Rules of the Day 2-18-08

Click here for a copy of the mechanism sheet used in lecture today.

1. Keto-enol equilibration is catalyzed by acid or base. Recall that catalysis does not change the position of equilibrium, but just accelerates the rate at which equilibrium is achieved. The keto form is almost always favored! In acid, the mechanism involves protonation of the carbonyl oxygen atom, followed by loss of a proton by the a-carbon atom.

2. The alpha-hydrogens of aldehydes and ketones are relatively acidic with a pKa around 20. Deprotonated aldehydes or ketones are resonance stabilized and are called enolates. The resonance stabilization of the enolate explains the relative acidity of aldehydes/ketones.

3. Base-catalyzed keto-enol equilibrium occurs via an enolate intermediate.

4. The enol form of a carbonyl reacts like an alkene, so even though it is only present in small amounts, some reactions characteristic of alkenes can be made to occur with aldehydes and ketones such as halogenation with X2 at the alpha position.

5. Because acid catalysis increases the rate at which keto and enol forms equilibrate, reactions that proceed through the enol form such as halogenation on the alpha carbon atom are accelerated in acid.

Homework: Finish working on the practice homework problem set that is meant as a helpful recview. You will NOT turn it in. Click here to download the pdf of the practice homework. The answers have now been posted. Click here for the answers to the practice homework.

Begin studying for the exam. You are responsible for all material covered up through Friday's lecture. Keto-enol equiolibria will not be on the first exam. What we cover next week will be on the second exam. Problems: Good review problems 16.44, 16.52, 16.55 To get ready for Wednesday's lecture Read: 17.1-17.3.

For tips on how to prepare for the exam, click here