Chemistry - GOC Question with Solution | TestHub

A spontaneous reaction needs the $\mathrm{C}-\mathrm{Cl}$ bond to ionise easily, so the carbocation formed must be very stable (aromatic).

(A) 5-Chlorocyclopenta-1,3-diene would give the cyclopentadienyl cation. It has 4 π electrons in a planar cyclic conjugated ring (antiaromatic), so it is very unstable. No reaction.

(B) 7-Chlorocyclohepta-1,3,5-triene would give the tropylium cation. It is planar and cyclic with 6 π electrons (aromatic, 4n + 2 with n = 1), so ionisation is easy and $\mathrm{AgCl}$ precipitates at once.

(C) Here $\mathrm{Cl}$ is on an $\mathrm{s}{\mathrm{p}}^2$ carbon (vinylic), the $\mathrm{C}-\mathrm{Cl}$ bond has partial double-bond character and the vinyl cation is unstable. No reaction.

(D) Chlorobenzene is an aryl halide: no reaction.

Answer: (B).