BSc Nursing GNM 2020 Solved Paper

In haloarenes and vinyl halides, the phenyl cation or the vinyl cation formed is not stabilised by resonance because the sp2-hybridised orbital of carbon having positive charge is perpendicular to the p-orbitals of the phenyl ring or the vinyl group. Therefore, these are highly unstable cations and aryl and vinyl halide do not undergo hydrolysis i.e., nucleophilic substitution reaction (SN1 mechanism).

CH2

Vinyl

=

CH

Chloride

−Cl⟶

CH2

Vinyl

=

CH++Cl−

Cation

On the other hand, due to greater stabilisation of allyl and benzylic carbocation intermediates by resonance, even primary allylic and primary benzylic halides show higher reactivity in SN1 reactions. Therefore, allyl and benzylic cations readily undergo hydrolysis and combines with OH−ions to form allyl and benzylic alcohol.
CH2=CH−CH2+↔CH2+−CH=CH2
(resonance stabilisation of allyl carbocation)