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: Explain the mechanism for the isomerization of (R)-glyceraldehyde to an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone in a dilute aqueous base. Answer: The isomerization mechanism involves the formation of an enolate ion by the removal of a proton from the alpha-carbon of (R)-glyceraldehyde by a dilute aqueous base. The enolate ion then undergoes tautomerization to form dihydroxyacetone. The base can also abstract a proton from dihydroxyacetone to form another enolate ion, which tautomerizes to produce (S)-glyceraldehyde. The reaction is reversible, leading to an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone.

Step-by-step solution

Identify the reactant and products

The reactant in this reaction is (R)-glyceraldehyde, which is a simple carbohydrate with the formula CH2OH-CHOH-CHO. The products are a mixture of (R,S)-glyceraldehyde and dihydroxyacetone, which is a ketone with the formula CH2OH-C(=O)-CH2OH.

Understand the role of the dilute aqueous base

A dilute aqueous base, like hydroxide ion (OH⁻), can remove a proton (H⁺) from a molecule, resulting in a negative charge on the oxygen atom. In the case of glyceraldehyde, the most acidic protons are connected to the carbons adjacent to the aldehyde functional group. When the base abstracts a proton from this position, an enolate ion will be formed.

Formation of enolate ion

The mechanism begins with the removal of a proton from the alpha-carbon of (R)-glyceraldehyde (the carbon adjacent to the aldehyde) by the base (OH⁻). This results in the formation of an enolate ion and an H2O molecule. (R)-Glyceraldehyde + OH⁻ → Enolate ion + H2O

Tautomerization

The enolate ion can undergo tautomerization, which is the migration of the double bond and movement of a proton, to form dihydroxyacetone. Enolate ion → Dihydroxyacetone

Formation of (S)-glyceraldehyde

Now, the base can abstract a proton from the other side of dihydroxyacetone to form another enolate ion. This enolate ion can then undergo tautomerization to produce (S)-glyceraldehyde. Dihydroxyacetone + OH⁻ → Enolate ion + H2O Enolate ion → (S)-glyceraldehyde

Establishing the equilibrium

Finally, it is important to note that this reaction is reversible, and an equilibrium mixture of (R, S)-glyceraldehyde and dihydroxyacetone will be formed. Since (R) and (S) forms can interconvert through dihydroxyacetone, the isomerization process is complete. (R)-Glyceraldehyde ↔ Dihydroxyacetone ↔ (S)-Glyceraldehyde