Chapter 25, Problem 25.61P

Interpretation Introduction

(a)

Interpretation:

The mechanism that leads to the formation of the major product when toluene undergoes Birch reduction is to be drawn.

Concept introduction:

In a dissolving metal reduction, solvated electrons behave like 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 accomplished in liquid ammonia with dissolved sodium metal. The anionic intermediates is protonated by ethanol as a solvent. The initial step of the Birch reduction is a one-electron transfer into the aromatic system. The product is a radical anion. This product is protonated by the solvent, which gives cyclohexadienyl radical. By an additional one-electron transfer, the allyl radical is resonance stabilized and transformed into a cyclohexadienyl anion. Afterward, the cyclohexadienyl anion is also protonated by the solvent molecules. This produces the final product. Electron-donating substituents such as alkoxy or alkyl groups are located at one of the double bonds of the final product. The carboxyl or amide is electron-withdrawing group. These groups are situated at one of the ${\text{sp}}^3$ hybridized carbon atoms in the product ring.

Interpretation Introduction

(b)

Interpretation:

It is to be explained whether the Birch reduction of toluene occurs faster or slower than the Birch reduction of benzene itself.

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 accomplished in liquid ammonia with dissolved sodium metal. The anionic intermediates is protonated by ethanol as a solvent. 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.