Rules of the Day 2-11-08

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

1. 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.

2. When the aldehyde and an alcohol group are 4 or 5 carbons away on the same molecule, a relatively stable five- or six-membered ring cyclic hemiacetal is formed.

3. Carbohydrates exist in the cyclic, hemiacetal form in solution. Click here for a "molecule of the day" that discusses much of this. The mechanism involves the same steps as the cyclic hemiacetal formation mechanism.

4. Diols can be used to create cyclic acetals. This usually confuses students although it is simply a special case of the same acetal reaction described above.

5. Protecting groups are chemical functions that can reversibly mask a reactive functional group to prevent an unwanted reaction. The protecting group is added prior to the reaction in question, then removed following the reaction in question.

6. Cyclic acetals are good protecting groups for use with Grignard reagents, since the cyclic acetals can be reversibly attached, and they have an sp3 carbon atom that won't react with nucleophiles like carbonyls do.

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

No new homework problems or reading so that we can catch up with the new material.