Q.Compare Friedel-Crafts alkylation and acylation of benzene, and explain why acylation is often preferred when a clean, single, unrearranged substitution is needed.

Both reactions are Lewis-acid-catalysed (typically anhydrous $\text{AlCl}_3$) electrophilic aromatic substitutions that install a carbon-based group on the ring, but they differ sharply in reliability. Alkylation's electrophile is a genuine (or near-genuine) carbocation, generated from an alkyl halide; because a carbocation can undergo a hydride or alkyl shift to reach a more stable cation before it ever attacks the ring, the alkyl group actually installed can be REARRANGED relative to the one present in the starting halide, making the product hard to predict for anything beyond a simple, already-stable (e.g. tertiary or methyl) alkyl halide. Alkylation ALSO tends toward polyalkylation, because the alkylbenzene product is more electron-rich, and therefore more reactive toward further EAS, than benzene itself -- so unless the alkyl halide is carefully limited in amount, a mixture of di- and tri-alkylated products forms alongside the desired monoalkylated one. Acylation's electrophile, by contrast, is an acylium ion that is ALREADY resonance-stabilised (the positive charge is shared between carbon and oxygen), so it has essentially no comparable driving force to rearrange before attacking the ring -- the acyl group transfers intact. And because the resulting ketone's carbonyl group is electron-WITHDRAWING, the acylbenzene product is LESS reactive than benzene, so the reaction self-limits to a single acylation without any careful control of stoichiometry being needed. For these two reasons, chemists needing a specific, unrearranged alkyl group ins …

Compare the two reactions feature by feature -- electrophile stability/rearrangement risk, and product reactivity relative to benzene (activating vs deactivating) -- rather …

Assuming alkylation and acylation are equally reliable/clean because both use the same $\text{AlCl}_3$ catalyst and the same overall EAS mechanism; overlooking that it is specifically the ELECTROPHILE's stability (acylium vs carbocation) and the P …