Chapter 22, Problem 22.31P

Interpretation Introduction

Interpretation:

The detailed mechanism for the electrophilic aromatic substitution that takes place in the given reaction is to be drawn.

Concept introduction:

Friedel–Crafts acylation of formylation (replacing H by a formyl group HC=O) requires in situ formation of the very unstable $\text{Cl-CH=O}$. In Friedel–Crafts acylation, the aromatic species is treated with an acid chloride, also called an acyl chloride. (An acyl group is $\text{R-C=O}$.) The strong Lewis acid catalyst, ${\text{AlCl}}_{\text{3}}$, is responsible for generating the cationic electrophile, called an acylium ion, in the first two steps. In Step 1, the Cl atom of the acyl chloride coordinates to the electron-deficient Al atom. In Step 2, the $\text{C-Cl}$ bond undergoes heterolysis. Step 3 is electrophilic addition of the acylium ion to the benzene ring, producing the arenium ion intermediate. Finally, in Step 4, removing the proton from the arenium ion yields the aromatic ketone product.