Q27E - Organic Chemistry (Mcmurry)

Question

One consequence of the base-catalyzed isomerization of unsaturated ketones described in Problem 22-55 is that 2-substituted 2-cyclopentenones can be interconverted with 5-substituted 2-cyclopentenones. Propose a mechanism for this isomerization.

Step-by-Step Solution

Answer

                                    

                                                                 Mechanism 

Step 1: Mechanism of the reaction

The mechanism of the given isomerization of 2-methyl-2-cyclopentanone to 5-methyl-2-cyclopentanone is 

                                                              Mechanism

Step 2: Explanation of the mechanism

In 2-cyclopentanone, deprotonation of the acidic hydrogens forms the enolate ion. Then, the enolate ion can be protonated at three different positions. 

When the partially positive carbon is protonated, it forms 2-methyl-3-cyclopentanone.

Abstraction of the hydrogen of C2 in 2-methyl-3-cyclopentanone does not form the desired product.

Abstraction of the hydrogen of C5 in 2-methyl-3-cyclopentanone forms the desired product.

The enolate forms by the abstraction of hydrogen of C5 can be stabilized by resonance and protonated at three different positions. This yields 5-methyl-3-cyclopentanone.