Hydrocarbons 2 Question 22
22. In the presence of peroxide, hydrogen chloride and hydrogen iodide do not give anti-Markownikoff’s addition to alkenes because
$(2001,1 \mathrm{M})$
(a) both are highly ionic
(b) one is oxidising and the other is reducing
(c) one of the steps is endothermic in both the cases
(d) all the steps are exothermic in both the cases
Show Answer
Answer:
Correct Answer: 22. (b)
Solution:
- In addition of $\mathrm{HBr}$ to an alkene, in the presence of peroxide, both the propagation steps are exothermic :
$$ \mathrm{HBr}+\mathrm{HO}^{\bullet} \longrightarrow \mathrm{H}_{2} \mathrm{O}+\mathrm{Br}^{\bullet} $$
Propagation
$$ \left{\begin{aligned} & \mathrm{CH}{3}-\mathrm{CH}=\mathrm{CH}{2}+\mathrm{Br}^{\bullet} \longrightarrow \mathrm{CH}{3}-\dot{\mathrm{C}} \mathrm{H}-\mathrm{CH}{2} \mathrm{Br} ; \Delta H<0 \ & \mathrm{CH}{3}-\stackrel{\bullet}{\mathrm{C}} \mathrm{H}-\mathrm{CH}{2} \mathrm{Br}+\mathrm{HBr} \longrightarrow \mathrm{CH}{3}-\mathrm{CH}{2}-\mathrm{CH}_{2} \mathrm{Br} \ &+\mathrm{Br}^{\bullet} ; \Delta H<0 \end{aligned}\right. $$
In case of addition of $\mathrm{HCl}$ and $\mathrm{HI}$, one of the propagation step is endothermic, reaction fail to occur.
