Chapter 17.6A, Problem 17.5P

(a)

Interpretation Introduction

To determine: A detailed mechanism for the ${\text{FeBr}}_{\text{3}}$-catalysed reaction of ethylbenzene with bromine and the reason for low energy of sigma complex for substitution at ortho and para position in comparison than it is for substitution at the meta position.

Interpretation: A detailed mechanism for the ${\text{FeBr}}_{\text{3}}$-catalysed reaction of ethylbenzene with bromine is to be drawn and the reason for low energy of sigma complex for substitution at ortho and para position in comparison than it is for substitution at the meta position is to be stated.

Concept introduction: One of the most important reactions of aromatic compound is electrophilic aromatic substitution. The pi electron cloud of benzene readily attacks a strong electrophile which results in the formation of a carbocation. This carbocation is resonance stablised. It is known as a sigma complex. This complex is not aromatic due to ${\text{sp}}^{\text{3}}$ hybrid carbon atom. The aromaticity is regained by the loss of proton on the tetrahedral carbon atom.

(b)

Interpretation Introduction

To determine: The reason for much faster nitration of m-xylene in comparison to p-xylene.

Interpretation: The reason for much faster nitration of m-xylene in comparison to p-xylene is to be stated.

Concept introduction: Nitrobenzene is formed when benzene reacts with hot concentrated nitric acid. This reaction is explosive as well as hazardous. A mixture of nitric acid and sulfuric acid is used to avoid such type of explosion. Sulfuric acid acts as a catalyst and allows the reaction to occur at lower temperatures as well as at high rate.