Rules of the Day 3-19-08
Click here for a copy of the mechanism sheet used in lecture today.
1. There are four new concepts associated with chapter 19. Click on these one at a time to get more information.
C) Beta-Dicarbonyl species are especially acidic
D) Conjugate addition to alpha,beta unsaturated carbonyl species.
2. Important acid-base concepts to remember are that 1) acid-base reactions favor formation of the weaker acid/weaker base (quantitatively, the equilibrium constant can be derived from the difference of the pKa values of the acids), 2) Weaker conjugate bases are derived from stronger acids and vice versa, and 3) stronger acids have lower pKa values. Putting 2) and 3) together means that conjugate bases of acids with lower pKa values are favored at equilibrium.
3. Enolates are resonance stabilized. By far the most important reactions of enolates are with carbonyl groups to make carbon-carbon bonds. See how the concept of enolates was ripped of in an old popular movie staring Mike Myers.
4. Enolates react at the carbon atom rather than the O atom for two reasons:A) When attacking a carbonyl carbon (like an aldol), attack by the enolate O atom would give a relatively unstable hemiacetal. B) By attacking on the enolate C atom, the product contains a C=O rather than a somewhat weaker C=C.
5. The classic aldol reaction combines two molecules of the same aldehyde. A complex mixture results (four different consititutional isomers plus stereoisomers) when two different aldehydes are used and both of them can make enolates. You can see a movie of the aldol reaction by clicking here.
6. Aldol products can dehydrate to give a mixture of E and Z alpha,beta unsaturated aldehydes when heated in dilute acid. The more stable alkene (usually E) generally predominates.
7. Aldols between two different aldehydes can be run in high yield if one of the carbonyls is a ketone and the other is an aldehyde that cannot make an enolate. This is generally only relevant to formaldehyde and benzaldehyde.
Homework: Begin studying for next week's midterm! Read: 19.4-19.5 Problems: 19.7-19.9, 19.32, 19.35, 19.38 You will not turn these in but they are very helpful to PREPARE for the next lecture.
