Chapter 22.SE, Problem 56AP

Interpretation Introduction

Interpretation:

Given that 2-substituted-2-cyclopentenones are in a base catalyzed equilibrium with their 5-substituted-2-cyclopentenone isomers. Why such analogous isomerization is not observed for 2-substituted -2-cyclohexenones is to be explained.

Concept introduction:

The isomerization of cycloalkenones occurs through an enolate anion formed by the abstraction of acidic hydrogen by the base. Hydrogens on carbon α- to a carbonyl group and those on γ- carbon in a conjugated enone are acidic. If these hydrogens are removed, isomerization will occur. Otherwise isomerization is not possible.

To explain:

Why 2-substituted -2-cyclohexenones are not in a base catalyzed equilibrium with their 6-substituted -2-cyclohexenone isomers like 2-substituted-2-cyclopentenones.