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Chapter 16: Problem 66

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

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Question: Describe the mechanism for the isomerization of (R)-glyceraldehyde to an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone in dilute aqueous base. Answer: The mechanism consists of four steps: 1. (R)-glyceraldehyde reacts with the aqueous base to form an (R)-glyceraldehyde enolate ion. 2. The (R)-glyceraldehyde enolate ion tautomerizes to form dihydroxyacetone. 3. Dihydroxyacetone reacts with water to generate a dihydroxyacetone enol. 4. The dihydroxyacetone enol tautomerizes to form both (R)-glyceraldehyde and (S)-glyceraldehyde.

Step by step solution

01

Identify the reactant and products

The reactant in this case is (R)-glyceraldehyde, and the products of the isomerization are (R, S)-glyceraldehyde and dihydroxyacetone. We will now focus on proposing a mechanism that leads to these products.
02

Propose the first step of the mechanism

In the first step, the hydroxyl group of the (R)-glyceraldehyde reacts with the aqueous base to form an enolate ion. This reaction can be represented as follows: (R)-glyceraldehyde + OH⁻ → (R)-glyceraldehyde enolate ion + H₂O
03

Propose the second step of the mechanism

In the second step, the (R)-glyceraldehyde enolate ion tautomerizes to form dihydroxyacetone. This involves the transfer of a proton between the oxygen atoms of the enolate ion, resulting in a keto-enol tautomerization. The reaction can be written as: (R)-glyceraldehyde enolate ion → dihydroxyacetone + OH⁻
04

Propose the third step of the mechanism

In the third step, dihydroxyacetone reacts with water to generate a dihydroxyacetone enol. This reaction involves the formation of a new enolate ion, which is in equilibrium with dihydroxyacetone. It can be represented as follows: dihydroxyacetone + H₂O → dihydroxyacetone enol + OH⁻
05

Propose the final step of the mechanism

In the final step of the mechanism, the dihydroxyacetone enol tautomerizes to form both (R)-glyceraldehyde and (S)-glyceraldehyde. Since the enol is an intermediate form between the two stereoisomers, this tautomerization will generate both products. The reaction can be written as: dihydroxyacetone enol → (R)-glyceraldehyde + (S)-glyceraldehyde + OH⁻ By combining all these steps, we can propose the complete isomerization mechanism of (R)-glyceraldehyde to an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone in dilute aqueous base.

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