Explain the mechanism of halogenations of methane.

Explanation

A. Chlorination of ethane takes place in thre step known as (1) Chain Initiation (2) Chain Propagation and (3) Chain Termination.
(I) Chain initiation: In this step `Cl_(2)` molecule absorbs energy and splits into Cl free radicals. (C-C bonds and C-H, bonds of ethane being relatively stronger, they do not break at this stage.)

2. Chain propagation : Chlorine free radicals react with ethane molecule.
`CH_(3)-CH_(3)+overset(*)(Cl) to CH_(3)-overset(*)(CH_(2))+HCl`.........(a)
`CH_(3)-overset(*)(CH_(2)) +Cl-Cl to CH_(3)CH_(2)Cl+overset(*)(Cl) .............(b)`
(a) and (b) repeat several times making the reaction a chain reaction and these steps are called propagation steps. In these steps the main products are formed.
In addition to (a) and (b) other reaction to replace other hydrogen atoms of ethane also takes place.
(3) Chain termination: When free radicals directly combine, the chain reaction stops down
`overset(*)(Cl)+overset(*)(Cl) to Cl_(2)`
`overset(*)(Cl)+overset(*)(C_(2)H_(5) to C_(2)H_(5)Cl`
`overset(*)(C)_(2)H_(5)+overset(*)(C)_(2)H_(5) to C_(4)H_(10)`