Question

Which of the following compounds undergoes addition reaction with $HBr$ most easily?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Rate of addition of HBr reaction depends on the stability of carbocation intermediate formed during reaction.
Stable the intermediate formed, more is the rate of reaction for that compound.


The substitutent in para position shows resonance effect and hyperconjugation effect. In compound (b) and (c), the presence of electron donating group will stabilise the carbocation than the compound (a) and (d).
Comparing (b) and (c), $-OC{H}_3$ is a powerful electron donating group due to the resonance effect which is prior to hyperconjugation effect shown by $-C{H}_3$.
Thus, compound (c) forms a stable carbocation and reacts most easily with HBr.