WEBVTT

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Prof: Okay,
I need two volunteers here at

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the beginning,
while we're waiting for

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everybody else to come in.

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You have to come up here and do
something.

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Come on up.

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Okay.

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So each of you -- actually I
don't want two people that are

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sitting together.

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So Lucas, you;
and Julia stay.

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But Rick, why don't you come up?

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So here, come here, come here.

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So your job is to grind -- this
is a spice -- so you grind it up

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good, and then we're going to
take it back and show it to the

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people around you;
we're going to smell it.

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Okay?

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See if you can -- you have to
really grind it,

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like this, look.

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Okay?

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So we've got two spices here
that we need to identify.

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Okay, you can go back and grind
them at your seat and then show

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it your neighbors.

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Later on I'm going to ask you
whether you know what it is.

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Okay, so here's how we're going
through the history of organic

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chemistry and the development of
the molecular model.

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So generation by generation.

01:29.810 --> 01:34.330
who actually did experiments to
allow people to figure out

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something about structure,
not only what was connected to

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what --
that's constitution;

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remember, the nature and the
sequence of bonds.

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So we had composition,
what elements are there,

01:46.250 --> 01:48.590
what ratio they're in,
how many atoms.

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Right?

01:49.660 --> 01:52.850
Then constitution,
the nature and sequence in

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bonds.

01:53.430 --> 01:56.450
And now we're going to see if
you can go beyond that,

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without cheating and using
X-ray diffraction,

01:59.000 --> 02:01.960
to figure out how things are
arranged in space;

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so structure.

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And we talked last time about
Koerner's proof of positions in

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benzene,
and his proof that all the

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positions in benzene were
equivalent to one another.

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And today we're going to go
really to the next generation.

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Koerner wasn't all that much
younger than -- or he was at

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least halfway in the generation
there.

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But van't Hoff is a full
generation beyond.

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So we're going to look at him.

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It says under him
"tetrahedron",

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and it says down here
"tetrahedral carbon".

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And also we'll see something
about Albert Ladenburg and about

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your, whatever,
great-great-chem

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ical-grandfather,
Peter Kohler at Harvard.

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Okay, so as we were closing
last time we were looking at

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this definite picture of the
arrangement of atoms in space,

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where the models are clearly
real models,

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held together by rubber tubing,
and they're tetrahedral,

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and it explains why there can
be three isomers of

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disubstituted --
symmetrically disubstituted --

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ethane.

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Of course it was wrong.

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Do you know why it was wrong,
why there are not three

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different models?

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Yeah.

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Because you can rotate about
the middle bond freely;

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it's not frozen in one rotation
here.

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Or is it?

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We'll see something about that
later on.

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That turns out to be what's
called conformation.

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Remember, that's one of the C's.

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But now we're in configuration,
how things are attached to the

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tetrahedron.

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he got a letter from his former
sponsor,

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who had moved on,
Adolf Lieben,

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an Austrian who was at this
time in Italy;

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first in Palermo and then,
at this time I think he was in

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Northern Italy,
I think, in Pisa -- oh,

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Torino;
it says up at the top,

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"Torino,
25th of June,

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1869."

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And you can see a translation
there if you click.

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But anyhow it says:
"My dear Emmanuele."

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(They corresponded,
like several of these people

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who were great friends;
were for a very long time.)

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"Permit me to make an
observation regarding the

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isomerism pointed out by you and
by Cannizzaro."

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(It's a little bit misleading
to say "by

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Cannizzaro."

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He was very cautious about
this, as you know,

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but he introduced the paper.)

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"Between"
(and he shows those two

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structures here.

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Which look like type
structures, with those brackets.

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And you can see what's fastened
on the two carbons is H^2Cl

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H^2Cl, and H^2Cl,
H^2Cl.

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But he wrote the Cl's
differently, obviously meaning

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to denote this difference in
phase of rotation.)

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"Certainly it's not
absurd, this kind of isomerism,

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even without any differences
among the four valences of

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carbon."

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(Remember one could've been
straight, one crooked,

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short, long or something like
that.)

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"I believe, however,
that I don't err when I suppose

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that the same cause which keeps
me from accepting it will also

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keep all the other chemists from
doing so,

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in a silent agreement with one
another.

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You perhaps haven't thought
that accepting this kind of

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isomerism crosses the Rubicon
which separates speculation

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that's considered legitimate
about the mode of combination of

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atoms from speculation which is
less legitimate about the

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position of the atoms in
space."

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(And notice here that he added
as an afterthought,

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vero;
the true position of the atoms

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in space, rather than just
chemical position.

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Right?)

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"In all speculations that
have been made up to this time,

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people have always been careful
never to let enter in either the

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relative positions in space
(Distances and surroundings of

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the atoms).

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This idea doesn't even enter
into the speculations of

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aromatic series."

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(So he draws the picture of the
hexagon and numbers 1 to 6.)

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chlorine 2 positions is
different from chlorine 1,3 and

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from 1,4,
because in the first case the

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two carbon chlorines find
themselves combined with one

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another,
but in the second case they're

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separated by CH,
and in the third case they're

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separated by two CH's.

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And to accept this,
one has no need to make

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hypotheses about the
surroundings of the atoms."

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That's really weird isn't it?

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He says in one case they're
connected to one another,

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in another case there's a CH in
between, and in another case

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there are two CH's in between.

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But to talk about this you
don't have to talk about the

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surroundings of the atoms.

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That doesn't make sense to us,
right?

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But it's the difference between
drawing a graph,

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which is just a graph,
and drawing something that's

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supposed to show something in
space.

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He goes on and says,
"It could be that the two

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chlorines in positions 1,
2 are in truth more distant

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from one another than the ones
in positions 1,4!"

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(And he draws an exclamation
point there.

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Right?)

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Making hypotheses about the
relative positions of the atoms

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in space would make possible two
isomers of dichloromethane,

07:41.470 --> 07:44.550


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even accepting the symmetry of
the four valences…"

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(That is,
that all the valences of a

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single carbon are equal to one
another.)

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"…and the existence
of a single isomer of methyl

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chloride.

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For example,"
And he draws the two structures

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here, the ones where the
chlorines are far from one

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another and when they're near
one another;

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but that's just the graph,
that's not where they are in

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space.

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Okay.

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He added as an afterthought
between the lines here,

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"There would also result
two isomers of ethyl

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chloride."

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"I repeat that I don't
consider this kind of isomerism

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absurd.

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But since we have no means to
know the topographic position of

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the atoms,
while we have many to know how

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they are connected to one
another,

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I consider it a bit dangerous
for science.

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Shooting off into space in
search of atoms,

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one risks losing the ground
under his feet."

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(Right?

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So be careful former student.)

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"Greeting to Cannizzaro,
and Koerner,

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Amato.

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I'll write soon to Compisi.

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Your friend, Adolf Lieben."

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Okay, so be careful.

08:57.408 --> 08:59.978
This isn't what structures are
about, right?

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They're just graphs that show
constitution.

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So here's Koerner,
you remember,

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And on the right of this group
is Albert Ladenburg who,

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once you know,
once you've read what he wrote,

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you can sort of read into his
attitude there,

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that he was a hard guy to deal
with.

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Okay?

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He thought he knew a lot,
and he did know a lot,

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but not as much as he thought.

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So he published this paper the
next year, in 1869;

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published his thing.

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(1) The 6 hydrogen atoms of
benzene are equivalent;"

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(that's what Koerner actually
proved) "and (2) Each

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hydrogen atom of benzene has two
sets of two others which are

09:50.085 --> 09:53.615
symmetrically arranged with
respect to it;

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that is, 1.2 is the same as
1.6, and 1.3 is the same as 1.5,

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although 4 is unique with
respect to 1."

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(Okay?

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So there are two that are
symmetrically arranged.)

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"Already several years ago
I had occasion to make

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formula for benzene is not
adequate,

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since here 1.2 and 1.6 must be
inequivalent,

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while one could have various
views about whether positions 3

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and 5 are equivalent or
not."

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Why would one and two be
certainly -- two and six,

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that is, be certainly
inequivalent,

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but three and five might or
might not?

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Yeah, Kate?

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Student:  Because
between one and two is a double

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bond and between one and six is
a single bond.

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Prof: Both have a single
and a double.

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But what about 1.3 and 1.5?

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Are they 100% equivalent?

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Student:
>.

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Prof: Ah ha!

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because one is single-double
and the other is double-single,

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going the other way.

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So that might or might not be
equivalent, Ladenburg says.

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"Both conditions could,
however, be fulfilled through

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alternative formulas which,
so far as I know,

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have not been proposed
before."

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So here are Ladenburg's
formulas.

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And the one in the middle,
the triangular prism,

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actually came to be called,
100 years later,

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or a little,
seventy years later,

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Ladenburg benzene,
right?

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Because someone was able to
synthesize that.

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It's also called prismane,
because it looks like a prism,

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right?

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And then there's this funny
structure on the right.

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Okay, so this is 1872,
and we're going to --

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there's van't Hoff,
who is an eighteen-year-old

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student,
come to Bonn to see the great

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And he writes,
in 1876, at the age of 24,

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a reply to Ladenburg.

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"Proof that the prism
formula"

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(of Ladenburg) "suffers
from the same problem as the

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hexagon with fixed double bonds
removes the previously asserted

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superiority from the
former" (from the prism)

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notation not only simpler,
but also the presentation with

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which the facts conform
best."

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lines really meant was how often
the bonds collide --

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how often the atoms collide
with one another.

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They collide more often,
he thought, when it's a double

12:39.990 --> 12:41.570
bond than when it's a single
bond.

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So he thought that in this
case, although you would draw

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single,
double, single,

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double, that the number of
collisions actually was the same

12:47.551 --> 12:48.721
one way and the other.

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So you'd draw both formulas to
be in between what we would call

12:52.852 --> 12:54.522
now resonance formulas.

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So anyhow, this young student,
twenty-four years old now,

12:57.899 --> 13:01.139
says Ladenburg's off base in
saying the prism is better,

13:01.139 --> 13:05.589
because it has the same problem
and you can't get out of it with

13:05.586 --> 13:07.206
this vibration thing.

13:07.210 --> 13:11.870
Okay, "The 1,2; 5,6;
and 3,4 derivatives are

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completely similar,
although differing from 4,5;

13:15.519 --> 13:17.139
2,3; 6,1."

13:17.139 --> 13:22.039
So 1,2; 5,6;
and 3,4 are completely similar.

13:22.039 --> 13:28.839
So here's 1,2; 3,6;
-- 1 5,6; and 3,4.

13:28.840 --> 13:29.210
Right?

13:32.181 --> 13:34.101
you go from one to the next.

13:34.100 --> 13:37.750
So those he says "are
completely similar,

13:37.754 --> 13:41.494
although differing from 4,5;
2,3; 6,1."

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Do you see how they differ?

13:45.294 --> 13:47.414
4,5; 2,3; 6,1.

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4,5; 2,3; 6,1.

13:50.129 --> 13:54.409
Now are the red and the blue
different or not?

13:54.408 --> 13:57.358
The reds, obviously you just
rotate and you get from one to

13:57.360 --> 13:57.920
the next.

13:57.919 --> 13:59.099
How about from a red to a blue?

13:59.100 --> 14:03.020


14:03.019 --> 14:06.729
Can you rotate to get from one
to the other,

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from a red to a blue?

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Say the one's in front?

14:11.860 --> 14:15.820
What happens if you rotate to
put a red line on a blue line?

14:15.820 --> 14:18.830
Student:  It's sitting
on a square face.

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Prof: Sam?

14:20.038 --> 14:22.068
Student:  It's sitting
on a square face instead of --

14:22.070 --> 14:24.170
Prof: Ah,
then it's sitting on a square

14:24.166 --> 14:25.746
face instead of triangular face.

14:25.750 --> 14:27.500
It's not the same anymore.

14:27.500 --> 14:29.920
Everybody see that?

14:29.918 --> 14:30.328
Right?

14:30.325 --> 14:33.155
This, as drawn,
it has a triangle on top.

14:37.291 --> 14:40.811
in front, on the red,
then you no longer have a

14:40.808 --> 14:42.338
triangle on top.

14:42.340 --> 14:47.520
It's not the same thing,
van't Hoff, twenty-four years

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old, says.

14:48.690 --> 14:53.190
What do you think Ladenburg
would say about that?

14:53.190 --> 14:54.420
Well we'll see.

14:54.419 --> 14:57.079
But van't Hoff goes on.

14:57.080 --> 15:01.720
He says: "A 1,3 product is
different according to whether A

15:01.719 --> 15:04.149
or B occupies position 1."

15:04.149 --> 15:07.299
(In the right,
there's single double,

15:07.302 --> 15:10.372
double single,
when you go from A to

15:10.365 --> 15:11.675
B.***Okay?)

15:11.678 --> 15:14.718
"Exactly the same thing
happens in Ladenburg's

15:14.716 --> 15:15.686
formula."

15:15.690 --> 15:17.420
(With the prism.)

15:17.418 --> 15:21.188
"An adequate consideration
shows that I and II are

15:21.192 --> 15:23.222
absolutely different."

15:23.220 --> 15:24.890
So one.

15:24.889 --> 15:28.289
And notice he puts dashed lines
in.

15:28.289 --> 15:31.319
Why?

15:31.320 --> 15:33.710
Why is one bond shown dashed?

15:33.710 --> 15:35.310
Says so, right?

15:35.308 --> 15:37.468
Because it's behind,
it's hidden.

15:37.470 --> 15:40.630
So he's definitely showing this
in three dimensions.

15:40.629 --> 15:43.829
And if it's in three
dimensions, then AB goes left to

15:43.826 --> 15:45.486
right,
in one, and right to left in

15:45.493 --> 15:47.773
the other,
and you can't superimpose them,

15:47.772 --> 15:51.462
they're not the same thing;
according to van't Hoff.

15:51.460 --> 15:54.110
Now what would Ladenburg say
about this?

15:54.110 --> 15:56.280
He wouldn't take it lying down.

15:56.279 --> 16:00.049
He wrote back and said,
"van't Hoff finds the two

16:00.046 --> 16:03.666
formulas below absolutely
different."

16:03.668 --> 16:09.228
How is what he wrote different
from what van't Hoff wrote?

16:09.230 --> 16:12.150
How are Ladenburg's formulas
different from van't Hoff's?

16:12.149 --> 16:19.179


16:19.179 --> 16:20.929
Pardon me?

16:20.929 --> 16:21.979
It's not what?

16:21.980 --> 16:23.480
Student:  It isn't in 3D.

16:23.480 --> 16:25.760
Prof: How do you know
whether it's in 3D or not?

16:25.759 --> 16:27.359
Student:  Well,
then he'd use a dashed line.

16:27.360 --> 16:30.930
Prof: Ah,
he doesn't use the dashed line.

16:30.928 --> 16:31.258
Okay?

16:31.264 --> 16:35.014
So he says they're absolute --
that van't Hoff finds them

16:35.005 --> 16:36.605
absolutely different.

16:36.610 --> 16:39.040
But he's just drawing
constitution,

16:39.042 --> 16:40.762
what's linked to what.

16:40.759 --> 16:42.989
Right?

16:42.990 --> 16:45.530
"I cannot agree with him
in this.

16:45.529 --> 16:49.389
Van't Hoff is dragging
something into the formulas

16:49.389 --> 16:53.959
which I, together with most
chemists, expressly exclude.

16:53.960 --> 16:57.800
I refer to the arrangement in
space."

16:57.799 --> 17:02.029
(Right? So keep that out. Okay?)

17:02.028 --> 17:05.188
"The formula takes account
of the composition,

17:05.192 --> 17:07.792
molecular weight,
and mode of union of the

17:07.787 --> 17:08.797
atoms."

17:08.799 --> 17:11.849
So composition and what else?

17:11.849 --> 17:13.389
What other C?

17:13.390 --> 17:15.030
Student:
>.

17:15.028 --> 17:18.698
Prof: Composition,
that's composition of molecular

17:18.695 --> 17:21.135
weight he says;
and the "mode of

17:21.141 --> 17:22.411
union," right?

17:22.410 --> 17:23.920
Which is constitution.

17:23.920 --> 17:26.820
It doesn't talk about
arrangement in space.

17:26.819 --> 17:27.979
Right?

17:27.980 --> 17:31.340
"If van't Hoff's view of
space really will not tolerate

17:31.337 --> 17:33.727
the two formulas above to be
identical,

17:33.730 --> 17:37.400
then I invite him,
for his own special purposes,

17:37.400 --> 17:39.490
to use a different benzene
formula,

17:39.490 --> 17:43.330
which was proposed by me in the
year 1869,

17:43.328 --> 17:46.488
when I compared the hexagon and
the prism for the first time,

17:46.490 --> 17:51.440
together with the prism,
to which it is for me certainly

17:51.440 --> 17:52.520
identical.

17:52.519 --> 17:56.129
It is the so-called Cross of
David.

17:56.130 --> 18:00.610
Even van't Hoff will have to
find that these two formulas are

18:00.605 --> 18:02.765
absolutely identical."

18:02.769 --> 18:09.349
Okay, now how could the prism
be identical to this?

18:09.349 --> 18:10.189
There's the prism.

18:10.190 --> 18:12.420
They certainly don't look
identical.

18:12.420 --> 18:13.610
But watch this.

18:13.609 --> 18:15.719
Let's twist it;
keep all the bonds,

18:15.717 --> 18:16.147
but twist it.

18:16.150 --> 18:20.960


18:20.960 --> 18:21.850
Right?

18:21.848 --> 18:27.068
So as long as you don't have
dashed lines for 3D,

18:27.071 --> 18:32.021
they're the same;
one is just twisted.

18:32.019 --> 18:32.289
Okay?

18:32.285 --> 18:34.615
So Ladenburg is expressly
keeping this out.

18:34.618 --> 18:37.718
Now this point of view survived
for quite awhile.

18:37.720 --> 18:41.710
Here's some notes taken by a
former Yale colleague,

18:41.711 --> 18:43.631
R.M. Fuoss,
in the 1920s,

18:43.627 --> 18:48.257
when he was a student at
Harvard in Kohler's course.

18:48.259 --> 18:51.189
His notes say:
"Benzene gives one

18:51.190 --> 18:55.230
monosubstitution product and
three disubstituted.

18:55.230 --> 18:58.110
Cyclohexatriene"
(if you had the ring with

18:58.114 --> 19:01.374
double single,
double single) "apparently

19:01.366 --> 19:04.116
requires four disubstitution
products.

19:04.118 --> 19:10.178
should be different,
it was argued."

19:10.180 --> 19:13.680
But then he shows another
example of an experiment that's

19:13.678 --> 19:15.988
been done here,
with this molecule.

19:15.990 --> 19:21.740
Now these hydrogens can be --
it turns out that those

19:21.740 --> 19:27.050
hydrogens we'll call later
α hydrogens,

19:27.048 --> 19:30.328
because they're the first,
next to a carbonyl group.

19:30.328 --> 19:32.638
So here's a carbon-oxygen
double bond, and then a C that

19:32.635 --> 19:34.475
has the H;
carbon-oxygen double bond,

19:34.479 --> 19:35.509
a C that has the H.

19:35.509 --> 19:37.279
Those are called α.

19:37.279 --> 19:41.039
And it was known that it's
possible to easily substitute

19:41.038 --> 19:43.498
α carbons with other
things;

19:43.500 --> 19:46.660
more easily than other carbons
typically, other hydrogens.

19:46.660 --> 19:49.200
So you'd expect to get two
possible structures.

19:49.200 --> 19:52.370
You could do this α
one and get the R in that

19:52.373 --> 19:55.613
position, or this one and get R
in that position.

19:55.609 --> 19:57.979
So you'd get two products.

19:57.980 --> 19:59.980
Right?

19:59.980 --> 20:01.910
Those two.

20:01.910 --> 20:05.190
"We can alkylate"
(that is, put the R group on)

20:05.189 --> 20:07.589
"glutaconic acid in several
ways,

20:07.588 --> 20:10.908
but we always get the same
product 1.

20:10.910 --> 20:14.520
2 cannot be made,
no matter how we go.

20:14.519 --> 20:18.719
Hence it is not surprising that
only one ortho disubstituted

20:18.722 --> 20:20.792
product for benzene exists.

20:20.788 --> 20:24.328
The double bonds shift until
the stable system is

20:24.328 --> 20:25.508
reached."

20:25.509 --> 20:27.849
Right? So you can't make one.

20:27.848 --> 20:29.698
Or if you make it,
it immediately transforms to

20:29.703 --> 20:30.353
the other one.

20:30.348 --> 20:33.158
So there could be two,
but there aren't.

20:33.160 --> 20:35.190
Okay? So this is what?

20:35.190 --> 20:39.720
This is forty years later,
almost half a century later,

20:39.719 --> 20:42.989
and he's still saying the same
thing.

20:42.990 --> 20:43.910
Okay?

20:43.910 --> 20:46.080
Now, is it really that?

20:46.078 --> 20:49.348
Well you can calculate by
Spartan that the minimum energy

20:49.353 --> 20:52.103
really is symmetrical with equal
bond distances,

20:52.099 --> 20:53.269
but it vibrates.

20:53.269 --> 20:57.029
It can vibrate like this --
right?

20:57.029 --> 21:00.399
-- in and out,
and the frequency of that is

21:00.403 --> 21:01.933
1276 wavenumbers.

21:01.930 --> 21:04.460
You've taken spectra,
you've seen that kind of thing.

21:04.460 --> 21:07.890
Although this particular one is
not active in the infrared.

21:07.890 --> 21:09.030
You can't see it in IR.

21:09.029 --> 21:10.359
It's called breathing.

21:10.359 --> 21:12.639
>

21:12.640 --> 21:13.970
Right?

21:13.970 --> 21:16.450
Okay, but there's another way
it can vibrate,

21:16.452 --> 21:19.222
like this: distort in that
direction, then back to

21:19.217 --> 21:21.697
symmetrical, and in the other
direction.

21:21.700 --> 21:22.370
See what that is?

21:22.369 --> 21:25.679


21:27.571 --> 21:29.721
direction, single double,
single double.

21:29.720 --> 21:31.280
Right?

21:31.278 --> 21:35.558
So that one is at 1367
wavenumbers;

21:36.950 --> 21:39.330
But it's around a single
minimum.

21:39.328 --> 21:43.258
So at any given time the thing
is probably vibrating.

21:43.259 --> 21:45.499
So it is single double,
single double.

21:45.500 --> 21:50.860
But the lowest energy is all
the same intermediate distance.

21:50.858 --> 21:55.388
Okay, now might you be able to
push on benzene and make it

21:59.368 --> 22:00.878
bond distances?

22:00.880 --> 22:02.210
Well that's a challenge.

22:02.210 --> 22:09.650
So a chemist named Jay Siegel
and his colleagues synthesized

22:09.646 --> 22:12.966
this one,
which you'll notice has a

22:12.969 --> 22:16.309
four-membered ring,
with funny bond angles,

22:16.309 --> 22:19.239
attached to one side of the
benzene.

22:19.240 --> 22:22.760
That means those angles are
going to be stretched and so on.

22:22.759 --> 22:25.629
So they're going to distort and
make that bond different.

22:25.630 --> 22:28.930
And indeed that bond,
1,6, is a little

22:28.930 --> 22:29.830
different.

22:29.828 --> 22:33.098
But on the other side…
if it made that one -- let's

22:33.096 --> 22:35.766
say it's a little bit long;
make it single and the

22:35.773 --> 22:37.843
neighbors double,
because of the way it's being

22:37.836 --> 22:40.936
stretched --
then if that propagated around

22:40.944 --> 22:42.484
the ring,
single double,

22:42.483 --> 22:43.803
single double,
single double,

22:43.798 --> 22:47.098
the ones on the left would be
different lengths.

22:47.099 --> 22:49.239
But in fact they're the same.

22:49.240 --> 22:50.360
Right?

22:50.359 --> 22:52.649
So it doesn't happen here.

22:52.650 --> 22:56.410
So that's the kind of thing
that people sometimes do to test

22:56.413 --> 22:59.223
out the limits of these kinds of
theories.

22:59.220 --> 23:03.100
Okay, so we've seen the
position determination.

23:03.098 --> 23:07.128
And now more about tetrahedral
carbon.

23:07.130 --> 23:13.020
And to get there we're going to
step back just a bit to look at

23:13.023 --> 23:17.213
Louis Pasteur,
who was in a different line of

23:17.208 --> 23:19.678
business at this time.

23:19.680 --> 23:23.110
Now it's hard to prove that two
samples are identical because

23:23.114 --> 23:26.724
there could always be some other
test in which they appear to be

23:26.721 --> 23:29.321
different,
even though everything you've

23:29.318 --> 23:31.298
done so far shows them the same.

23:31.298 --> 23:34.318
But let's take carvone as an
example.

23:34.318 --> 23:38.578
Here's a sample of carvone,
a bottle of carvone.

23:38.578 --> 23:41.698
And if I crack it a little bit,
I can smell it.

23:41.700 --> 23:43.580
It has a very nice smell.

23:43.579 --> 23:46.289
Here, I'll pass it around here.

23:46.289 --> 23:49.379
Don't spill it obviously.

23:49.380 --> 23:52.100
Do you recognize the odor?

23:52.098 --> 23:54.038
Actually I have another bottle
too.

23:54.039 --> 23:57.699


23:57.700 --> 23:59.170
Here let's start this one over
here.

23:59.170 --> 24:01.820
I'll start it with Catherine.

24:01.819 --> 24:04.279
So just smell that one.

24:04.278 --> 24:05.718
Don't put your nose right down
in it.

24:05.720 --> 24:07.730
But it's not that strong.

24:07.730 --> 24:11.100
It's very pleasant actually.

24:11.098 --> 24:14.858
Now, this stuff is called
carvone.

24:14.858 --> 24:17.508
There's a reason it's called
carvone.

24:17.509 --> 24:20.999
How are you doing Rick,
with your grinding?

24:21.000 --> 24:23.040
Can you smell anything on it?

24:23.038 --> 24:24.408
Student:  Definitely can
smell it.

24:24.410 --> 24:25.910
Prof: What does it smell
like?

24:25.910 --> 24:29.400
Student:  It smells to
me like -- I'm not sure if I

24:29.402 --> 24:33.142
know the exact name but it's a
little like sesame seeds --

24:33.140 --> 24:34.690
Prof: A little like
sesame seeds.

24:34.690 --> 24:37.460
But there's another -- pass it
to Shai next to you here,

24:37.462 --> 24:38.272
let him smell.

24:38.269 --> 24:41.879


24:41.880 --> 24:45.020
Is it familiar?

24:45.019 --> 24:45.719
How about anybody?

24:45.720 --> 24:47.270
How about the bottle here
coming?

24:47.269 --> 24:48.289
Do you notice it?

24:48.289 --> 24:50.079
Is it a familiar scent?

24:50.078 --> 24:51.538
Student:  Smells like
peppermint.

24:51.538 --> 24:53.818
Prof: It's spearmint,
not peppermint.

24:53.819 --> 24:55.529
But you're close.

24:55.529 --> 24:56.479
Shai, how are you doing?

24:56.480 --> 24:59.620
Does it smell like peppermint
to you?

24:59.618 --> 25:04.368
Student:  I couldn't say
what kind mint it smells like.

25:04.368 --> 25:05.548
Prof: A little bit like
mint.

25:05.549 --> 25:08.449
Okay, how are doing Yoonjoo?

25:08.450 --> 25:10.110
You're grinding up a different
stuff.

25:10.108 --> 25:14.698
You're grinding dill seed,
in fact.

25:14.700 --> 25:16.540
Does it smell?

25:16.539 --> 25:18.619
Student:  Yes it does.

25:18.618 --> 25:20.508
Prof: What does it smell
like?

25:20.509 --> 25:22.699
Dill seed maybe, right?

25:22.700 --> 25:28.520
But carvone comes from -- this
is a familiar scent.

25:28.519 --> 25:30.459
I'm surprised you didn't get
it, Shai.

25:30.460 --> 25:33.260
Did anybody else recognize?

25:33.259 --> 25:35.569
Did you get it, Andrew?

25:35.569 --> 25:36.699
No.

25:36.700 --> 25:39.390
How about the bottle that's
coming along the back row there,

25:39.391 --> 25:40.351
that little bottle?

25:40.349 --> 25:42.189
Yeah, notice it?

25:42.190 --> 25:43.190
Student:  Rye.

25:43.190 --> 25:44.020
Prof: It's rye.

25:44.019 --> 25:45.079
Rye bread has that.

25:45.078 --> 25:47.738
It's caraway,
those seeds are caraway.

25:47.740 --> 25:53.600
And caraway is -- carv
is the Latin root for it.

25:53.599 --> 25:55.169
It's like caraway, right?

25:55.170 --> 25:58.600
So carvone, this substance
we're passing around,

25:58.602 --> 25:59.992
carvone, caraway.

25:59.990 --> 26:00.950
Okay?

26:00.950 --> 26:03.710
Now, so let's look at the --
suppose we took these two

26:03.709 --> 26:06.729
bottles and you distilled them,
found the boiling point.

26:10.369 --> 26:11.809
Right?

26:11.808 --> 26:15.958
The densities are exactly the
same.

26:15.960 --> 26:18.340
The refractive indices for
those two bottles,

26:18.344 --> 26:20.784
how much they bend light when
it goes through,

26:20.781 --> 26:21.921
exactly the same.

26:21.920 --> 26:24.270
The infrared spectra are
identical;

26:24.269 --> 26:27.329
to the extent that any IR
spectra are identical.

26:27.328 --> 26:30.398
You know from experiment that
if you put more in one sample

26:30.395 --> 26:32.665
than in another they don't look
the same.

26:32.670 --> 26:33.160
Right?

26:33.160 --> 26:35.450
Did you notice that in lab?

26:35.450 --> 26:38.300
If you ground up more stuff and
less stuff, the spectra don't

26:38.300 --> 26:40.720
really look the same,
but then you can find there is

26:40.723 --> 26:44.063
a peak to peak correspondence;
the intensities are never going

26:44.058 --> 26:45.388
to be exactly the same.

26:45.390 --> 26:47.970
But to the extent IR spectra
can be the same,

26:47.969 --> 26:49.199
these are the same.

26:49.200 --> 26:51.880
If you take their NMR spectra
they're the same.

26:51.880 --> 26:53.620
So they must be the same.

26:53.618 --> 26:58.798
Or is there any property in
which they differ?

26:58.798 --> 27:01.458
Any property that those two
bottles differ in?

27:01.460 --> 27:02.560
Students:  Smell.

27:02.558 --> 27:04.908
Prof: The smell is
different.

27:04.910 --> 27:08.520
Now, those bottles -- they're
in dark bottles,

27:08.521 --> 27:10.851
to protect them from light.

27:10.848 --> 27:13.868
But if I poured them out --
they should be colorless liquids

27:13.867 --> 27:16.117
but I bet anything they're brown
-- right?

27:16.118 --> 27:18.728
-- because they've sat around
and a little bit is oxidized.

27:18.730 --> 27:21.650
So it's very difficult with
scent, because little bits of

27:21.652 --> 27:24.472
other things can make them smell
a little different.

27:24.470 --> 27:24.800
Right?

27:24.801 --> 27:27.961
You don't need much stuff for
scent of the proper thing.

27:27.960 --> 27:31.640
So it's hard to be confident
that this difference,

27:31.643 --> 27:35.783
that one of them is rye bread,
caraway, and the other is

27:35.777 --> 27:38.057
spearmint;
it's hard to be sure that

27:38.057 --> 27:39.087
that's a difference.

27:39.088 --> 27:42.488
But it turns out there's
another property you can measure

27:42.486 --> 27:42.846
too.

27:42.848 --> 27:46.518
And Karo syrup has that
property.

27:46.519 --> 27:48.719
And I'm going to show it to you.

27:48.720 --> 27:51.940
It's the ability to rotate
polarized light.

27:51.940 --> 27:54.170
Now -- oh, did I bring?

27:54.170 --> 27:59.420
-- oh I forgot to bring my
Polaroids.

27:59.420 --> 28:04.660
So what I'm going to do is ask
a TA to go back to my office,

28:04.660 --> 28:08.040
and on the table in my office
there's a brown package about

28:08.037 --> 28:10.887
that big that has two sheets of
Polaroid in it.

28:10.890 --> 28:15.060
There's the key.

28:15.059 --> 28:19.119


28:19.119 --> 28:20.059
Yeah, it's not here.

28:20.059 --> 28:22.519
Sorry.

28:22.519 --> 28:27.479
Let me think a second,
what we can go on to.

28:27.480 --> 28:32.320
I'll tell you the measure of
this, which is you have light

28:32.317 --> 28:33.927
that's polarized.

28:33.930 --> 28:35.690
Have you all dealt with
polarized light?

28:35.690 --> 28:38.640
You probably have polarized
sunglasses and you can turn it

28:38.636 --> 28:41.116
and see different light
reflecting off things.

28:41.119 --> 28:42.639
Have you done that?

28:42.640 --> 28:44.850
Okay, and that's why you have
polarized sunglasses,

28:44.847 --> 28:47.007
because light that reflects
doesn't go through the

28:47.010 --> 28:49.220
particular direction that's in
the sunglasses.

28:49.220 --> 28:52.320
So you get rid of reflected
light.

28:52.318 --> 28:56.448
Okay, so but what you measure
is -- when light goes through a

28:56.452 --> 28:59.112
polarized filter,
it's as if it were -- you were

28:59.113 --> 29:01.513
shaking a rope and it was going
through a picket fence.

29:01.509 --> 29:04.309
It can vibrate one way but it
can't vibrate the other way.

29:04.309 --> 29:07.279
What vibrates in light?

29:07.279 --> 29:09.769
What is vibrating?

29:09.769 --> 29:11.119
What's light?

29:11.119 --> 29:12.729
We went through this before.

29:12.730 --> 29:13.740
Student:
Electromagnetic --

29:13.740 --> 29:14.890
Prof: Electromagnetic
radiation.

29:14.890 --> 29:16.650
What's vibrating?

29:16.650 --> 29:18.330
Prof: The electric field.

29:18.328 --> 29:21.288
And it's set up so one of
these, if you have a Polaroid

29:21.291 --> 29:23.761
filter, gets absorbed,
and the other one comes

29:23.760 --> 29:24.420
through.

29:24.420 --> 29:26.870
Okay, so light that comes
through is vibrating this way.

29:26.868 --> 29:30.038
And if you put another one,
that's crossed with it,

29:30.042 --> 29:33.092
like this, what do you get --
what do you see?

29:33.089 --> 29:34.089
Student:  Nothing.

29:34.088 --> 29:36.108
Prof: Nothing,
because the first one stops one

29:36.114 --> 29:37.714
and the second one stops the
other one.

29:37.710 --> 29:41.320
If you turn it like this,
then it goes on through.

29:41.319 --> 29:42.139
Okay?

29:42.140 --> 29:45.700
But suppose that you put
something in the middle that

29:45.702 --> 29:48.172
rotated the plane of
polarization.

29:48.170 --> 29:50.820
So the light's vibrating like
this.

29:50.819 --> 29:53.059
Suppose it's going like this.

29:53.058 --> 29:53.338
Right?

29:53.337 --> 29:55.977
Suppose by the time it gets to
the second filter it's like

29:55.979 --> 29:56.349
this.

29:56.348 --> 29:56.758
Right?

29:56.760 --> 29:59.440
Now you have the second one
like this.

29:59.440 --> 30:02.300
What do you see?

30:02.298 --> 30:04.788
Light comes right on through,
because that's the way it's

30:04.788 --> 30:07.418
vibrating now;
if you rotated the plane of

30:07.415 --> 30:08.405
polarization.

30:08.410 --> 30:08.690
Okay?

30:08.693 --> 30:10.463
That's what
"rotation"

30:10.458 --> 30:10.968
means.

30:10.970 --> 30:19.620
Okay, now you measure this
property of a substance and call

30:19.618 --> 30:25.458
it specific rotation;
which is how many degrees it

30:25.455 --> 30:26.225
rotates.

30:26.230 --> 30:33.580
And you have to say what light
rotates, so that the units of

30:33.577 --> 30:39.797
α are degrees -- how many
degrees it changes,

30:39.803 --> 30:43.293
per gram per milliliter.

30:43.288 --> 30:48.108
If you put more stuff in,
then you get a bigger effect.

30:48.108 --> 30:48.448
Right?

30:48.448 --> 30:50.478
So it depends on the
concentration.

30:50.480 --> 30:54.070
So how many degrees per mole,
you might say.

30:54.068 --> 30:54.398
Okay?

30:54.404 --> 30:58.364
But then it's per decimeter,
because the longer path you go,

30:58.357 --> 31:00.767
if it's rotating,
the further you go,

31:00.769 --> 31:02.579
the further it twists.

31:02.578 --> 31:09.138
So the old style cells were --
oh, no, that's not it.

31:09.140 --> 31:13.240
Nice try.

31:13.240 --> 31:14.760
This is really embarrassing.

31:14.759 --> 31:16.469
It's an envelope.

31:16.470 --> 31:17.870
Teaching Assistant:
Oh it's an envelope.

31:17.874 --> 31:18.184
Oh okay.

31:18.180 --> 31:19.450
Prof: Did you see it?

31:19.450 --> 31:20.690
Teaching Assistant:
I can take another look.

31:20.690 --> 31:25.400
Prof: Okay,
good >

31:25.400 --> 31:29.220
Okay, let me show you the other
things I'm going to show you.

31:29.220 --> 31:32.510
Okay here -- actually here's --
we'll do a trick.

31:32.509 --> 31:37.069
Here are models of carvone.

31:37.069 --> 31:38.169
Okay?

31:38.170 --> 31:42.150
Now I'm going to give them to
you guys, and I want you to

31:42.148 --> 31:44.848
compare and see if they're the
same.

31:44.848 --> 31:48.838
Stand up and let people see
what -- so decide how you --

31:48.837 --> 31:52.317
start maybe at the top,
with the methyl group.

31:52.318 --> 31:55.408
Each of you -- turn -- both of
you turn and face the thing,

31:55.410 --> 31:56.370
so they can see.

31:56.368 --> 31:58.778
Now hold -- start with the
methyl group at the top.

31:58.779 --> 32:00.249
It starts with the methyl group.

32:00.250 --> 32:04.400
Now Russell,
you describe it and then check

32:04.397 --> 32:09.727
and see whether -- Eric,
you check and see whether your

32:09.731 --> 32:13.191
model is the same as Russell's.

32:13.190 --> 32:13.990
Okay?

32:13.990 --> 32:18.310
While you're doing that,
I'll go help find the thing.

32:18.309 --> 32:21.809
So tell me when I get back.

32:21.808 --> 33:37.678
<<Professor McBride
leaves the class briefly>>

33:37.680 --> 33:39.780
Prof: Okay what's the
answer?

33:39.779 --> 33:40.979
Are they the same?

33:40.980 --> 33:43.690
Student:  One of them's
pointing down.

33:43.692 --> 33:45.792
I can't see it on the other
one.

33:45.788 --> 33:48.638
Prof: So is that real,
do you think?

33:48.640 --> 33:54.650
Is that just a peculiarity of
the models, or does that relate

33:54.654 --> 33:56.164
to molecules?

33:56.160 --> 33:57.880
Student:  I think it
should relate to the smell of

33:57.881 --> 33:58.251
the thing.

33:58.250 --> 33:59.620
Prof: Okay.

33:59.615 --> 34:04.165
Well anyhow -- if I can catch
my breath -- these are Polaroid

34:04.170 --> 34:04.930
sheets.

34:04.930 --> 34:10.210
So here you see me,
and here you don't.

34:10.210 --> 34:12.450
Students:  Ohhh.

34:12.449 --> 34:13.699
Prof: Okay?

34:13.699 --> 34:17.499
Now, let's see a case of
rotation.

34:17.500 --> 34:20.840


34:20.840 --> 34:29.840
So here you see crystals and
I'll put this -- I'll get light

34:29.836 --> 34:32.426
coming through.

34:32.429 --> 34:37.729
And there's the Polaroid
filter, and here's another

34:37.731 --> 34:39.641
Polaroid filter.

34:41.949 --> 34:46.179
And now let me put the crystals
in there.

34:46.179 --> 34:51.589
Now, so there,
most of it's coming through.

34:51.590 --> 34:55.270
But now watch.

34:55.269 --> 34:57.419
What do you notice?

34:57.420 --> 35:05.910
Student:
>.

35:05.909 --> 35:10.159
Prof: What's happening?

35:10.159 --> 35:18.639
Student:
>.

35:18.639 --> 35:20.669
Prof: What's surprising
about this?

35:20.670 --> 35:24.700
Obviously they're rotating the
plane of polarization -- right?

35:24.699 --> 35:27.769
-- so that I have to set a
different angle in order to stop

35:27.773 --> 35:28.413
the light.

35:28.409 --> 35:31.059
But what's special about this
set of crystals?

35:31.059 --> 35:35.019
<<Students speak over one
another>>

35:35.018 --> 35:37.898
Prof: Some are
right-handed and some are

35:37.902 --> 35:38.782
left-handed.

35:38.780 --> 35:40.820
Right?

35:40.820 --> 35:41.910
That's sodium chlorate.

35:41.909 --> 35:44.999
That's the stuff that
Gay-Lussac used.

35:45.000 --> 35:45.380
Right?

35:45.380 --> 35:48.180
So this property was
discovered, of crystals,

35:48.175 --> 35:49.695
early in the century.

35:49.699 --> 35:51.279
Now how about liquids?

35:51.280 --> 35:53.400
Will liquids do it?

35:53.400 --> 35:56.180
Well here's glass.

35:56.179 --> 36:01.439
Glass doesn't do anything
special, it's just like the air

36:01.442 --> 36:02.572
around it.

36:02.570 --> 36:08.700
But let me pour water in here,
and see if water does the

36:08.697 --> 36:09.587
trick.

36:09.590 --> 36:14.430


36:14.429 --> 36:16.949
Nothing special, right?

36:16.949 --> 36:18.799
So let's try Karo syrup.

36:18.800 --> 36:25.300


36:25.300 --> 36:27.340
It's not quite January,
but it takes a little while.

36:27.340 --> 36:34.820


36:34.820 --> 36:35.970
It's a little bit yellow.

36:35.969 --> 36:37.739
It sat around awhile.

36:37.739 --> 36:50.509


36:50.510 --> 36:51.540
What's happening?

36:51.539 --> 36:56.839


36:56.840 --> 37:00.700
Well there's an underlying
yellow that biases this.

37:00.699 --> 37:03.139
But we can get different colors.

37:03.139 --> 37:07.739
So different colors in the
spectrum are being blocked out.

37:07.739 --> 37:12.209
How can that be,
for different angles different

37:12.206 --> 37:13.076
colors?

37:13.079 --> 37:17.269
So the Karo syrup is rotating
the polarization,

37:17.269 --> 37:22.279
but it's rotating different
colors different amounts.

37:22.280 --> 37:24.260
Right?

37:24.260 --> 37:26.310
So we block off different
colors at different angles.

37:26.309 --> 37:30.039


37:30.039 --> 37:34.829
Okay, so if we're going to make
a measurement of the angle,

37:34.829 --> 37:39.039
how much it rotates,
we're going to have to say what

37:39.041 --> 37:41.521
color we're talking about.

37:41.519 --> 37:44.999
And that's what the D is.

37:45.000 --> 37:46.470
The D is the color.

37:46.469 --> 37:49.699
There's a so-called D line of
sodium, a yellow color.

37:49.699 --> 37:52.479
The yellow street lights are
that color.

37:52.480 --> 37:52.760
Right?

37:52.760 --> 37:55.380
So that's the color that was
used, because it was easy to

37:55.382 --> 37:56.742
generate in the old times.

37:56.739 --> 38:00.839
And twenty is how many degrees;
because it's different for

38:00.836 --> 38:02.336
different temperatures.

38:02.340 --> 38:06.290
Okay, so you can measure the
specific rotation of a sample.

38:06.289 --> 38:09.749
And if it's in a solvent,
you have to say what solvent it

38:09.750 --> 38:12.160
is, because it can vary with
solvent.

38:12.159 --> 38:14.149
So anyhow, back to our
comparison.

38:14.150 --> 38:17.390
These odors were the same,
but we might not believe it.

38:17.389 --> 38:21.139
But if we measure the specific
rotation, it's equal and

38:21.144 --> 38:21.914
opposite.

38:27.539 --> 38:32.409
So these things somehow are
mirror images of one another.

38:32.409 --> 38:34.709
Now here is what you've already
seen.

38:34.710 --> 38:38.260
You can write the structure of
carvone, either longhand or

38:38.262 --> 38:39.012
shorthand.

38:39.010 --> 38:40.360
And there are the two models.

38:40.360 --> 38:43.420
And you correctly identified a
difference between them,

38:43.422 --> 38:46.542
that one has a hydrogen
pointing backwards and the other

38:46.543 --> 38:48.023
coming out toward you.

38:48.019 --> 38:50.779
And is this difference real?

38:50.780 --> 38:53.830
Of course there were
differences in the sausage

38:53.831 --> 38:56.421
formulas too,
but those weren't real;

38:56.420 --> 38:58.980
remember, those two compounds
turned out to be the same

38:58.978 --> 38:59.498
compound.

38:59.500 --> 39:02.540
The question is,
is this difference real?

39:02.539 --> 39:07.169
Are there two carvones,
but with different impurities

39:07.172 --> 39:12.252
in them, so they smell
different, or is there just one?

39:12.250 --> 39:14.530
Oh pardon me,
vice-versa from what I said

39:14.525 --> 39:14.975
there.

39:14.980 --> 39:18.310
Okay, so tartaric acid turns
out to be the same thing.

39:18.309 --> 39:22.819
Remember, Berzelius coined the
name isomer to talk about

39:22.817 --> 39:27.077
tartaric acid and racemic acid,
which both came as byproducts

39:27.079 --> 39:29.299
of the wine industry and had
very,

39:29.300 --> 39:31.220
very different melting points.

39:31.219 --> 39:33.929
So obviously they're quite
different from one another.

39:33.929 --> 39:36.859
And one of them turns out to
rotate light;

39:36.860 --> 39:38.720
if you make an experiment like
this.

39:43.554 --> 39:46.654
per milliliter,
per decimeter.

39:46.650 --> 39:47.130
Okay.

39:47.130 --> 39:51.930
But if you heat it,
you get another compound called

39:51.932 --> 39:56.162
pyrotartaric acid,
or sometimes meso-tartaric

39:56.159 --> 39:57.119
acid.

39:57.119 --> 40:00.889
Meso doesn't mean anything,
except "between".

40:00.889 --> 40:05.169
Do you know any cases where
meso means between?

40:05.170 --> 40:08.440
Mesopotamia means between the
Tigris and the Euphrates River,

40:08.443 --> 40:08.883
right?

40:08.880 --> 40:09.990
So it just means in between.

40:09.989 --> 40:13.179
So there was another one in
between tartaric and racemic

40:13.182 --> 40:13.592
acid.

40:13.590 --> 40:15.960
It wasn't between in melting
point;

40:18.320 --> 40:20.540
And it didn't rotate light
either.

40:20.539 --> 40:23.399
So there were these three
different forms of tartaric

40:23.402 --> 40:27.092
acid, but all were found to have
the same constitutional formula;

40:27.090 --> 40:30.690
the same things linked to the
carbons.

40:30.690 --> 40:34.630
So here's a problem with the
model, in terms of isomer

40:34.626 --> 40:35.366
numbers.

40:35.369 --> 40:35.699
Okay?

40:39.835 --> 40:40.965
dibromoethane.

40:40.969 --> 40:42.719
He had different phases of
rotation;

40:42.719 --> 40:44.079
said those were different.

40:44.079 --> 40:45.709
In fact, they weren't
different, it was just

40:45.711 --> 40:48.181
experimental errors of someone
who thought they were different.

40:48.179 --> 40:52.249
So sometimes the model predicts
too many isomers.

40:52.250 --> 40:56.440
But often, or sometimes at
least, it predicts too few;

40:56.440 --> 40:59.900
as in the case of maleic and
fumaric acid where you have a

40:59.900 --> 41:00.750
double bond.

41:00.750 --> 41:03.960
There are two different
substances with different

41:03.960 --> 41:06.100
melting points and properties.

41:06.099 --> 41:10.559
With lactic acid -- remember
that Scheele had found lactic

41:10.561 --> 41:15.021
acid from milk,
but Liebig had found it from

41:15.018 --> 41:17.668
meat --
and they were the same,

41:17.673 --> 41:22.333
it was found out about this
time by a guy named Wislicenus.

41:22.329 --> 41:25.859
They had the same constitution,
but one rotated to the right

41:25.856 --> 41:28.006
and the other rotated to the
left.

41:28.010 --> 41:30.320
There was tartaric and racemic
acid;

41:30.320 --> 41:33.890
which was even more complicated
because there was also

41:33.893 --> 41:38.073
meso-tartaric and also what came
to be called l-tartaric

41:38.072 --> 41:38.682
acid.

41:38.679 --> 41:41.349
And that was the discovery of
Pasteur.

41:41.349 --> 41:45.939
So in 1848, which was another
revolutionary time in France,

41:45.940 --> 41:49.500
Pasteur was twenty-six years
old and was becoming an

41:49.498 --> 41:52.428
interdisciplinary scientist:
chemistry,

41:52.429 --> 41:55.459
physics, crystallography,
all he was studying.

41:55.460 --> 41:59.450
And he wrote a paper called
"On the relations that can

41:59.451 --> 42:03.241
exist among crystalline
form," (so crystallography)

42:03.235 --> 42:05.435
"chemical
composition,"

42:05.436 --> 42:08.666
(chemistry) "
and the direction of rotatory

42:08.672 --> 42:10.532
polarization."

42:10.530 --> 42:15.480
That is this stuff we're
talking about,

42:15.476 --> 42:17.946
which is physics.

42:17.949 --> 42:21.529
Now remember we talked about
Mitscherlich and the ability to

42:21.527 --> 42:25.227
measure angles on crystals and
tell -- distinguish crystals by

42:25.228 --> 42:26.258
their angles.

42:26.260 --> 42:32.620
So these are pictures that
Pasteur was using to draw his

42:32.619 --> 42:35.509
salts of tartaric acid.

42:35.510 --> 42:39.060
In particular sodium -- it's a
diacid, and so you can have

42:39.063 --> 42:42.173
different cations;
sodium ammonium tartrate is

42:42.170 --> 42:42.860
this one.

42:42.860 --> 42:47.280
And there's his picture that he
drew of it.

42:47.280 --> 42:51.380
And he said -- mostly you see
the faces that are shown in that

42:51.376 --> 42:55.136
picture, but sometimes you can
see other little faces.

42:55.139 --> 42:56.319
Right?

42:56.320 --> 42:59.850
And you notice that the
crystals have symmetry.

42:59.849 --> 43:03.609
They have a horizontal plane of
symmetry and also a vertical

43:03.606 --> 43:04.876
plane of symmetry.

43:04.880 --> 43:06.300
Right?

43:06.300 --> 43:09.160
So if you can see one of those
edges,

43:09.159 --> 43:12.309
you should see the others that
are related by symmetry this

43:12.313 --> 43:14.023
direction,
symmetry this direction,

43:14.016 --> 43:15.426
and also symmetry this
direction.

43:15.429 --> 43:20.139
So you should see all eight,
in the tartrate.

43:20.139 --> 43:23.229
But always only four of them
are observed,

43:23.228 --> 43:27.048
in sodium ammonium tartrate;
not all eight.

43:27.050 --> 43:29.940
Mitscherlich had studied this
stuff, and he was sort of the

43:29.938 --> 43:31.878
father of measuring angles like
this.

43:31.880 --> 43:34.650
But when Pasteur was trying to
learn the technique and repeat

43:34.652 --> 43:36.382
it,
he noticed that you don't very

43:36.380 --> 43:38.170
often get these edges all
together,

43:38.170 --> 43:39.490
these little tiny ones.

43:39.489 --> 43:41.629
It usually looks like the
original picture did.

43:41.630 --> 43:45.570
But when those sharp edges are
truncated by little additional

43:45.567 --> 43:48.977
faces, you only see four,
you don't see all eight.

43:48.980 --> 43:52.720
So Mitscherlich had reported
that the -- now how…?

43:52.719 --> 43:55.219
-- That was tartrate, right?

43:55.219 --> 43:57.719
Now they had the idea that
racemate,

43:57.719 --> 43:59.769
which remember didn't rotate
light --

43:59.768 --> 44:03.398
Pasteur's idea was that
molecules were twisted in a

44:03.396 --> 44:06.076
right-hand or a left-handed
helix,

44:06.079 --> 44:08.459
and therefore they had this
effect on light,

44:08.460 --> 44:09.790
to make it twist.

44:09.789 --> 44:10.739
Right?

44:10.739 --> 44:14.999
But one's that aren't twisted,
like racemic acid --

44:15.000 --> 44:16.880
Where the molecules aren't
twisted,

44:16.880 --> 44:20.130
then, at least in those,
you should see all eight,

44:20.130 --> 44:23.040
because you will have mirror
images.

44:23.039 --> 44:24.029
Okay?

44:24.030 --> 44:26.600
So he thought untwisted
molecules would be what's called

44:26.601 --> 44:29.781
"holohedral";
that is, have all their faces.

44:29.780 --> 44:34.350
So he studied the sodium
ammonium racemate and was quite

44:34.347 --> 44:38.997
surprised to find that sometimes
he saw the red ones,

44:39.000 --> 44:42.810
sometimes he saw the green
ones, but never on the same

44:42.809 --> 44:43.529
crystal.

44:43.530 --> 44:47.470
Some crystals were right,
red, and some were left,

44:47.467 --> 44:48.107
green.

44:48.110 --> 44:51.800
So he separated those crystals,
the right ones from the left

44:51.800 --> 44:52.240
ones.

44:52.239 --> 44:54.929
He wrote: "I carefully
separated the right from the

44:54.931 --> 44:57.871
left crystals and observing
their dissolution separately,

44:57.869 --> 45:00.799
with Monsieur Biot's
polarization apparatus"

45:00.795 --> 45:04.085
(the thing that measures the
rotation) "I saw with

45:04.086 --> 45:07.496
surprise and delight that the
right crystals deviated the

45:07.501 --> 45:10.731
plane of polarization to the
right and the left to the

45:10.731 --> 45:11.891
left."

45:11.889 --> 45:15.869
So why did racemic acid not
rotate light?

45:15.869 --> 45:19.269
Student:
>

45:19.268 --> 45:21.618
Prof: Because it was a
50:50 mixture of things that

45:21.617 --> 45:24.147
would rotate it one way --
it's not that it was untwisted

45:24.146 --> 45:26.846
-- it was that some of them
would rotate it one way and some

45:26.853 --> 45:29.083
would rotate it the other,
and they'd cancel out.

45:29.079 --> 45:32.509


45:32.510 --> 45:32.990
Okay.

45:32.987 --> 45:37.967
Oops I improved this and forgot
to take that one out;

45:37.967 --> 45:38.827
sorry.

45:38.829 --> 45:41.389
Okay, so now he knows what's
going on,

45:41.389 --> 45:45.739
that racemic acid is actually a
50:50 mixture of right-handed,

45:45.739 --> 45:49.529
dextro-tartaric acid,
d, that has a plus

45:49.530 --> 45:53.070
rotation,
and left-handed tartaric acid,

45:53.072 --> 45:55.442
l,
levo, which has a minus

45:55.438 --> 45:57.858
rotation, but the same melting
point,

45:57.860 --> 46:01.950
the same rotation,
but opposite in direction.

46:01.949 --> 46:04.569
So those are mirror images of
one another.

46:04.570 --> 46:07.320
And the racemic acid is a 50:50
mixture.

46:07.320 --> 46:09.510
Now this was 1848.

46:09.510 --> 46:12.360
So that's a long time from
where we've gotten to.

46:12.360 --> 46:12.770
Right?

46:12.773 --> 46:15.883
That was ten years before the
idea of bonds.

46:15.880 --> 46:19.470
Okay?

46:19.469 --> 46:21.599
So that was
"resolution",

46:21.601 --> 46:24.991
the separation of a mixture
into the two components.

46:24.989 --> 46:25.479
Right?

46:25.476 --> 46:28.066
But what is meso-tartaric acid?

46:28.070 --> 46:30.060
Why doesn't it rotate?

46:30.059 --> 46:33.189
So now we go forward,
twenty-four years,

46:37.914 --> 46:39.924
at the age of twenty.

46:39.920 --> 46:42.950
So that's a quarter of a
century.

46:42.949 --> 46:44.389
That's a long time, right?

46:44.389 --> 46:46.439
It's longer than you've lived.

46:46.440 --> 46:47.770
So here's van't Hoff.

46:47.769 --> 46:51.109
He had some opinion of himself.

46:51.110 --> 46:56.620
He won the first Nobel Prize --
I mean, he had a high opinion

46:56.617 --> 47:02.307
that was completely justified --
in 1901, the first Nobel Prize

47:02.309 --> 47:03.869
in Chemistry.

47:03.869 --> 47:06.259
And he didn't get it for this,
which he should've gotten it

47:06.262 --> 47:06.512
for.

47:06.510 --> 47:09.280
He got for inventing physical
chemistry.

47:09.280 --> 47:16.360
The most admired trait he had
was imagination.

47:16.360 --> 47:18.140
So he liked poets and artists.

47:18.139 --> 47:22.459
In fact, his hero was Lord
Byron.

47:22.460 --> 47:26.330
Look at that.

47:26.329 --> 47:28.629
Okay?

47:28.630 --> 47:32.660
And in 1874,
as we said, he was a student.

47:32.659 --> 47:36.459
And he wrote this,
a pamphlet, in Dutch,

47:36.460 --> 47:40.100
which was then translated into
German and called "The

47:40.103 --> 47:43.493
Arrangement of Atoms in
Space," and was published

47:43.492 --> 47:44.262
in 1877.

47:44.260 --> 47:47.820
It was fifty-three pages long
and heavily illustrated.

47:47.820 --> 47:50.340
And look at the website for
this.

47:50.340 --> 47:53.940
One of the Wikis I've assigned
is that website.

47:53.940 --> 47:59.210
And also this website about
criticism of van't Hoff by a guy

47:59.208 --> 48:00.458
named Kolbe.

48:00.460 --> 48:01.730
So here's Hermann Kolbe.

48:01.730 --> 48:04.960
He was twice,
more than twice van't Hoff's

48:04.956 --> 48:08.496
age, and a pillar of traditional
chemistry.

48:08.500 --> 48:12.030
He wrote, in a review of this
book: "It is completely

48:12.030 --> 48:15.560
impossible to criticize this
booklet in any detail because

48:15.561 --> 48:19.031
the fancy trifles in it are
totally devoid of any factual

48:19.030 --> 48:22.070
reality and are completely
incomprehensible to any

48:22.067 --> 48:24.047
clear-minded researcher.

48:24.050 --> 48:28.000
The brochure begins with the
words: 'The modern chemical

48:27.996 --> 48:29.966
theory has two weak points.

48:29.969 --> 48:33.229
It says nothing either about
the relative position or the

48:33.228 --> 48:36.198
motion of the atoms within the
molecules.'"

48:36.195 --> 48:37.705
This is absurd,
right?

48:37.710 --> 48:40.660
But he was wrong.

48:40.659 --> 48:41.679
Right?

48:44.059 --> 48:46.549
Because I'm more than twice as
old as you are.

48:46.550 --> 48:46.940
Right?

48:46.940 --> 48:50.600
So those are three websites you
should read about this.

48:50.599 --> 48:53.419
But, in fact,
it's not true that van't Hoff

48:53.416 --> 48:55.896
was devoid of any factual
reality,

48:55.900 --> 48:57.900
because we've already seen some
of his evidence,

48:57.900 --> 49:00.420
and we'll see more next time.

49:00.420 --> 49:07.000