The correct order of reactivity of the following benzyl halides towards reaction with KCN is :

Solution

Chemistry ma’am · 4 steps

Picture the benzyl carbocation: the group that feeds it the most electrons makes the reaction fastest.

1. What sets the rate. Benzyl bromides lose Br⁻ easily, and the positive charge of the benzylic carbocation spreads into the ring. The question treats the reaction with CN⁻ as S_N1, so a more stable carbocation means a faster reaction.
2. Donors at para. -NH₂ and -OH at the para position push a lone pair into the ring by +R and stabilise the cation. N is less electronegative than O, so it gives up its lone pair more readily, and -NH₂ is the stronger donor. That puts b ahead of a.
3. Nitro position matters. -NO₂ destabilises the cation. At para it withdraws by both −R and −I. At meta it cannot conjugate with the benzylic carbon, so it withdraws only by −I. Para-nitro is therefore the worst, and d beats c.

   Compound	Substituent	Effect on benzyl cation	Rank
   b	NH₂ at para	strong +R, most stabilised	1
   a	OH at para	+R, weaker than NH₂	2
   d	NO₂ at meta	−I only	3
   c	NO₂ at para	−R and −I, least stable	4

4. So. The order of reactivity is b > a > d > c, which is option (b).

The idea to keep. For benzylic substitution, rank by carbocation stability. +R donors at ortho or para speed it up. A nitro group slows it down, and it does so most at para, where −R adds to −I.

Answer from the NTA final answer key. Solution by PrepWiser.