Question

Consider the following reaction:

Methane + Cl₂/heat -->

At what step during the mechanism of chlorination that the methyl radical is generated?

	A.	First propagation step
	B.	Second initiation step
	C.	First initiation step
	D.	Third propagation step
	E.	Second propagation step

Answer 1

A. First propagation step

Explanation:

The reaction product is chloromethane CH3Cl. It is the substitution reaction in which hydrogen atom is replaced by chlorine atom , follows free radical mechanism. So the overall process is called free radical substitution reaction , involves three steps as explained below

Initiation step:

the chain is initiated by the presence of heat/sunlight breaking of chlorine molecule into chlorine free radical

Cl2-------->Cl• + Cl•

Propagation step :

these are the reaction which keeps the chain going further . Free radical generated in the initiation step reacts with methane and forms a methyl free radical (CH3•) and hydrogen chloride (HCl) in the first step of the propagation step of free radical mechanism.

CH4 + Cl• ----------->CH3• + HCl

So formed methyl radical further reacts with Cl2 and forms CH3Cl and another Chlorine free radical (Cl•)

CH3• + Cl2 ----------> CH3Cl + Cl•

termination step:

These are the reaction which removes the free radicals from the system and thereby stops the process with out generating any free radicals

2Cl• -------------->Cl2

CH3•+ Cl ---------> CH3Cl

2CH3• ----------->CH3CH3

Therefore generation of methyl free radical is the first propagation step of free radical mechanism .