Friedel–Crafts Alkylation and Acylation

The Friedel-Crafts reactions are two closely related electrophilic aromatic substitutions,

discovered by Charles Friedel and James Crafts, that attach a carbon-based substituent -- an alkyl

group (alkylation) or an acyl group (acylation) -- directly to the benzene ring, both requiring a

Lewis acid catalyst, most commonly anhydrous aluminium chloride, $\text{AlCl}_3$.

Friedel-Crafts alkylation. An alkyl halide reacts with benzene in the presence of anhydrous

$\text{AlCl}_3$; the Lewis-acidic aluminium coordinates to the halide's lone pair and helps pull

it away, generating a carbocation electrophile (or a strongly polarised, carbocation-like complex):

This carbocation then attacks the ring exactly as in the general EAS mechanism -- forming the

arenium ion and then losing $\text{H}^+$ to give the alkylbenzene, e.g. benzene $+$

$\text{CH}_3\text{Cl}$ / $\text{AlCl}_3 \rightarrow$ toluene $+ \text{HCl}$. The reaction has

two well-known practical limitations. First, because a genuine (or carbocation-like) intermediate

is involved, the alkyl group can rearrange (via a hydride or alkyl shift) to a more stable

carbocation before it attacks the ring, so a primary alkyl halide often gives a rearranged,

more-branched product rather than the "expected" straight-chain one. Second, the alkylbenzene

product is more electron-rich (more reactive toward further EAS) than benzene itself, because

the newly installed alkyl group is electron-donating -- so the reaction is prone to

polyalkylation, installing a second and even third alkyl group unless the alkyl halide is

used in a strictly controlled, limited amount.

Friedel-Crafts acylation. An acyl (acid) chloride or acid anhydride reacts with benzene, again

catalysed by anhydrous $\text{AlCl}_3$, which coordinates to the acyl chloride's chlorine and

helps generate a resonance-stabilised acylium ion,

$\text{R--C}{\equiv}\text{O}^+ \leftrightarrow \text{R--}\overset{+}{\text{C}}{=}\text{O}$,

as the electrophile; this attacks the ring by the same arenium-ion mechanism to give an aryl

ketone, e.g. benzene $+ \text{CH}_3\text{COCl} / \text{AlCl}_3 \rightarrow$ acetophenone

$+ \text{HCl}$. Acylation avoids both of alkylation's drawbacks: the acylium ion is already

resonance-stabilised and has no comparably favourable rearrangement pathway available, so the acyl

group transfers intact without skeletal rearrangement; and the ketone product is less reactive …