JEE Main · 2021 · 26 Aug 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

Bromine with aluminium bromide generates the electrophile $\mathrm{B}{\mathrm{r}}^{+}$, which attacks the benzene ring. The acyl group withdraws electron density both inductively and by resonance, so the ring is deactivated and the direction of attack is set by the arenium ion: for ortho and para attack one resonance structure places the positive charge on the ring carbon bearing the carbonyl, which is strongly destabilising, while for meta attack no such structure arises. Bromination therefore occurs at the position meta to the acyl group, and the major product is 1-(3-bromophenyl)pentan-1-one.