Question

# Methane does not react with Chlorine in dark(A) True(B) False

Hint: Methane reacts with Chlorine in the presence of light .Since the reaction proceeds through a free radical intermediate, it requires some kind of energy for forming the free radicals. When in dark, there is no such energy for the free radicals formation to take place.

Methane reacts with ${ Cl }_{ 2 }$ only in the presence of sunlight.

It is because the reaction of methane with chlorine proceeds through free radicals.

Given below is the reaction of methane with chlorine.

${ CH }_{ 4 }+{ Cl }_{ 2 }\xrightarrow [ sunlight ]{ } { CH }_{ 3 }{ Cl }+{ HCl }$

So, in order to make free radicals, we need to provide some energy in the form of sunlight.

Therefore, the statement given in the question, methane does not react with chlorine in dark is True.

So, the correct option is Option (A) true.

We can now look at the mechanism of reaction of methane with chlorine in order to get a better idea about this. This reaction is a ‘free radical substitution’.

This reaction mechanism mainly contains three steps:

(i) Initiation

(ii)Propagation

(iii)Termination.

Step one is initiation, in which the ${ Cl }_{ 2 }$ gets converted into chlorine radicals.

Step two is propagation, in which one of the chlorine radical initiates the homolytic cleavage of methane and forms a methyl radical and HCl.

Step three is termination, in which the methyl radical and chlorine radical combines to form the methyl chloride.

This reaction is known as a ‘free radical substitution’. It is because of the fact that a free radical intermediate is formed during the reaction.

And for the most, we have used a term called ‘homolytic cleavage’ over here.

So, basically there are two types of cleavage: Homolytic and Heterolytic cleavage.

Homolytic cleavage means when the bond breaks equally between the two atoms.i.e, each atom gets one electron each. This happens when the electronegativity difference between both atoms are zero or minimal.

For example :${ O }_{ 2 }$ , ${ H }_{ 2 }$, ${ Cl }_{ 2 }$ etc..

Heterolytic cleavage occurs biased, when there is a considerable electronegativity difference between the atoms in that molecule. This cleavage leads to the formation of charged anions and cations.

For example :${ H }_{ 2 }{ O }$ , HF, HCl etc..

Note: The reactivity of different halogens with methane are different. Among halogens, fluorine reacts the fastest, followed by chlorine, bromine and iodine. Fluorination is fast and explosive whereas iodination is slow and reversible.This reversible nature of iodination can be prevented by removing one of the products (HI) by some oxidising agents like iodic acid, per-iodic acid etc.