Question

In presence of peroxide, hydrogen chloride and hydrogen iodide do not give anti - Markownikoff's addition to alkenes because

• A. Both are highly ionic
• B. One is oxidising and other is reducing
• C. One of the steps is endothermic in both the cases
• D. All the steps are exothermic in both the cases

Answer 1

The correct option is C One of the steps is endothermic in both the cases

In the presence of peroxide, $HCl$ and $HI$ reaact with alkenes as follows.
$1^{st}step$:
$\left(a\right)\underset{peroxide}{R-O-O-R}\stackrel{\Delta }{⟶}2R-O^{\cdot }\left(b\right)R{O}^{\cdot }+H-X\to R-OH+X^{\cdot }$
$I{I}^{nd}step$:
$R^{\prime }CH=C{H}_2+X^{\cdot }\to \underset{\text{more stable}}{R^{\prime }CH-C{H}_{2}X}+R\underset{\underset{X}{|}}{C}H-C{H}_{2}X\text{less stable}{R}^{\prime }CH-C{H}_{2}X+XH\to R^{\prime }C{H}_2-C{H}_{2}X+X$
For $HCl$ step $I$ (b) is endothermic and step $II$ is exothermic. While for $HI$ step $I$
(b) is exothermic and step $II$ is endothermic. Hence, in each case, one of the steps is exothermic. Due to this reason $HCl$ and $HI$ do not give anti -Markownikoff's addition to alkenes.