Question

When the enzymatic decarboxylation of acetoacetate is carried out in${{\mathbf{H}}_2}^{18}\mathbf{O}$, all the acetone that is formed contains${}^{18}\mathbf{O}$. What does this tell you about the mechanism of the reaction?

Short answer

If the reaction proceeds without the imine formation then the source of oxygen in acetone is${\text{H}}_{\text{2}}{\text{O}}^{\text{18}}$.

If the decarboxylation reaction occurs without imine formation, most of the acetone will contain ${\text{O}}^{\text{18}}$.

Step-by-step solution

Step-by-Step SolutionStep 1: Mechanism of the reaction having imine as an intermediate

It is seen that in the first step of the reaction imine is formed as an intermediate as imine is formed in the reaction mechanism. Hence, the only source of oxygen in the acetone formation is ${\text{H}}_{\text{2}}{\text{O}}^{\text{18}}\text{.}$

Mechanism without imine

The mechanism of the reaction without  imine as an intermediate

If the decarboxylation reaction occurs without imine formation most of the acetone contains 18-O. There would be some ${\text{O}}^{\text{18}}$incorporated into acetone because acetone would react with ${\text{H}}_{\text{2}}{\text{O}}^{\text{18}}$and form a hydrate. The ${\text{O}}^{\text{18}}$would become incorporated into the acetone when the hydrates will refer to the acetone.

Mechanism of enzymatic decarboxylation