Which of the following compound will most easily be attacked by an electrophile?

Solution

Chemistry ma’am · 3 steps

An electrophile wants electrons, so pick the ring whose substituent pushes the most electron density into it.

1. What the electrophile wants. An electrophile attacks the π cloud of the ring. The richer the ring is in electrons, the faster the attack. So rank the substituents by how much electron density they give to the ring.

   Compound	Substituent effect	Net result on the ring
   (a) chlorobenzene	-I of Cl is stronger than its weak +R	deactivated
   (b) phenol	strong +R from a lone pair on O, which outweighs its -I	strongly activated
   (c) benzene	no substituent	reference
   (d) toluene	+I and hyperconjugation from CH₃	mildly activated

2. Why OH wins. In phenol, a lone pair on oxygen is delocalised into the ring. This puts extra negative charge at the ortho and para carbons. Resonance donation like this is far stronger than the hyperconjugation of a methyl group.
3. So. The order of reactivity is phenol > toluene > benzene > chlorobenzene. The answer is (b).

The idea to keep. For electrophilic substitution, a lone pair donated by resonance (-OH, -NH₂, -OR) beats alkyl hyperconjugation. Halogens are the odd case: they direct ortho/para but still deactivate.

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