Vinyl chloride and chlorobenzene are unreactive in `S_(N)2` reaction. Explain these observations.

Explanation

Vinyl chloride `(H_(2)C=CHCl)` is unreactive in `S_(N)2` reaction due to resonance.

In vinyl chloride, the lone pair of electrons on chlorine is in resonance with C=C double bond. Hence there is paritial double bond character in C-Cl bond. As a reasult, removel of `Ci^(Theta)` is not feasible here.
Again, chlorobenzene is also unreactive in `S_(N)2` reaction because of the resonance of chlorobenzene. The lone pair of electrons on chlorine atom is in conjugation with the `pi` -electrons of benzene ring which results in delocalisation of electrons of C-Cl bond. Hence a partial double bond character develops in this bond which makes it difficult for the nucleophile to cleave the C-Cl bond.

Again the `sp^(2)`-hybridised carbon atom involved in C-Cl bond in both vinyl chloride and chlorobenzene is more electronegative than the `Sp^(3)` -hybridised carbon atom in alkyl halide. therefore, this `sp^(2)`-hybridised carbon atom has less tendency to donate the C-Cl bond pair of electorns to the CL atom. Thus, both vinyl chloride and chlorobenzene are unreactive towards `S_(N)2` reaction because of the lower polarity of C-Cl bond.