Rules of the Day 2-8-08

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

1. Wittig reactions form C=C bonds from C=O bonds through reaction of the nucleophilic ylide via the famous four-membered ring intermediate (See Pictures of the Day). This is a useful reaction because carbonyl compounds are easily made, and the ylides are made from alkyl halides, also easy to make. Unique mechanism! (See Pictures of the Day) Carbon-carbon bond forming!

2. Alkyl Wittig reagents give predominantly "Z" (cis) alkene products, while Wittig reagents with a carbonyl adjacent to the negative charge give predominantly "E" (trans) products.

3. Protonation of a carbonyl oxygen atom in acid makes the carbonyl carbon atom much more electrophilic (i.e. able to react with weaker nucleophiles).

4. Alcohols are weak nucleophiles so they add to aldehydes/ketones with acid catalysis. Mechanism D is first part of mechanism. This mechanism is important! First one, then two alcohol molecules add to give hemiacetals and acetals, respectively. Only the acetal is thermodynamically more stable than the carbonyl species unless a cyclic hemiacetal is formed. These reactions are reversible by adding H3O+ to drive the reaction back to the carbonyl compound and alcohols.

Homework: Start working on the fourth homework problem set, due Wed, 2-13-08, BEFORE CLASS. Click here to download the pdf.

Read: Sections 16.8, 16.9 Problems: 16.8-16.10, 16.38, 16.39 . You will not turn these in but they are very helpful to PREPARE for the next lecture.