Question

In a strongly acidic solution, cyclohexa-1, 4-diene tautomerizes to cyclohexa-1, 3-diene. Propose a mechanism for this rearrangement, and explain why it is energetically favorable.

Short answer

The cyclohexa-1, 3-diene is in conjugated form and is energetically favorable.

Step-by-step solution

Tautomerization

Tautomerization is a transfer of protons from one compound site to the other site in steps.For example, enol form when tautomerizes, it changes to keto form. In some places, keto form is more stable, and in some, the enol form is stable.

Mechanism for the rearrangement

When cyclohexa-1, 4-diene in a strongly acidic solution gets tautomerized to cyclohexa-1, 3-diene. A strong acid (we take general acid HA) has a protonated base, where is a conjugate base.

In a strongly acidic solution, a cyclohexa-1, 4-diene generates a positive charge by removing one double bond. A conjugate base formed acts as a nucleophile and attacks the hydrogen at C3, neutralizing the positive charge.

The cyclohexa-1, 3-diene formed is in conjugated form and hence, energetically favorable. On the other hand, cyclohexa-1, 4 –diene is not in the conjugate form and is less energetically favorable.

Mechanism for the rearrangement