Ans In 1st SN 1 reaction is possible so by-product is HBr in 2nd SN 1 reaction is not possible.

1st can give SN 2 Ar but 2nd can not give because –m of –NO 2 is not opperating.

2nd product has two antiaromatic rings but 1st does not have antiaromatic system.

–NO 2 is metadirecting but –N=O group is ortho-para directing due to +m of –N=O.

Sol. + HBr (strong acid)

+

(SN 1 reaction)

No reaction

Because aryl halide have resonance stabilized C–X bond, and do not give SN reaction.

Due to presence of p-NO 2 group, (– I , –m group) the SN 2 Ar reaction is accelerated (due to stabilization of intermediate carbanion. In the second case NO 2 can not exert its –m effect to stabilize the carbanion.

In the formation of first product the antiaromaticity due to the presence of three “ ” rings of the reactant is finished and the product becomes more stable. While in 2nd case the product is thermodynamically less stable.

Arenium ion is stabilised by + m effect of – = O

etc.

– NO 2 exerts -m effect, so incoming electrophile can attack at meta position only.