Question

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

• A. b \(>\) a \(>\) c \(>\) d
• B. a \(>\) b \(>\) c \(>\) d
• C. a \(>\) b \(>\) d \(>\) c
• D. b \(>\) a \(>\) d \(>\) c

Hint:

For benzylic substitution, the rate is often determined by the ability of the substituent to stabilize the charge in the transition state. Strong donors (\( +M \)) are generally the most reactive.

Answer 1

The Correct Option is D

The reactivity of benzyl halides towards nucleophilic substitution reactions with KCN is heavily influenced by the electronic nature of substituents on the benzene ring. The substituents can either be electron-donating or electron-withdrawing. 

1. Compound (b) (having an \(\text{-NH}_2\) group) contains an electron-donating substituent. The \(\text{-NH}_2\) group is a strong activator and increases the electron density on the benzene ring, making it more reactive towards nucleophilic substitution.
2. Compound (a) (with a \(\text{-OH}\) group) is also an electron-donating group but weaker than \(\text{-NH}_2\). Therefore, it is the second most reactive after compound (b).
3. Compound (d) (with a single \(\text{-NO}_2\) group) has an electron-withdrawing group that moderately decreases reactivity.
4. Compound (c), with two \(\text{-NO}_2\) groups, has the strongest electron-withdrawing effect, which decreases the reactivity of the benzene ring further compared to compound (d).

Thus, the correct order of reactivity of the benzyl halides towards KCN is b > a > d > c.