Among the following organic halides in the increasing order of their dehydrohalogenation reactions in the…
Among the following organic halides in the increasing order of their dehydrohalogenation reactions in the presence of alcoholic $\mathrm{KOH}$
(A) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br}$
(B) $\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br}$
(C) $\mathrm{CH}_3 \mathrm{CH}(\mathrm{Br}) \mathrm{CH}_3$
(D) $\left(\mathrm{CH}_3ight)_3 \mathrm{CBr}$
A $ < $ B $ < $ C $ < $ D
D $ < $ B $ < $ C $ < $ A
A $ < $ C $ < $ B $ < $ D
B $ < $ A $ < $ C $ < $ D
Solution
The elimination reaction (dehydrohalogenation)
is favoured by those substrates that form a more stable carbocation intermediate.
Thus, the order of reactivity will be :-
\(\begin{array}{c}\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{Br} < \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{Br} < \begin{array}{l}\mathrm{Br} \\ ~| \\ \mathrm{CH}_3 \\ ~| \\ \mathrm{CH}_3 \end{array} \mathrm{CH} < \left(\mathrm{CH}_3ight)_3 \mathrm{C} \mathrm{Br} \\ 1^{\circ} \mathrm{ethyl} \qquad 1^{\circ} \mathrm{propyl} \qquad 2^{\circ} \mathrm{halide} \qquad 3^{\circ} \mathrm{halide} \\ \text{(A)} \qquad \qquad \text{(B)} \qquad \qquad \text{(C)} \qquad \qquad \text{(D)} \end{array}\)