Q26E - Organic Chemistry (Mcmurry)

Question

Nonconjugated $\mathit{\beta }$,$\mathit{\gamma }$-unsaturated ketones, such as 3-cyclohexenone, are in an acid-catalyzed equilibrium with their conjugated $\mathit{\alpha }$,$\mathit{\beta }$-unsaturated isomers. Propose a mechanism for this isomerization.

Step-by-Step Solution

Answer

Step 1: Mechanism of the reaction

The mechanism of the given isomerization is

                                               Mechanism

Step 2: Explanation of the mechanism

The isomerization of 3-cyclohexanone to form 2-cyclohexanone takes place through the formation of its enol.

 

The acid protonates the carbonyl oxygen in the ketone. Then the partially positively charged hydrogen is abstracted, which forms the enol. Protonation of the enol with the electron pairs of oxygen yields a protonated 2-cyclohexenone. Lastly, deprotonation gives our desired product.