31.1.2—Difference in reactivity

The C–Cl bond in chlorobenzene is less susceptible to nucleophilic substitution than the C–Cl bond in chloroethane. In chlorobenzene, the chlorine lone pair overlaps with the aromatic π system, giving the bond partial double-bond character.

The aryl carbon is sp² and the bond is shorter and stronger; attack is also hindered by the ring. Chloroethane has a more accessible sp³ carbon with a normal polar C–Cl bond.

Aqueous hydroxide readily substitutes chloroethane on heating, whereas chlorobenzene needs much harsher conditions and is not treated as an ordinary halogenoalkane.

Both molecules contain C–Cl, but functional context changes reactivity; do not transfer the halogenoalkane mechanism automatically to an arene.