Question

Which of the following is maximum reactive towards E.S.R. :-

• A.
• B.
• C.
• D.

Answer 1

Answer: (B)

2

Answer 2

The reactivity of substituted benzenes towards Electrophilic Substitution Reaction (ESR) is primarily determined by the electron density of the benzene ring. Electron-donating groups (EDGs) increase the electron density of the ring, thereby activating it towards electrophilic attack. Alkyl groups are known EDGs due to their inductive effect (+I) and hyperconjugation.

For alkyl groups, the activating effect on the benzene ring towards ESR is primarily governed by hyperconjugation. The more alpha-hydrogens present on the carbon atom directly attached to the benzene ring, the greater the hyperconjugative effect, leading to a more activated ring and higher reactivity towards ESR.

Let's compare the number of alpha-hydrogens for each substituted benzene:

• Toluene (-CH₃): 3 alpha-hydrogens
• Ethylbenzene (-CH₂CH₃): 2 alpha-hydrogens
• Cumene (-CH(CH₃)₂): 1 alpha-hydrogen
• Benzene: 0 alpha-hydrogens (no alkyl substituent)

Based on the number of alpha-hydrogens, Toluene (Option 2) is the most reactive towards Electrophilic Substitution Reaction.