SN2 substitution provides an example of establishing the mechanism of a chemical reaction by disproving all the alternatives. Five general pathways are envisioned (two-step involving either pentavalent or trivalent carbon intermediates, and one-step).
The coincidentally substantial extent of ionic dissociation of water provides an example of Brønsted acidity, or nucleophilic substitution at hydrogen. Relative pKa values are insensitive enough to solvent that they provide insight on the role of energy-match, overlap, and resonance in ionic dissociation. The titration of alanine in water illustrates the experimental determination of pKa values and the phenomenon of buffering.
Most organic reactions occur in solution, and particularly in the case of ions, one must consider non-bonded interactions with neighboring molecules. Non-bonded interactions, including hydrogen-bonding, also determine such physical properties as boiling point. For the most part these interactions may be understood in terms of electrostatics and polarizability. Artificial or natural ion carriers (ionophores) can be tailored to bind specific ions. Energetically the ionic dissociation of water in the gas phase is prohibitively expensive.
A student provides insight on fractional-order rate laws. Bonds involving atoms with lone-pair electrons are weakened by electron-pair repulsion. Electronegativity differences between atoms make ionic dissociation (heterolysis) easier and radical dissociation (homolysis) harder, although Pauling’s definition of electronegativity makes the logic of the latter effect somewhat circular. The course transitions from free-radical reactions to ionic reactions by discussing solvent properties, in particular the electrostatic properties of alkyl halides and alkanes.
The reactivity-selectivity principle explains why bromine atoms are more selective that chlorine atoms in abstracting hydrogen atoms from carbon. A free-radical mechanism for adding HBr to alkenes explains its anti-Markovnikov regiospecificity. Careful analysis is required to understand kinetic order for reactions involving catalysts. Termination of radical-chain reactions can make their rate half-order in the initiator. Selectivity due to protonation of radicals and their reaction partners illustrates the importance of ionic charge in determining reaction rates.
Curious kinetic orders can be mechanistically informative. Fractional kinetic orders suggest dissociation of a dominant aggregate to give a smaller reactive species. An apparent negative kinetic order, due to competition with a second-order process, leads to spontaneous deracemization of chiral crystals. Changes in bond dissociation energies can be due to differences in bonds or in radicals.
This second semester of Freshman Organic Chemistry builds on the first semester’s treatment of molecular structure and energy* to discuss how reaction mechanisms have been discovered and understood. It also treats the spectroscopy and synthesis of organic molecules. Reactions and their rates can be understood in terms of reaction-coordinate diagrams involving the passage of a set of atoms through the “transition state” on the potential-energy surface. Analysis of bond-dissociation energies suggests a chain mechanism for free-radical halogenation of alkanes. Experimen
Professor Holloway offers an introduction to the course. He explains the organization of the course and summarizes some of the key concepts that will be explored over the course of the semester. Professor Holloway uses the African American experience as a prism to understand American history, because, as he notes, the African American experience speaks to the very heart of what it means to be American. He highlights specific examples of the linkage between freedom, citizenship, and the denial of citizenship, including an ex-slave's epitaph and Confederate scrip.
Three issues related to the impending end of the novel define this lecture. The first one is improvisation, as we see it in the confluence of actual geography with current historical events: the expulsion of the moriscos, and the Turkish and Huguenots menaces. With the story of Ricote, a kind of morisco novel in a nutshell, Cervantes provides a smorgasbord of narrative possibilities, and presents the consequences that political decisions have on common people.
In this lecture, the Priestly source (P) found primarily in Leviticus and Numbers is introduced. The symbolism of the sacrificial cult and purity system, the differences between moral and ritual impurity, as well as holiness and purity are explained within the Priestly context. The concept of holiness and imitatio dei, or human imitation of God, is explained.