Question

In dilute aqueous base, \((R)\)-glyceraldehyde is converted into an equilibrium mixture of \((R, S)\)-glyceraldehyde and dihydroxyacetone. Propose a mechanism for this isomerization.

Short answer

Question: Propose a mechanism for the isomerization of (R)-glyceraldehyde into an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone in a dilute aqueous base. Answer: The mechanism involves 4 steps: (1) Deprotonation by the base to form an enolate intermediate, (2) Tautomerization to form dihydroxyacetone, (3) Protonation by the base to form a new enolate intermediate, and (4) Reprotonation to form a mixture of (R, S)-glyceraldehyde.

Step-by-step solution

Identify the compounds involved in the reaction

In this reaction, we have (R)-glyceraldehyde isomerizing into a mixture of (R, S)-glyceraldehyde and dihydroxyacetone. First, let's understand the structure of these compounds: (R)-glyceraldehyde has a chiral carbon and the structure is HOCH2-CH(OH)-CHO. Dihydroxyacetone has no chiral carbon and the structure is HOCH2-CO-CH2OH. Notice that they are both aldo-ketone isomers.

Identify the role of the base

In a dilute aqueous base, the base provides hydroxide ions (OH-) that can act as a nucleophile and catalyze the reaction. In this case, the base will help to facilitate the isomerization of (R)-glyceraldehyde into an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone.

Propose the mechanism

Now, let's propose a mechanism for this isomerization: 1. Deprotonation: The hydroxide ion (OH-) acts as a nucleophile and abstracts the acidic hydrogen from the carbon next to the carbonyl group of (R)-glyceraldehyde (the alpha-carbon), forming an enolate intermediate and water as a byproduct. 2. Tautomerization: The enolate intermediate can then undergo keto-enol tautomerization, which involves the movement of the double bond and the migration of the hydrogen atom. This leads to the formation of the keto form, dihydroxyacetone. 3. Protonation: In the presence of a dilute aqueous base, the hydroxide ion (OH-) can act as a nucleophile again and attack the carbonyl carbon of dihydroxyacetone, forming a new enolate intermediate. 4. Reprotonation: Finally, the new enolate intermediate can be reprotonated, forming a mixture of (R, S)-glyceraldehyde. By following these steps, we have proposed a mechanism for the isomerization of (R)-glyceraldehyde into an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone in a dilute aqueous base.