Question

Account for the fact that the chain-initiating step in thermal chlorination of $CH_{4}$ is $Cl_{2} \underrightarrow{heat} 2Cl^{.}$ and not $CH_{4} \underrightarrow{heat} CH_{3}^{.} + H^{.}$

Answer 1

The “radical chain mechanism” involves the three steps of “initiation”, "propagation", and "termination".

In the initial chain initiation step, radicals are formed. Chain initiation step of halogens is endothermic and involves input energy in the form of heat or light.

Hence, chlorine also gets dissociated homolytically by the action of heat or light in chain initiation step in the following process:

${ Cl }_{ 2 }\quad \xrightarrow [ heat ]{ \quad  } \quad 2{ Cl }^{ - }$

The other steps, propagation and termination, take place without any input energy and is exothermic.

Answer 2

The “radical chain mechanism”involves the three steps of “initiation”,"propagation", and "termination".

In the initial chain initiation step, radicals are formed. Chain initiation step of halogens is endothermic and involves input energy in the form of heat or light.

Hence, chlorine also gets dissociated homolytically by the action of heat or light in chain initiation step in the following process:



The other steps, propagation and termination, take place without any input energy and is exothermi