2020 H2 Chem P1 Q21 — Halogenoalkane Solution

The answer is C (chlorobenzene).

With ethanolic silver nitrate, water (and ethanol) act as nucleophiles and hydrolyse the C-Cl bond. The \(\text{Cl}^-\) released forms a white precipitate of AgCl. The faster the hydrolysis, the faster the precipitate appears.
In chlorobenzene, a lone pair on Cl is delocalised into the benzene ring. This gives the C-Cl bond partial double bond character, so it is stronger and less polar, and the electron-rich ring also repels the incoming nucleophile. Chlorobenzene is not hydrolysed and gives no precipitate, so it is the least reactive.

• A is wrong: benzoyl chloride is an acyl chloride. Its carbonyl carbon is bonded to both O and Cl, so it is very electron-deficient and is hydrolysed rapidly, giving a white precipitate at once. It is the most reactive of the four.
• B is wrong: 2-chloro-2-methylpropane is a tertiary halogenoalkane, hydrolysed readily by \(\text{S}_\text{N}1\) through a stable tertiary carbocation, so a precipitate forms quickly.
• D is wrong: 1-chloropropane is a primary halogenoalkane. It is hydrolysed more slowly (\(\text{S}_\text{N}2\)) but still gives a precipitate on warming, so it is more reactive than chlorobenzene.