Rules of the Day 320N

1. Aldehydes and ketones can be reduced by H2/Pd to give alcohols. (Watch for racemic mixtures!) Note that this method will also reduce any carbon-carbon double or triple bonds that happen to be in the molecule.

2. Metal hydride reagents can be used to reduce aldehydes and ketones to alcohols, but in this case carbon-carbon double or triple bonds are NOT reduced. In these reagents, NaBH4 (less reactive) and LiAlH4 (more reactive), the hydrogen atom is the nucleophile, that attacks a carbonyl carbon atom to give a tetrahedral intermediate, followed by addition of water to the reaction flask that protonates the oxyanion to give the alcohol product. Mechanism A.

Click here to see a short video that explains Lewis Acid-Lewis Base complexes and Dative bonds, which are important to understand for the Metal Hydride Reagent reactions

3. Aldehydes are oxidized to carboxylic acids by chromic acid (Jones Reagent, H2CrO4 in H2O).

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

5. Alcohols are weak nucleophiles so they add to aldehydes/ketones only with acid catalysis. Mechanism D is the first part of the 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 H2O to drive the reaction back to the carbonyl compound and alcohols. See the movie by clicking here Remember, there is a "hemi" in this mechanism, a hemiacetal that is. Sweet!!! This process is reversible and can be run in either direction depending on the ratio of water vs. alcohol in the solution.

6. Cyclic acetals are formed from 1,3 and 1,4 diols to make 5- or 6-membered rings, respectively. Cyclic acetals are more stable than regular acetals because of the chelate effect, which derives from having both -OH groups of the acetal connected to each other in the diol.

7. Cyclic hemiacetals that form five- or six-membered rings are stable (as opposed to non-cyclic hemiacetals which are not stable species). Carbohydrates are actually found in the cyclic acetal state in solution (SWEET!). In carbohydrates, the new C-O bond is formed to the so-called anomeric carbon, and for glucose, alpha means the new OH group is axial, and beta means it is equatorial.

Homework:

Read: Sections 16.8, 16.11 in the ebook textbook. This text is part of the Longhorn Textbook access program.

Take the Daily Quiz 5 before 10 PM tomorrow. Click here to access the quiz. These quizzes are designed to review the important material from today's lecture. Together, they will count as 3% of your final grade..

Start working on the Homework Problem Set 3, due at 10 PM on Monday, February . Click here to access the Homework Problem Set 3. Note there are Aktiv Learning and Gradescope Questions, and you MUST DO BOTH. Homeworks will be worth 10% of your final grade (20% from the Aktiv Learning and 80% from the Gradescope portions of the homework.