In vinyl and aryl halides are less reactive because
(1) In vinyl chloride ,the lone pair of electrons on Cl atom conjugate with π – electron pair of double bond. The resonating structures impart a partial double bond character to C-Cl bond which also results in shortening of bond length and this imparts stability to vinyl halide and thus bond cleavage becomes difficult .
CH2 = CH – Cl → CH2 - – CH = Cl +
(2) Aryl halides in the same way are less reactive as the lone pair of halogen is in conjugation with benzene ring making a partial double bond between C and Cl.
Second reason
Different hybrid state of carbon atom
In haloalkanes(halohalides), the carbon atom bearing halogen is sp3 hybridized while halogen bearing carbon atom in haloarens and vinyl halide is sp2 hybridized. sp2 orbital is smaller in size due to greater s-character as compared with sp3 orbital. As a result bond formed by overlap of sp2 hybrid orbital is shorter in size and stronger than the bond formed by overlap of sp3 hybrid orbital. Therefore, C-X bond of haloalkanes is cleaved more easily than in haloarenes and vinyl halides.
