Question

explain substitution reaction of alkanes​

Answer 1

Answer:

Alkanes undergo a substitution reaction with halogens in the presence of light. For instance, in ultraviolet light , methane reacts with halogen molecules such as chlorine and bromine. This reaction is a substitution reaction because one of the hydrogen atoms from the methane is replaced by a bromine atom.

Answer 2

Answer:

Those reactions in which hydrogen atoms of alkanes are substituted are known as substitution reaction.

In alkanes, Substitution reaction involves Halogenation.

Halogenation takes place in the presence of ultraviolet light or sunlight or heat.

For example :

${\sf{CH_4 + Cl_2 \ \underrightarrow{ \ \ h \nu \ \ } \ CH_3Cl + HCl}}$

Mechanism :

(I) Initiation : This step involves the homolysis (a type of cleavage in which shared electrons goes with each of the bonded atoms) of chlorine molecule in the presence of light or heat.

${\sf{Cl-Cl \ \underrightarrow{ \ \ h \nu \ \ } \ \bullet Cl + Cl \bullet}}$

Why only Cl-Cl bond is cleaved ?

The Cl-Cl bond is weaker than the C-C and C-H bond and therefore, is easiest to break.

(II) Propagation : This step involves the attack of chlorine free radical on the methane molecule and generating the methyl free radical.

${\sf{CH_4 + Cl \bullet \ \underrightarrow{ \ \ h \nu \ \ } \bullet CH_3 + HCl}}$

This methyl radical attacks the second molecule of chlorine.

${\sf{ \bullet CH_3 + Cl-Cl \underrightarrow{ \ \ h \nu \ \ } CH_3-Cl + Cl \bullet }}$

Many propogations steps are possible -

${\sf{CH_3Cl + Cl \bullet \longrightarrow \bullet CH_2Cl + HCl}}$

${\sf{ \bullet CH_2Cl + Cl-Cl \longrightarrow CH_2Cl_2 + \bullet Cl}}$

(III) Termination : This step involves the combination of all the radicals and stop the reaction.

Possible reactions are :

${\sf{ \bullet Cl + \bullet Cl \longrightarrow Cl-Cl}}$

${\sf{CH_3 \bullet + \bullet CH_3 \longrightarrow CH_3 - CH_3}}$

${\sf{ \bullet CH_3 + \bullet Cl \longrightarrow CH_3Cl}}$

• The above mechanism helps us to understand that why ethane formed as a byproduct during chlorination of methane.