Chapter 22, Problem 22.10P

Interpretation Introduction

Interpretation:

It is to be explained why butylbenzene cannot be synthesized in good yield directly from benzene using a Friedel-Crafts alkylation. An alternative synthesis of butylbenzene that does not use a Friedel-Crafts reaction is to be proposed.

Concept introduction:

When benzene is treated with an alkyl halide in the presence of the Lewis acid (e.g., ${\text{AlCl}}_{\text{3}}$) as a catalyst, alkylation takes place at the benzene ring; it is known as a Friedel-Crafts alkylation reaction. A Friedel-Crafts alkylation reaction does not occur readily when the halogen atom of the alkyl halide is bonded to an ${\text{sp}}^{\text{2}}$ hybridized carbon. A Friedel-Crafts alkylation reaction is also susceptible to carbocation rearrangements. An organometallic compound has a polar covalent $\text{C-M}$ bond. For example, the Grignard reagent ($\text{RMgX}$) has a polar R-Mg bond; an alkyllithium ($\text{RLi}$) has a polar covalent $\text{C-Li}$ bond. Since carbon is bonded with an electropositive element, carbon possesses the partial negative charge and acts as the nucleophile as well as the base. Thus it is used for the new $\text{C-C}$ bond formation with electrophilic carbon. The nucleophilic part (${\text{R}}^{\text{-}}$) of the Grignard reagent shows a nucleophilic addition reaction with electrophilic carbon of the carbonyl compounds. Alkenes, on catalytic hydrogenation, are reduced to the corresponding alkane.