Answer to Question #94072 in Organic Chemistry for Megha Nautiyal

Question

In case of addition reaction of HBr to propenenitrile, the secondary
carbocation is less stable than the primary carbocation while in chloroethene
secondary carbocation is more stable than primary carbocation. Explain.

Answer 1

In propenenitrile the secondary carbocation is not stable because of strong electron withdrawing effect of the neighboring CN group. The primary carbocation is located 2 carbons away and CN group doesn't have such destabilising effect.

In case of chloroethene, secondary carbocation is stabilized by chlorine group by donating its lone electron pair, so the secondary carbocation is more stable.