Q.Outline the mechanism of the Friedel-Crafts acylation of benzene with acetyl chloride and anhydrous $\text{AlCl}_3$, and explain why this reaction avoids the problems seen in alkylation.

Anhydrous $\text{AlCl}_3$ coordinates to the chlorine of acetyl chloride, $\text{CH}_3\text{COCl}$, and helps it ionise to generate an acylium ion, $\text{CH}_3\text{--C}{\equiv}\text{O}^+$, which is significantly stabilised by resonance delocalisation of the positive charge between the carbon and the oxygen ($\text{CH}_3\text{--C}{\equiv}\text{O}^+ \leftrightarrow \text{CH}_3\text{--}\overset{+}{\text{C}}{=}\text{O}$). This acylium ion then attacks the ring by the usual EAS mechanism -- forming the resonance-stabilised arenium ion, then losing $\text{H}^+$ to rearomatise -- to give the aryl ketone, acetophenone, $\text{C}_6\text{H}_5\text{COCH}_3$. This avoids alkylation's two problems for two distinct reasons. Because the acylium ion is ALREADY resonance-stabilised, it has no comparably favourable rearrangement pathway available (unlike a simple alkyl carbocation, which can gain stability by rearranging), so the acyl group transfers to the ring intact, without skeletal rearrangement. And because the resulting ketone's carbonyl group is electron-WITHDRAWING (inductively pulling electron density out of the ring, unlike an alkyl group's electron-donating effect), the acetophenone product is LESS reactive toward further electrophilic substitution than benzene itself -- so the reactio …

Note the acylium ion's built-in resonance stabilisation (no rearrangement driving force) and the carbonyl group's electron-withdrawing character in the product (self-limiting to mon …

Assuming acylation also risks polysubstitution like alkylation does (it specifically does not, because the product is deactivated); missing that the acylium ion's resonance stabilisation, n …