Acyl chlorides are useful reagents in synthesis - AQA - A-Level Chemistry - Question 7 - 2018 - Paper 2

Formation of Electrophile:

• The acyl chloride, $CH_3CH_2COCl$, reacts with the Lewis acid, $AlCl_3$, to form the electrophile, an acylium ion $CH_3CH_2C^+=O$.

Electrophilic Attack:

• The benzene ring donates a pair of electrons to the electrophile, forming a sigma complex (also known as an arenium ion).

Deprotonation:

• A proton is removed from the sigma complex by the $AlCl_4^-$ ion, restoring the aromatic system and yielding the final product, $C_6H_5C(O)CH_2CH_3$.