Chapter 25, Problem 25.60P

Interpretation Introduction

(a)

Interpretation:

There are two isomeric $\text{cyclohexa-1, 4-diene}$ products when $\text{benzoic acid}$ undergoes the Birch reduction. The mechanism that leads to the formation of the major product is to be drawn.

Concept introduction:

In a dissolving metal reduction, solvated electrons behave like the free radical species. The dissolving metal reduction of a benzene ring is called Birch reduction which yields a $\text{cyclohexa-1, 4-diene}$ as the major product. It is processed in liquid ammonia with dissolved sodium metal. Ethanol used as solvent and it requires as a proton donor that protonates the anionic intermediates. This begins with a one-electron transfer into the aromatic system forming radical anions. After that, it protonated by ethanol and which result radical intermediate. By an additional electron transfer produced resonance stabilized ally radical and by protonation which result final product. Alkoxy or alkyl groups are electron-donating substituents and which are located at one of the double bonds of the final product. Carboxyl or amide groups are electron-withdrawing substituents, which are located at one of the ${\text{sp}}^{\text{3}}$ hybridized carbon atoms in the ring of the product.

Interpretation Introduction

(b)

Interpretation:

There are two isomeric $\text{cyclohexa-1, 4-diene}$ products when $\text{benzoic acid}$ undergoes the Birch reduction. The Birch reduction of $\text{benzoic acid}$ occur faster or slower than the Birch reduction of benzene itself is to be explained.

Concept introduction:

In a dissolving metal reduction, solvated electrons behave like the free radical species. The dissolving metal reduction of a benzene ring is called Birch reduction which yields a $\text{cyclohexa-1, 4-diene}$ as the major product. Birch reduction is carried out in liquid ammonia with dissolved sodium metal. The solvent used in the reaction is an alcohol, such as ethanol, which requires as a proton donor that protonates the anionic intermediates.  Since the nucleophile in this reaction is essentially free electrons, the reaction is faster on aromatic rings with electron-withdrawing substituents and slower on aromatic rings with electron-donating substituents.