Question

Decreasing order of rate of reaction of compounds towards electrophilic addition reaction with $HBr$ is :

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

Answer 1

The correct option is C $\left(d\right)>\left(b\right)>\left(c\right)>\left(a\right)$

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

Carbocation intermediates formed are :


In (d) ,(b) the carbocation is stabilized by the $+R$ effect of $-N{H}_2$ and $-OH$ group respectively.
$+R$ effect of $-N{H}_2$ group is more than that of $-OH$ group. So (d) is more stable than (b).
In (c) the carbocation is stabilized by hyperconjugation, so it is less stable than (b) and (d)
(a) is least stable because it is destabilized by $-R$ effect of $-N{O}_2$
Correct order is :
$\left(d\right)>\left(b\right)>\left(c\right)>\left(a\right)$