Question

Order of activity of halogens during the halogenation of alkane.
(a) $C{l_2} > B{r_2} > {I_2}$
(b) $B{r_2} > {I_2} > C{l_2}$
(c) ${I_2} > C{l_2} > B{r_2}$
(d) $C{l_2} > {I_2} > B{r_2}$

Answer 1

Hint: Alkanes are the hydrocarbons which are chemically least reactive in organic compounds. Alkanes despite having inert nature undergo several reactions like combustion, halogenation. The halogenation of alkanes depends on the lower positive value of energy of activation for the chain propagation step. Chlorination and bromination are exothermic in nature.

Complete answer:
> Halogenation of alkane is the reaction, where one or more hydrogen atoms of the organic compound are replaced by the halogen atom.
> The halogenation reaction takes place in three steps initiation, propagation and termination.
> Among the halogen atoms, fluorine is so explosively reactive that it is difficult to control the reaction. So, mainly halogens chlorine, bromine and iodine take part in the halogenation reaction of alkane. The halogen iodine is unreactive as compared to other halogen atoms.
> The chlorination reaction and bromination reaction are exothermic in nature as the positive activation energy of chain propagation reaction is lower.
> The halogenation reaction takes place either in gaseous phase or liquid phase.

> In the gaseous phase, if chlorination is done in the presence of oxygen, the reaction is inhibited. In the liquid phase, when the initiators like peroxides are used, it facilitates the reaction.
> Thus, the order of activity of halogens during the halogenation of alkane is shown below.
$C{l_2} > B{r_2} > {I_2}$

Therefore, the correct option is A.

Note: In the halogenation reaction, energy in the form of heat or light is used to execute the halogenation reaction. If light is applied to initiate the halogenation reaction, thousands of molecules get reacted with each proton of light which is absorbed.