,

15:32.706 --> 15:34.496
but the same number of isomers.

15:34.500 --> 15:38.520
So there'll be two
monosubstituted isomers,

15:38.519 --> 15:40.529
either an A or a B.

15:40.529 --> 15:43.729
And now we've got the di's to
figure out, and it turns out

15:43.729 --> 15:44.739
there'll be six.

15:44.740 --> 15:46.890
I'll let you do that yourself.

15:46.889 --> 15:48.879
Okay, so two six there.

15:48.879 --> 15:51.689
Now how about if it were this,
which is AA,

15:51.688 --> 15:55.098
BB, CC, because it doesn't have
as much symmetry.

15:55.100 --> 15:58.120
The red ones don't have as much
symmetry as the blue ones,

15:58.120 --> 16:00.400
although they're the same as
one another.

16:00.399 --> 16:03.829
So it turns out there are three
monos, A or B or C,

16:03.826 --> 16:07.286
and there are nine di's;
and you can work those out if

16:07.287 --> 16:08.037
you want to.

16:08.038 --> 16:10.578
Okay, so you can't distinguish
among the red ones,

16:10.580 --> 16:13.980
or between the blue ones,
but you can distinguish a black

16:13.975 --> 16:17.275
one from an aqua one,
from a blue, from a red.

16:17.278 --> 16:19.998
Okay, so again you can get
information about structure by

16:19.998 --> 16:22.088
counting isomers,
if you're good enough,

16:22.086 --> 16:26.546
and you can prepare them,
and you can tell them apart.

16:26.548 --> 16:29.468
Okay, now here's a problem for
you to do for Monday,

16:29.465 --> 16:31.975
and you can work in groups if
you want to.

16:31.980 --> 16:37.380
But take -- this is planar and
two-dimensional,

16:37.379 --> 16:38.319
right?

16:38.320 --> 16:41.210
But in truth,
because carbons are

16:41.205 --> 16:45.255
tetrahedral, when they form four
single bonds,

16:45.264 --> 16:47.974
the others are not planar.

16:47.970 --> 16:48.340
Right?

16:48.336 --> 16:51.686
And I've drawn here how each of
them would look in three

16:51.693 --> 16:53.633
dimensions,
with some things,

16:53.625 --> 16:55.775
the wedges, coming out toward
you,

16:55.779 --> 16:58.339
and other things,
the dotted ones,

16:58.336 --> 17:00.426
going back into the page.

17:00.429 --> 17:01.309
Right?

17:01.308 --> 17:05.528
So if you consider these in
three dimensions,

17:05.528 --> 17:08.498
how many isomers can you get?

17:08.500 --> 17:10.070
Right? That's the question.

17:10.069 --> 17:12.729
Mono and disubstituted isomers.

17:12.730 --> 17:15.430
Will you be able to distinguish
these, if they're

17:15.425 --> 17:16.545
three-dimensional?

17:16.548 --> 17:20.028
And -- that one,
remember, is called Dewar

17:20.030 --> 17:23.070
benzene --
and the one that's not included

17:23.067 --> 17:26.317
was prismane or what's called
"Ladenburg"

17:26.323 --> 17:27.043
benzene.

17:27.038 --> 17:28.768
Remember I said we'd talk about
Ladenburg;

17:28.769 --> 17:30.029
he's coming up.

17:30.028 --> 17:34.038
So that's another possibility,
a three-dimensional symmetrical

17:34.042 --> 17:34.572
prism.

17:34.568 --> 17:34.878
Right?

17:34.875 --> 17:37.475
Which obviously Dewar couldn't
make with his models,

17:37.481 --> 17:40.551
because they would -- unless he
distorted them and bent them,

17:40.547 --> 17:41.107
right?

17:41.108 --> 17:43.488
Okay, so the question is,
how many mono- and

17:43.491 --> 17:45.931
disubstituted isomers can you
get from these,

17:45.931 --> 17:47.761
if they're three-dimensional?

17:47.759 --> 17:52.669
So you guys can get together
and talk that over and tell me

17:52.673 --> 17:53.693
on Monday.

17:53.690 --> 17:55.980
Okay?

17:55.980 --> 18:00.460
Now there's another way to get
-- to infer structure,

18:00.463 --> 18:02.883
and that's from synthesis.

18:06.865 --> 18:09.555
molecules,
shown on the left there -- you

18:09.563 --> 18:13.173
see the C double bond O and the
two methyls attached to each

18:13.169 --> 18:14.269
central carbon.

18:14.269 --> 18:19.859
So three of those come together
at a, at b,

18:19.861 --> 18:22.361
at c,
losing water,

18:22.356 --> 18:27.146
and they form the double bonds
on the right.

18:27.150 --> 18:32.050
So if that's the way the stuff
forms, then the three carbons in

18:32.047 --> 18:36.937
this compound called mesitylene
should be on every other carbon

18:36.944 --> 18:38.924
of the ring -- right?

18:38.920 --> 18:41.900
-- because of the way it was
put together by losing waters

18:41.904 --> 18:42.694
between them.

18:42.690 --> 18:47.150
So that's another way of
figuring out where something is,

18:47.154 --> 18:48.354
by synthesis.

18:48.348 --> 18:54.418
Notice incidentally that there
are little dots in the middle of

18:54.416 --> 18:56.566
some of these bonds.

18:56.568 --> 18:58.628
Why do you think those dots are
there?

18:58.630 --> 19:07.370


19:07.368 --> 19:11.818
I think it's because this is a
picture of a real model,

19:11.818 --> 19:14.528
that was made with wire coming
out of balls,

19:14.528 --> 19:20.748
and that was the thing that
screwed them together.

19:22.672 --> 19:25.832
when he wanted to come work
with him, a set of these

19:25.834 --> 19:27.264
brass-strip models.

19:30.490 --> 19:32.490
from brass-strip models.

19:32.490 --> 19:35.150
Notice the funny bonds between
the carbon and the oxygen,

19:35.150 --> 19:36.860
the double bonds,
those triangles.

19:36.858 --> 19:40.718
Those are made of stiff wire
that comes straight out and then

19:40.719 --> 19:43.099
were fastened together at an
angle.

19:43.098 --> 19:43.398
Okay?

19:46.789 --> 19:49.209
used in his lectures to show
benzene.

19:49.210 --> 19:53.860
And you can see that the carbon
is tetrahedral.

19:53.859 --> 19:54.599
Okay?

19:54.599 --> 19:55.299
Yes?

19:57.299 --> 19:59.009
also had a dream about a snake
eating itself?

19:59.009 --> 20:02.219
Prof: Yeah,
that he made up in 1890.

20:02.220 --> 20:04.370
This is long after this.

20:04.368 --> 20:07.498
It's conceivable that he had
such a dream,

20:07.502 --> 20:10.562
but it was a story told for
amusement.

20:10.558 --> 20:13.978
Okay, so here he is at the time
he did this, in 1865,

20:13.984 --> 20:14.714
in Ghent.

20:14.710 --> 20:19.310
And he was surrounded with some
hotshots.

20:19.308 --> 20:22.878
Koerner, for example,
who was a young man.

20:25.992 --> 20:29.482
as well as a lab worker,
and suffered from rheumatism.

20:29.480 --> 20:33.080
He came from Central Germany,
which is a very sort of damp

20:33.083 --> 20:36.373
place a lot of the time,
and cloudy, and so it wasn't

20:36.369 --> 20:39.859
good for his rheumatism;
nor was Ghent in Belgium.

20:39.858 --> 20:45.738
So, on his doctor's advice,
he went to sunny Palermo in

20:45.740 --> 20:46.720
Sicily.

20:46.720 --> 20:50.710
So Palermo is down there on the
northwest coast of Sicily.

20:50.710 --> 20:55.010
And Sicily is a very
interesting place in history.

20:55.009 --> 20:58.799
It was liberated in 1860,
just before this time,

20:58.797 --> 21:03.307
after 2700 years of occupation
by successive cultures.

21:03.309 --> 21:04.899
Okay?

21:04.900 --> 21:10.830
And the most recent one was
Garibaldi, who came down with

21:10.832 --> 21:16.662
his Red Shirts in 1860 to free
Sicily from the Bourbons,

21:16.659 --> 21:19.519
from the Spanish crown.

21:19.519 --> 21:20.449
Okay?

21:20.450 --> 21:26.560
And Cannizzaro came along and
established this laboratory in

21:26.563 --> 21:27.603
Palermo.

21:27.598 --> 21:32.598
In 1860, he helped both
Garibaldi, and the late

21:32.596 --> 21:35.526
Avogadro, and Gay-Lussac.

21:35.529 --> 21:38.459
Both of these were
revolutionaries,

21:38.463 --> 21:41.143
although in different senses.

21:41.140 --> 21:44.270
Garibaldi was revolutionary
politically,

21:44.269 --> 21:48.509
and Avogadro was revolutionary
about knowing the atomic weight

21:48.508 --> 21:52.398
and molecular weight of
compounds through gas density.

21:52.400 --> 21:56.050
But in his way,
Cannizzaro was a cautious

21:56.048 --> 21:56.868
person.

21:56.868 --> 22:00.348
This is what he said when he
spoke in London,

22:00.348 --> 22:02.958
in 1872, on chemistry teaching.

22:02.960 --> 22:06.730
He said: "Above all we
should take care that the pupils

22:06.730 --> 22:10.180
do not form to themselves any
mechanical or geometrical

22:10.180 --> 22:12.800
conception of the cause and
effects of the

22:12.799 --> 22:15.989
quantivalence"
(that is, the valence) "of

22:15.994 --> 22:16.894
atoms.

22:16.890 --> 22:20.940
They must frequently be
reminded that chemical facts

22:20.935 --> 22:25.375
neither prove not disprove
anything relating to size,

22:25.380 --> 22:28.930
form, continuity,
distance, relative disposition,

22:28.930 --> 22:30.900
etcetera, etcetera,
of the atoms."

22:30.900 --> 22:34.170
That is, don't use these
arguments for this purpose of

22:34.173 --> 22:37.573
finding where things -- where
atoms are in molecules.

22:37.568 --> 22:41.718
This is just a fancy that will
appeal to students but isn't

22:41.715 --> 22:42.855
anything real.

22:42.858 --> 22:46.088
This was a very cautious,
conservative point of view,

22:46.093 --> 22:49.143
but was shared by all the
leaders in chemistry.

22:49.140 --> 22:52.480
"If we are sometimes
obliged to speak of the relative

22:52.480 --> 22:54.710
position of atoms in the
molecules,

22:54.710 --> 22:57.550
and even to give graphic
representation of these

22:57.553 --> 23:00.063
positions,
we must hasten to remark that

23:00.058 --> 23:03.258
these figures are nothing but
artifices of the mind,

23:03.259 --> 23:07.549
intended to represent to the
sight that which in reality we

23:07.548 --> 23:11.838
perceive only through the veil
of transformations."

23:11.838 --> 23:15.238
(So it's only through reactions
that you try to infer something

23:15.240 --> 23:17.380
about this; it's not something
real.)

23:17.380 --> 23:20.700
"But that we do not really
know what it is that corresponds

23:20.698 --> 23:23.578
to that which we call position,
either in space,

23:23.580 --> 23:26.790
or in the mutual action of
different portions of

23:26.788 --> 23:27.878
matter."

23:27.880 --> 23:30.400
But he was one of the leaders,
right?

23:30.400 --> 23:33.120
The followers,
the students were more --

23:33.124 --> 23:34.944
didn't take this,
right?

23:34.940 --> 23:37.410
So Koerner, who had just come
down,

23:37.410 --> 23:40.800
for his health,
and joined Cannizzaro --

23:40.798 --> 23:46.288
in 1869 he came down,
or he came down in 1866,

23:46.288 --> 23:51.408
I think, or seven -- he wrote a
paper called Facts Serving to

23:51.407 --> 23:56.037
Determine Chemical Position in
Aromatic Substances.

23:56.038 --> 23:56.428
Right?

23:56.434 --> 24:00.324
So a completely different point
of view from what Cannizzaro

24:00.323 --> 24:02.963
advocated three years later --
right?

24:02.960 --> 24:04.980
-- to determine position.

24:04.980 --> 24:07.870
Cannizzaro, to be fair to him,
was saying that you have to be

24:07.873 --> 24:10.673
different in what you teach and
what you do research on.

24:10.670 --> 24:13.590
Teaching should be things you
really know for sure,

24:13.586 --> 24:17.146
and research should be things
where you're trying to push back

24:17.146 --> 24:18.136
the frontier.

24:18.140 --> 24:21.140
But anyhow, Koerner wrote in
this paper, in 1869:

24:21.140 --> 24:25.120
"The dogma of the
impossibility of determining the

24:25.121 --> 24:27.851
atomic constitution of
substances,

24:27.848 --> 24:31.608
which until recently was
advocated with such fervor by

24:31.613 --> 24:35.593
the most able chemists,
is beginning to be abandoned

24:35.588 --> 24:38.538
and forgotten;
and one can predict that the

24:38.540 --> 24:42.360
day is not far in the future
when a sufficient collection of

24:42.357 --> 24:46.367
facts will permit determination
of the internal architecture of

24:46.368 --> 24:47.338
molecules.

24:47.338 --> 24:50.398
A series of experiments
directed toward such a goal is

24:50.402 --> 24:52.312
the object of this paper."

24:52.308 --> 24:52.598
Right?

24:52.598 --> 24:54.338
So that's what he was trying to
do.

24:54.338 --> 24:58.268
If he came along fifty years
later, what would he have used,

24:58.270 --> 25:01.270
to find the position of atoms
in molecules?

25:01.269 --> 25:06.939


25:06.940 --> 25:08.200
X-ray, right?

25:08.200 --> 25:13.200
But he's using only reactions
and counting isomers.

25:13.200 --> 25:13.590
Right?

25:13.588 --> 25:16.508
So only the properties of
things and elemental

25:16.507 --> 25:17.477
composition.

25:17.480 --> 25:20.500
Okay, "facts will permit
determination of the internal

25:20.497 --> 25:21.587
architecture."

25:21.589 --> 25:24.419
So this was a really new idea.

25:24.420 --> 25:26.960
And the paper,
when it was published,

25:26.960 --> 25:31.300
was introduced and warmly
endorsed by Cannizzaro,

25:31.298 --> 25:33.938
who went on to say,
later, that you shouldn't be

25:33.940 --> 25:37.010
teaching this to students,
but it is a good thing to study.

25:37.009 --> 25:37.369
Okay?

25:37.367 --> 25:41.527
So Koerner had -- there's a
thing called the Koerner Proof,

25:41.526 --> 25:43.386
which indeed he proved.

25:43.390 --> 25:45.880
But it's not what he did in
1869.

25:45.880 --> 25:47.210
But this is very interesting.

25:47.210 --> 25:50.400
So you have,
among disubstituted isomers,

25:50.397 --> 25:52.867
you have these possibilities.

25:52.869 --> 25:54.659
Okay?

25:54.660 --> 25:57.310
Now, how could you tell which
is which?

25:57.309 --> 25:58.699
Suppose you had three bottles.

25:58.700 --> 26:00.480
One is black,
one red, one blue,

26:00.483 --> 26:03.653
but they're not colored,
they're just white crystals;

26:03.650 --> 26:05.710
or liquids maybe in this case.

26:05.710 --> 26:08.450
How could you tell which one is
which?

26:08.450 --> 26:10.730
How could you put a label on
the bottle?

26:10.730 --> 26:14.310
Well you could label them just
with names, call them something

26:14.307 --> 26:17.647
like ortho, meta and para,
which doesn't mean anything.

26:17.650 --> 26:19.460
Right?

26:19.460 --> 26:25.270
But how do you know what
structure they have?

26:25.269 --> 26:28.649
Cory?

26:28.650 --> 26:29.920
Student:  Differences in
melting point.

26:29.920 --> 26:31.950
Prof: Oh,
differences in melting point

26:31.946 --> 26:33.326
will show they're different.

26:33.328 --> 26:35.378
But how do you know which one
will have the lowest melting

26:35.382 --> 26:35.672
point?

26:35.670 --> 26:37.920
You have to have some theory
about melting point as a

26:37.921 --> 26:40.331
function of structure;
which didn't exist.

26:40.329 --> 26:41.659
Or do you have such a theory?

26:41.660 --> 26:43.090
Student:  Polarity.

26:43.088 --> 26:44.918
Prof: Polarity.

26:44.920 --> 26:49.830
But notice, to be -- yeah,
one of them would be non-polar.

26:49.828 --> 26:52.028
But people weren't measuring
polarity at this time.

26:52.029 --> 26:54.589
So that was out of the question.

26:54.589 --> 26:56.729
Okay, here's how Koerner did it.

26:56.730 --> 27:01.580
He added a third identical
substituent and counted the

27:01.584 --> 27:02.504
isomers.

27:02.500 --> 27:04.300
So suppose you start with a
black one,

27:04.298 --> 27:07.278
you add a third one,
down to the right,

27:07.278 --> 27:11.868
or down at the bottom,
or -- and that's all,

27:11.868 --> 27:14.808
because if you added down to
the left it would be the same as

27:14.810 --> 27:17.510
down at the bottom,
and if you added over to the

27:17.510 --> 27:20.060
left it would be the same as
down to the --

27:20.058 --> 27:22.398
over to the left would be the
same as down to the right.

27:22.400 --> 27:24.020
Everybody see that?

27:24.019 --> 27:26.259
You can only get two isomers,
starting from there.

27:26.259 --> 27:30.549
If you start from the red one,
you can get either of those.

27:30.548 --> 27:37.478
Can you get anything else,
from the red one?

27:37.480 --> 27:38.640
Andrew?

27:38.640 --> 27:39.940
Student: You get another
one on the left.

27:39.940 --> 27:41.310
Prof: Yeah,
down to the left will be a

27:41.310 --> 27:41.810
different one.

27:41.808 --> 27:43.198
So you'll get three from that
one.

27:43.200 --> 27:45.780
And how about if you start with
the blue one and put a third one

27:45.775 --> 27:45.975
in?

27:45.980 --> 27:49.590


27:49.589 --> 27:51.249
Maria, what do you say?

27:51.250 --> 27:54.850
If you start with the blue one
and put a third blue substituent

27:54.848 --> 27:55.138
in?

27:55.140 --> 27:56.890
Student:  You have the
one in the middle.

27:56.890 --> 27:58.260
Prof: Speak up a little.

27:58.259 --> 28:01.199
Student:  You have the
one in the middle.

28:01.200 --> 28:04.210
Prof: Right,
you can get the one in the

28:04.208 --> 28:04.808
middle.

28:04.808 --> 28:07.968
Student:  And everything
else is just --

28:07.970 --> 28:09.120
Prof: Nothing else.

28:09.118 --> 28:09.658
Right?

28:09.663 --> 28:12.203
Only one possibility there.

28:12.200 --> 28:12.530
Okay?

28:12.529 --> 28:15.109
So one will give one,
one will give two,

28:15.108 --> 28:18.028
one will give three;
supposing that all the

28:18.025 --> 28:19.035
reactions work.

28:19.039 --> 28:20.119
Right?

28:20.118 --> 28:23.468
So it establishes the identity,
not only of the disubstituted,

28:23.465 --> 28:25.985
but only also of the
trisubstituted isomers.

28:25.990 --> 28:28.010
One comes from one of the
original bottles,

28:28.005 --> 28:30.255
one comes from two,
and one comes from three.

28:30.259 --> 28:31.599
So you know it.

28:31.599 --> 28:34.089
Pretty clever.

28:34.088 --> 28:39.688
And these were called -- these
are called now ortho,

28:39.685 --> 28:41.225
meta and para.

28:41.230 --> 28:44.210
But those aren't the names that
Koerner used.

28:44.210 --> 28:48.200


28:48.200 --> 28:50.430
Or more properly,
they are the names that he

28:50.432 --> 28:52.772
used, but not to name the way we
do it now.

28:52.769 --> 28:54.589
If anybody, he had the right to
name them,

28:54.588 --> 28:57.538
but he didn't get it because he
was off in left-field down in

28:57.538 --> 28:59.348
Palermo,
and the people up in Germany,

28:59.351 --> 29:03.591
who were influential,
were the ones that finally

29:03.588 --> 29:05.078
named them.

29:05.078 --> 29:09.878
But notice that this argument
holds if benzene is a hexagon.

29:09.880 --> 29:14.120
But if it were a pentagonal
pyramid, or some other geometric

29:14.119 --> 29:17.929
structure, then this argument
wouldn't work at all.

29:17.930 --> 29:18.230
Right?

29:18.234 --> 29:21.394
So you have first to know that
all the positions are equivalent

29:21.387 --> 29:23.337
in benzene;
that it's not like some of

29:23.343 --> 29:25.643
those Dewar structures we were
just looking at.

29:25.640 --> 29:27.230
So how can you do that?

29:27.230 --> 29:30.430
This was proven,
in this paper in 1869,

29:30.432 --> 29:31.532
by Koerner.

29:31.528 --> 29:33.888
The paper was published in
Palermo,

29:33.890 --> 29:37.050
in the Giornale di Scienze
Naturale ed Economiche di

29:37.050 --> 29:39.270
Palermo,
which is the Journal of

29:39.266 --> 29:41.626
Natural Science and Economics of
Palermo.

29:41.630 --> 29:44.950
It's a beautiful big journal
with big wide margins on the

29:44.951 --> 29:45.961
page and so on.

29:45.960 --> 29:47.930
But it had no circulation
whatever.

29:47.930 --> 29:50.060
People in Germany,
years later,

29:50.058 --> 29:53.608
were still trying to prove
this, what he proved.

29:53.609 --> 29:55.299
Okay, now here's a question.

29:55.298 --> 29:58.358
Are the four valences of carbon
equivalent;

29:58.358 --> 30:02.868
that is, are they all the same
or might one of them be a thick

30:02.867 --> 30:06.117
bond, say, different from the
other three?

30:06.118 --> 30:06.448
Right?

30:06.451 --> 30:09.671
Or, might it even be that two
of the bonds are different;

30:09.670 --> 30:13.070
one's long, or one of them's
curly.

30:13.069 --> 30:15.209
Might all four be different?

30:15.210 --> 30:18.710
Can you think of any evidence
to say that the bonds in methane

30:18.712 --> 30:19.692
are equivalent?

30:19.690 --> 30:22.920


30:22.920 --> 30:24.440
How could you prove this?

30:24.440 --> 30:27.790
Alison, you got any idea?

30:27.789 --> 30:30.809
What technique is Koerner using?

30:30.809 --> 30:31.749
Pardon me?

30:31.750 --> 30:32.810
Student:  To see if the
isomers are --

30:32.809 --> 30:33.989
Prof: Counting isomers.

30:33.990 --> 30:36.800
So what do you expect if you
have four different bonds in

30:36.798 --> 30:37.248
carbon?

30:37.250 --> 30:39.170
Student:  It matters
where you substitute.

30:39.170 --> 30:39.790
Prof: Right.

30:39.787 --> 30:41.767
How many isomers would you get
if there were four different?

30:41.769 --> 30:43.359
Student:  One substitute.

30:43.358 --> 30:45.808
Prof: You could have a
chlorine on the fat bond,

30:45.813 --> 30:47.723
the long bond,
the curly bond or the normal

30:47.721 --> 30:49.661
bond;
there'd be four isomers,

30:49.663 --> 30:53.373
if you could make them all,
and they didn't interconvert.

30:53.369 --> 30:54.269
Right?

30:54.269 --> 30:58.389
So there's only one methyl
chloride.

30:58.390 --> 31:00.620
Nobody's found a second methyl
chloride.

31:00.618 --> 31:06.148
So does that prove that they're
all equivalent?

31:06.150 --> 31:07.280
Yeah?

31:07.279 --> 31:08.519
Pardon me?

31:08.519 --> 31:09.569
Yeah?

31:09.568 --> 31:10.818
Student:  Not on its
own.

31:10.816 --> 31:13.306
But they can attach more than
one chlorine to all the bonds.

31:13.308 --> 31:16.598
So they know that they all sort
of -- they will work.

31:16.598 --> 31:17.718
Prof: Okay.

31:17.722 --> 31:21.342
That doesn't prove it alone,
because maybe there's only one

31:21.344 --> 31:23.034
that can be substituted.

31:23.029 --> 31:23.889
Right?

31:23.890 --> 31:26.550
That is, of the one -- if it
had the structure on the right,

31:26.545 --> 31:29.015
with four different bonds,
maybe you could make only one

31:29.019 --> 31:30.099
monosubstituted one.

31:30.098 --> 31:33.888
How many disubstituted ones
could you make then?

31:33.890 --> 31:35.430
Student:  The one.

31:35.430 --> 31:37.490
Prof: If the first one
went in one place,

31:37.494 --> 31:39.214
the second one could go any of
three.

31:39.210 --> 31:42.490
So you'd still say get three
disubstituted.

31:42.490 --> 31:45.200
Unless you could only make one
of those.

31:45.200 --> 31:46.770
Then there would only be two.

31:46.769 --> 31:50.629
So it doesn't really prove it,
the way a mathematician would

31:50.627 --> 31:53.437
like things proven,
but it's a plausibility,

31:53.439 --> 31:54.289
at least.

31:54.289 --> 31:55.389
Okay?

31:55.390 --> 32:00.290
But it turns out that -- so the
evidence is there's only one

32:00.290 --> 32:03.780
isomer known,
but it's not really proof.

32:03.778 --> 32:06.538
Now Koerner,
in order to make a real proof,

32:06.535 --> 32:10.265
had to make some assumptions,
and these he stated in 1867,

32:10.273 --> 32:13.623
two years before the paper
we're talking about.

32:13.618 --> 32:16.278
And the first is direct
replacement.

32:16.278 --> 32:18.888
And he wrote:
"If one grants that in

32:18.887 --> 32:22.867
simple transformation the new
substituent assumes the position

32:22.865 --> 32:25.665
of the element
displaced…."

32:25.670 --> 32:28.810
So you can take a substituent
off and put a new one there,

32:28.808 --> 32:32.168
but it will go in the same
place that the old one came off.

32:32.170 --> 32:34.150
Everything doesn't rearrange.

32:34.150 --> 32:37.520
You need that in order to make
arguments like he wants to make.

32:37.519 --> 32:38.339
Right?

32:38.339 --> 32:40.219
And in fact that's often true.

32:40.220 --> 32:42.490
It's certainly logically
parsimonious.

32:42.490 --> 32:44.410
It's the simplest assumption
you can make,

32:44.409 --> 32:45.579
and it usually is true.

32:45.578 --> 32:48.688
But there are rearrangements
that do sometimes occur,

32:48.693 --> 32:50.973
and we'll talk about those
later on.

32:50.970 --> 32:54.200
But anyhow, for his argument
you have to assume direct

32:54.204 --> 32:55.064
replacement.

32:55.058 --> 32:58.188
And the second assumption is
experimental distinguishability,

32:58.189 --> 32:59.909
which we've been talking about.

32:59.910 --> 33:02.160
"Most of the
demonstrations are based on

33:02.159 --> 33:05.229
establishing the identity or
difference of several substances

33:05.227 --> 33:07.877
of the same composition,
but obtained by different

33:07.882 --> 33:08.682
reactions."

33:08.680 --> 33:11.370
So you have to be confident
that you can tell when things

33:11.371 --> 33:13.391
are the same and when they're
different.

33:13.390 --> 33:14.960
We just were talking about that.

33:14.960 --> 33:17.600
But Koerner was in an
especially good position to do

33:17.596 --> 33:20.126
this, because he was really
skilled in the lab.

33:20.130 --> 33:24.130
In fact, he got a Silver Medal
at a Paris exposition,

33:24.130 --> 33:26.030
one of these international
expositions,

33:26.028 --> 33:29.458
for his collection of beautiful
crystals that he had lovingly

33:29.462 --> 33:31.262
prepared,
and they had different shapes

33:31.259 --> 33:31.679
and so on.

33:31.680 --> 33:33.790
That's one way you can tell
things apart.

33:33.788 --> 33:36.208
And, in fact,
this bottle here,

33:36.211 --> 33:40.571
says up at the top -- I don't
think you can read it.

33:40.568 --> 33:44.088
But it says,
Laboratorio di Chimica

33:44.092 --> 33:47.712
Organica,
and it says Acido,

33:47.709 --> 33:50.279
Ac., Salicilico.

33:50.279 --> 33:51.969
So what is it?

33:51.970 --> 33:53.370
Salicylic Acid.

33:53.368 --> 33:57.308
This is Koerner's sample --
right?

33:57.309 --> 33:58.969
-- of salicylic acid.

33:58.970 --> 34:03.270
And here are his beautiful
crystals of

34:03.269 --> 34:06.639
2,6-dinitro-3-bromotoluene.

34:06.640 --> 34:08.430
You can't see them so well here.

34:08.429 --> 34:11.329
Well you can see that they're
still beautiful crystals,

34:11.329 --> 34:11.759
right?

34:11.760 --> 34:14.910
So those are some of his prize
crystals.

34:14.909 --> 34:19.829
And here you can -- so crystals
are the best way to tell.

34:19.829 --> 34:23.189
And there are pictures of those
things that were Koerner's.

34:23.190 --> 34:27.530
Okay, now here's his proof,
based on those assumptions.

34:27.530 --> 34:32.590
Okay, so there are three known
isomers of

34:32.594 --> 34:35.764
hydroxycarboxylbenzene.

34:35.760 --> 34:37.050
So you don't know anything
about the structure.

34:37.050 --> 34:39.430
All you know is you is you have
C_6H_4.

34:39.429 --> 34:42.969
You originally had C_6H_6,
but one of the H's has been

34:42.974 --> 34:45.654
replaced by OH,
and another H by COOH.

34:45.650 --> 34:49.000
And the names of these things
are salicylic acid,

34:48.998 --> 34:52.068
which I just showed you,
hydroxybenzoic acid,

34:52.067 --> 34:54.297
and parahydroxybenzoic acid.

34:54.300 --> 34:56.880
So the names don't mean
anything, but those are just

34:56.880 --> 34:58.400
what the bottles are called.

34:58.400 --> 34:59.450
Right?

34:59.449 --> 35:03.639
Now, you want to use the
existence of at least three.

35:03.639 --> 35:07.189
You don't -- there might be
eighteen, for all you know,

35:07.188 --> 35:08.568
disubstituted ones.

35:08.570 --> 35:09.600
Right?

35:09.599 --> 35:12.279
But you know there are at least
these three, and you can do

35:12.280 --> 35:13.390
experiments with them.

35:13.389 --> 35:15.069
So these are the experiments.

35:15.070 --> 35:19.110
First you treat it with HCl and
heat it up -- this was done by

35:19.108 --> 35:21.888
Graebe in 1866 -- and it loses
the COOH.

35:21.889 --> 35:25.299
Remember, we talked about how
you could take benzoic acid,

35:25.297 --> 35:27.507
take the COOH off and have
benzene.

35:27.510 --> 35:29.280
Here it's done,
take the COOH off,

35:29.278 --> 35:31.208
but you're left with still the
OH.

35:31.210 --> 35:33.550
And that compound,
as we mentioned before,

35:33.547 --> 35:34.627
is called phenol.

35:34.630 --> 35:35.520
Right?

35:35.518 --> 35:39.518
But you can do the same trick
with parahydroxybenzoic acid,

35:39.516 --> 35:43.026
and you get the same phenol,
not a different one.

35:43.030 --> 35:45.210
What does that tell you?

35:45.210 --> 35:48.990
It tells you that the
difference between salicylic

35:48.992 --> 35:53.472
acid and parahydroxybenzoic acid
is not where the OH is.

35:53.469 --> 35:53.819
Right?

35:53.824 --> 35:57.264
They have to have the same
position of the OH because -- or

35:57.255 --> 36:00.685
at least equivalent ones --
because you get the same phenol

36:00.686 --> 36:01.216
out.

36:01.219 --> 36:04.929
And it was found also by Graebe
that you could convert

36:04.929 --> 36:09.199
hydroxybenzoic acid to methyl
anisate -- that's the OH becomes

36:09.199 --> 36:11.859
OCH_3;
and so same for the acid.

36:11.860 --> 36:15.840
And then you can convert it to
anisic acid, taking the CH_3 off

36:15.836 --> 36:19.026
on the acid group;
from the ester, that is to say.

36:19.030 --> 36:23.900
And you can convert phenol by
way of bromoanisole -- Koerner,

36:23.896 --> 36:28.106
I believe, did this -- and then
with sodium and CO_2,

36:28.114 --> 36:30.714
to get the same anisic acid.

36:30.710 --> 36:31.510
Right?

36:31.510 --> 36:35.950
So the OH position has to be
the same in hydroxybenzoic acid

36:35.945 --> 36:39.025
as it was in salicylic and
parahydroxy.

36:39.030 --> 36:39.310
Right?

36:39.311 --> 36:41.661
So the conclusion is the
difference among the three

36:41.663 --> 36:45.073
isomers that are known --
salicylic, hydroxybenzoic acid,

36:45.068 --> 36:48.568
parahydroxybenzoic --
is not where the OH is,

36:48.568 --> 36:52.518
since the O survives in
identical compounds.

36:52.518 --> 36:55.868
So, for example,
it couldn't be like these three

36:55.865 --> 36:58.425
isomers of Dewar's structure
here.

36:58.429 --> 37:01.779
Suppose that were one of the
acids, and that were another

37:01.782 --> 37:03.642
one, and that were the third.

37:03.639 --> 37:06.889
Then when you took the COOH
off, you'd have three different

37:06.889 --> 37:07.729
OH compounds.

37:07.730 --> 37:09.680
Everybody with me?

37:09.679 --> 37:11.069
You see that?

37:11.070 --> 37:12.630
So the difference among these
three,

37:12.630 --> 37:14.570
whatever it is that makes them
different,

37:14.570 --> 37:18.520
can't be where the OH is,
because you can remove the COOH

37:18.523 --> 37:20.433
and they're all the same.

37:20.429 --> 37:22.909
Okay, that's the first step.

37:22.909 --> 37:29.069
And the conclusion is the sites
occupied by OH are either

37:29.070 --> 37:32.040
identical, or equivalent.

37:32.039 --> 37:33.069
Here's what's equivalent.

37:33.070 --> 37:34.320
So suppose they could be this.

37:34.320 --> 37:35.910
There's OH, COOH.

37:35.909 --> 37:37.839
See did I? Yeah here.

37:37.840 --> 37:41.500
So you could have that OH,
that OH and that -- no,

37:41.501 --> 37:44.911
there's;
okay, sorry I'm messing this up.

37:44.909 --> 37:48.889
There's one,
one, one, two,

37:48.893 --> 37:50.123
three.

37:50.119 --> 37:52.869
The OHs in those are not the
same position -- one's the top

37:52.871 --> 37:55.571
right, the other's the top left
-- but they're equivalent,

37:55.574 --> 37:56.054
right?

37:56.050 --> 37:58.230
Once you got it,
you could flop if over and

37:58.231 --> 38:00.001
they're the same as one another.

38:00.000 --> 38:03.600
Okay, so we know at least that
much.

38:03.599 --> 38:07.239
So let's call the OH position,
which is either the same or

38:07.242 --> 38:09.482
exactly equivalent,
call it ω,

38:09.481 --> 38:10.761
just for a name.

38:10.760 --> 38:13.740
So OH is in the position of
Hω;

38:13.739 --> 38:16.419
that we know,
in all three compounds.

38:16.420 --> 38:18.090
Can you guess what you do next?

38:18.090 --> 38:25.550


38:25.550 --> 38:27.260
Next -- yeah?

38:27.260 --> 38:30.980
Student:  Probably take
the OH off and leave the COOH

38:30.983 --> 38:31.303
on.

38:31.300 --> 38:32.460
Prof: Ah!

38:32.463 --> 38:35.083
Take the OH off and leave the
COOH.

38:35.079 --> 38:35.659
Right?

38:35.659 --> 38:37.169
And that's possible too.

38:37.170 --> 38:39.550
You can treat it with
phosphorus pentachloride,

38:39.550 --> 38:44.750
which converts OH to Cl,
and then with sodium and water,

38:44.750 --> 38:47.760
which takes the chlorine off
and leaves hydrogen.

38:47.760 --> 38:49.710
So you get benzoic acid.

38:49.710 --> 38:53.110
And what do you do next?

38:53.110 --> 38:55.440
Do it to one of the other
compounds, right?

38:55.440 --> 38:58.290
There, and you get the same
benzoic acid.

38:58.289 --> 39:01.679
Or you do it here -- and I
think Koerner is the guy who did

39:01.675 --> 39:04.825
that, although I can't --
haven't found a paper to that

39:04.827 --> 39:05.467
effect.

39:05.469 --> 39:08.489
But anyhow, he says that you
get the same benzoic acid.

39:08.489 --> 39:09.099
So what does that prove?

39:09.099 --> 39:12.909


39:12.909 --> 39:16.679
So the COOH positions are not
fundamentally different in the

39:16.679 --> 39:17.829
three compounds.

39:17.829 --> 39:21.639
So the OH positions are
equivalent, and also the COOH

39:21.641 --> 39:23.621
positions are equivalent.

39:23.619 --> 39:26.359
How can they then be different,
the three compounds?

39:26.360 --> 39:29.550
Because they're clearly
different.

39:29.550 --> 39:32.750
It must be their relative
positions that are different.

39:32.750 --> 39:33.960
Okay?

39:33.960 --> 39:38.510
The three isomers do not differ
because of the absolute position

39:38.509 --> 39:39.519
of the COOH.

39:39.518 --> 39:45.388
So the three sites of COOH are
equivalent, and we can name them

39:45.393 --> 39:46.913
x and y and z.

39:46.909 --> 39:48.989
None of them can be exactly the
same,

39:48.989 --> 39:52.029
because if they were exactly
the same then those

39:52.034 --> 39:54.844
hydroxybenzoic acids,
two of them at least,

39:54.844 --> 39:58.094
would've been the same,
if the COOHs -- because we know

39:58.090 --> 39:59.390
the OHs were the same.

39:59.389 --> 40:00.219
Okay?

40:00.219 --> 40:03.429
So, for example,
it could be this way in Dewar's

40:03.427 --> 40:04.107
benzene.

40:04.110 --> 40:08.020
You could have that position
for x, that for y,

40:08.023 --> 40:09.133
that for z.

40:09.130 --> 40:13.120
And they're all different with
respect to ω.

40:13.119 --> 40:16.549
Okay?

40:16.550 --> 40:18.450
The ω's will all be the
same.

40:18.449 --> 40:23.189
So that would be a possible
structure, according to this.

40:23.190 --> 40:28.220
In this particular case,
ω would be the same as x,

40:28.224 --> 40:33.634
y and z, as a site for a single
substituent, if it were this

40:33.628 --> 40:34.908
structure.

40:34.909 --> 40:38.659
So Arppe, a chemist,
had made a compound which he

40:38.661 --> 40:42.931
called nitroaniline,
which was benzene that had NH_2

40:42.927 --> 40:45.287
and NO_2 in it,
and it was known by

40:45.291 --> 40:49.821
transformations,
that the NH_2 was ω,

40:49.820 --> 40:52.800
that position OHω,
and the NO_2_ was in

40:52.798 --> 40:55.078
y;
that is, you could interconvert

40:55.079 --> 40:58.799
groups and make it into one of
those known hydroxybenzoic

40:58.800 --> 40:59.400
acids.

40:59.400 --> 41:01.950
So you knew it was ω
and y.

41:01.949 --> 41:03.349
Okay?

41:03.349 --> 41:08.509
Now, you do a transfer -- and
it's related to that particular

41:08.514 --> 41:10.414
hydroxybenzoic acid.

41:10.409 --> 41:11.899
Okay?

41:11.900 --> 41:15.400
Now, you replace the NH_2 by Br
-- and we'll talk about these

41:15.398 --> 41:18.138
next semester,
what particular reactions did

41:18.144 --> 41:21.474
that --
and then you change NO_2 to

41:21.472 --> 41:26.632
NH_2, and then you replace the
NH_2 by chlorine.

41:26.630 --> 41:29.680
So now you've got benzene with
bromine;

41:29.679 --> 41:33.069
that must be in the ω
position, because it replaced

41:33.070 --> 41:34.100
the first NH_2.

41:34.099 --> 41:38.059
And then chlorine is in y
because it substituted for the

41:38.059 --> 41:42.089
second NH_2, which came from the
NO_2, which was in y.

41:42.090 --> 41:44.280
Okay?

41:44.280 --> 41:46.050
So it's ω and y.

41:46.050 --> 41:47.130
Can you see what he's going to
do next?

41:47.130 --> 41:55.400


41:55.400 --> 42:00.400
He replaces the NH_2 by Cl,
and then changes NO_2 to NH_2,

42:00.403 --> 42:02.953
and then replaces it by Br.

42:02.949 --> 42:04.149
Why?

42:04.150 --> 42:07.590


42:07.590 --> 42:10.890
So now he's prepared a compound
which clearly has chlorine in

42:10.891 --> 42:13.591
the position ω
and bromine in position y.

42:13.590 --> 42:17.370


42:17.369 --> 42:21.449
I see pursing of brows and so
on.

42:21.449 --> 42:22.169
Is everybody clear?

42:22.170 --> 42:26.350
So he's prepared two -- from
the same compound he's prepared

42:26.351 --> 42:27.841
a bromide chloride.

42:27.840 --> 42:28.540
Right?

42:28.539 --> 42:31.869
One of them is ω
bromide y chloride,

42:31.867 --> 42:34.707
and the other has them
exchanged.

42:34.710 --> 42:37.040
And what do you think he
observed?

42:37.039 --> 42:38.529
They're the same.

42:38.530 --> 42:42.240


42:42.239 --> 42:43.879
What does that show?

42:43.880 --> 42:46.690
Student:
>.

42:46.690 --> 42:49.360
Prof: It shows that
ω is the same as y,

42:49.360 --> 42:51.450
because otherwise these would
be different when you put

42:51.445 --> 42:53.765
different atoms in them;
assuming you have the

42:53.768 --> 42:57.408
experimental technique to tell
when things are different.

42:57.409 --> 42:58.129
Okay?

42:58.130 --> 43:00.400
So now -- so it couldn't be,
for example,

43:00.400 --> 43:04.310
a pentagonal pyramid,
which one had bromine in y,

43:04.309 --> 43:08.049
and the other had chlorine --
chlorine in y,

43:08.050 --> 43:10.290
the other had chlorine and
ω and bromine in y.

43:10.289 --> 43:13.019
It couldn't be that,
those would be different.

43:13.019 --> 43:14.769
Okay?

43:14.768 --> 43:20.038
So there must be at least four
equivalent substituted

43:20.036 --> 43:23.376
positions: ω,
y, x and z.

43:23.380 --> 43:23.950
Okay?

43:23.949 --> 43:27.579
And that would be consistent
with that structure we showed

43:27.579 --> 43:28.789
before, this one.

43:28.789 --> 43:30.919
So it could be that.

43:30.920 --> 43:34.600
It doesn't have to be a hexagon.

43:34.599 --> 43:36.419
Now, here's the next step.

43:36.420 --> 43:40.220
So you start with a compound
that's known to be in the meta

43:40.219 --> 43:40.809
series.

43:40.809 --> 43:46.289
It's got OH in ω,
and nitro, NO_2 -- he wrote

43:46.293 --> 43:48.053
AzO_2 -- in x.

43:48.050 --> 43:52.270
And you have another one that
has again OH in the position

43:52.271 --> 43:55.681
ω, but bromine in x,
instead of nitro.

43:55.679 --> 43:59.109
And now you start doing
substitutions.

43:59.110 --> 44:00.780
Remember, he used
"meta"

44:00.775 --> 44:02.435
differently from the way we do.

44:02.440 --> 44:06.020
Okay, so first he put in a
bromine someplace,

44:06.016 --> 44:08.776
and then a nitro someplace,
and got

44:08.780 --> 44:11.220
bromonitrometanitrophenol.

44:11.219 --> 44:13.049
Okay, he added a bromo and a
nitro group.

44:13.050 --> 44:15.950
So you know that's the benzene
core, with H_2,

44:15.954 --> 44:19.124
with a nitro someplace,
with a bromo someplace.

44:19.119 --> 44:20.829
Those are the ones you just put
in.

44:20.829 --> 44:23.419
But the others are still where
they were.

44:23.420 --> 44:28.890
OH is in ω
and nitro is in x position.

44:28.889 --> 44:32.859
And now you go to the other
compound and put in two nitro

44:32.860 --> 44:33.500
groups.

44:33.500 --> 44:36.970
So you don't know where those
two nitro groups are,

44:36.971 --> 44:40.231
but you know ω
's -- that OH is in ω

44:40.233 --> 44:41.903
and bromine is in x.

44:41.900 --> 44:44.590
Everybody see what you've done
now?

44:44.590 --> 44:48.260
And you know what he observed?

44:48.260 --> 44:51.730
Those two compounds are
identical.

44:51.730 --> 44:53.590
Can you see what conclusion you
get from that?

44:53.590 --> 44:58.350


44:58.349 --> 45:01.409
These are identical, right?

45:01.409 --> 45:07.039
But one of them has bromine in
x, and the other has nitro in x.

45:07.039 --> 45:07.999
How can they be identical?

45:08.000 --> 45:12.840


45:12.840 --> 45:17.110
There must be a second x
position that's -- there must be

45:17.112 --> 45:20.032
two positions that are
equivalent, right?

45:20.030 --> 45:23.620
Now, could any of those
positions be ones we already

45:23.617 --> 45:24.107
know?

45:24.110 --> 45:27.370
Could it be ω?

45:27.369 --> 45:30.699
We already know ω,
x, y and z are the same,

45:30.695 --> 45:31.245
right?

45:31.250 --> 45:34.950
Could this new x be one of
those?

45:34.949 --> 45:36.769
Could it be ω?

45:36.768 --> 45:40.258
No, because we already got
ω, so it can't be ω.

45:40.260 --> 45:43.320
Could it be y or z?

45:43.320 --> 45:46.040
No, because those would've
started from a different

45:46.036 --> 45:48.696
compound in the first place,
if it were y or z.

45:48.699 --> 45:49.129
Right?

45:49.125 --> 45:51.105
You know that this is in x.

45:51.110 --> 45:52.490
It's not y, not z.

45:52.489 --> 45:53.069
Right?

45:53.070 --> 45:56.550
So there must be a second x,
right?

45:56.550 --> 45:59.630
We'll call it x'.

45:59.630 --> 46:02.280
There must be two positions,
x and x'.

46:02.280 --> 46:05.080
They're not equivalent to x,
or y.

46:05.079 --> 46:10.029
They're not the same as x or y
or z or ω.

46:10.030 --> 46:12.230
It's a new one.

46:12.230 --> 46:13.750
Okay?

46:13.750 --> 46:15.550
So that's what they are.

46:15.550 --> 46:19.090
So there must be at least five
equivalent positions.

46:19.090 --> 46:23.920
Now that would be consistent
with that pentagonal pyramid,

46:23.920 --> 46:24.600
right?

46:24.599 --> 46:27.459
All the five around the base of
it are at five equivalent

46:27.456 --> 46:28.066
positions.

46:28.070 --> 46:33.780
But you can't get three
different disubstituted ones

46:33.782 --> 46:36.472
among those positions.

46:36.469 --> 46:39.889
Right? So it couldn't be that.

46:39.889 --> 46:43.069
So none of Dewar's models work
with this, although the perfect

46:43.068 --> 46:44.948
hexagon will work;
a perfect one,

46:44.949 --> 46:47.409
not alternating single and
double bonds.

46:47.409 --> 46:51.859
And the equilateral triangular
prism, the Ladenburg benzene

46:51.856 --> 46:53.156
will also work.

46:53.159 --> 46:55.899
So as I said,
it would work with either of

46:55.902 --> 46:59.982
those, except that the one on
the right doesn't give the right

46:59.981 --> 47:02.391
number of disubstituted isomers.

47:02.389 --> 47:04.819
Now Koerner's own argument,
that the sixth position is

47:04.818 --> 47:07.418
equivalent,
was faulty, because it hinged,

47:07.416 --> 47:10.656
in a subtle way,
on assuming that the benzene

47:10.664 --> 47:13.634
was hexagonal before you proved
it was.

47:13.630 --> 47:15.730
It wasn't obvious,
but it's true if you look at it

47:15.733 --> 47:16.253
carefully.

47:16.250 --> 47:18.640
But it is possible,
using group theory,

47:18.641 --> 47:22.041
to construct a logically
rigorous argument on the basis

47:22.041 --> 47:23.301
of his evidence.

47:23.300 --> 47:26.600
His last step of his argument
was wrong but his evidence was

47:26.601 --> 47:27.891
adequate to prove it.

47:27.889 --> 47:30.439
So he was right in his
intuition that this proved it,

47:30.443 --> 47:32.703
and he was right in his
conclusion that all the

47:32.704 --> 47:34.134
positions are equivalent.

47:34.130 --> 47:37.720
So he deserves enormous credit
for formulating the logic of the

47:37.715 --> 47:40.825
first true structural proof;
not just one of these

47:40.827 --> 47:43.517
plausibility things,
like there's only one

47:43.516 --> 47:46.136
monosubstituted methane --
chloromethane,

47:46.141 --> 47:49.291
therefore all the positions are
equivalent.

47:49.289 --> 47:50.849
Right? That's just plausibility.

47:50.849 --> 47:54.509
But this is a real rigorous
proof.

47:54.510 --> 48:02.090
Now how many isomers do you
think there are of C_2H_4Br_2?

48:02.090 --> 48:02.990
Think about it.

48:02.989 --> 48:06.259
Ethane with two bromines,
how many isomers?

48:06.260 --> 48:07.980
Student:  Two.

48:07.980 --> 48:10.010
Prof: Pardon me?

48:10.014 --> 48:11.434
Somebody guess.

48:11.429 --> 48:13.079
Student:  Two.

48:13.079 --> 48:14.199
Prof: John?

48:14.199 --> 48:15.169
Student:  Three.

48:15.170 --> 48:17.400
Prof: And how would you
get three?

48:17.400 --> 48:21.380
Student:  Because
there's a double bond on the --

48:21.380 --> 48:23.060
Prof: No double bond.

48:23.059 --> 48:24.299
There's six things on it.

48:24.300 --> 48:25.230
So forget that one.

48:25.230 --> 48:25.920
Prof: Yeah.

48:25.922 --> 48:27.502
If it were a double bond you'd
be right.

48:27.500 --> 48:29.570
But there are no double bonds.

48:29.570 --> 48:30.450
Corey?

48:30.449 --> 48:31.389
Student:  Two.

48:31.389 --> 48:33.159
Prof: And what would you
call them?

48:33.159 --> 48:35.359
How would you distinguish them,
or describe them?

48:35.360 --> 48:38.810
Student:  One would have
one bromine on each carbon and

48:38.809 --> 48:41.299
one would have two bromines on
one carbon.

48:41.300 --> 48:42.190
Prof: Good.

48:42.193 --> 48:45.223
So you could have two bromines
on one carbon or one bromine on

48:45.224 --> 48:45.974
each carbon.

48:45.969 --> 48:47.699
So there should be two isomers.

48:47.699 --> 48:50.559
Now, in that same journal,
the same volume of the same

48:50.556 --> 48:52.796
journal,
in 1869, in Palermo,

48:56.996 --> 49:00.296
dibromoethane,
supposing they actually exist,

49:00.304 --> 49:03.954
can easily be explained without
assuming any difference among

49:03.954 --> 49:05.844
the four valences of carbon.

49:05.840 --> 49:09.430
You don't need curly bonds,
long bonds and stuff like that.

49:09.429 --> 49:12.489
And here's how he explained it,
with this picture.

49:12.489 --> 49:12.929
Right?

49:12.925 --> 49:17.055
So both, on the leftmost they
have two on the same atom.

49:17.059 --> 49:19.689
The other two have one on each
atom.

49:19.690 --> 49:23.450
And how are they different --
one on each carbon atom -- but

49:23.451 --> 49:24.791
how do they differ?

49:24.789 --> 49:29.339
In the phase of rotation,
around the carbon-carbon single

49:29.340 --> 49:29.910
bond.

49:29.909 --> 49:30.929
Right?

49:30.929 --> 49:35.129
And at the bottom he says -- or
actually notice there's a little

49:35.132 --> 49:37.002
black thing in the middle.

49:37.000 --> 49:37.990
What do you think it is?

49:37.989 --> 49:43.069


49:43.070 --> 49:44.640
Dana, what do you think?

49:44.639 --> 49:46.379
Student:  Some way to
join the --

49:46.380 --> 49:48.240
Prof: Right,
it's a case of rubber tubing.

49:48.239 --> 49:51.759
Because this is an artist's
picture of an actual model made

49:51.760 --> 49:53.340
out of sticks and balls.

49:53.340 --> 49:54.510
Right?

49:54.510 --> 49:57.750
And sure, those things aren't
super-imposable.

49:57.750 --> 49:58.570
Right?

49:58.570 --> 50:02.570
But he says at the bottom:
"It is superfluous to say

50:02.567 --> 50:06.347
that this is only a way of
representing the facts,

50:06.349 --> 50:08.789
and that all these ideas need
to be tested

50:08.789 --> 50:10.159
experimentally."

50:10.159 --> 50:14.469
So we'll see next time what
kind of reception this received.

50:14.469 --> 50:19.999