Alkanes, Free Radical Substitution Reactions of Alkanes and Mechanism of Halogenation of Alkanes

Free Radical Substitution Reactions of Alkanes / Mechanism of Halogenation of Alkanes

Contents

• Substitution reaction

• Free radicals

• Chain initiation

• Chain propagation

• Chain termination

• Uses of paraffins

Learning
Objectives

At the end of this lecture, student will be able to

• Explain the mechanism involved in free radical
substitution reactions of alkanes

Alkanes or Paraffins

Alkanes are
also known as saturated hydrocarbons and sometimes as paraffins

The name
“paraffin” is based on the Latin words parum + affinis =
“little affinity”.

The
“little affinity” behind their Latin name referring to their
relatively low reactivity.

They have
“little affinity” for other elements or compounds

Uses of alkanes

Methane.
The largest amount of methane produced is used for heating purposes

Ethane is
being used as heating fuel

Propane
is an important raw material in petrochemistry

Butane is
an important feedstock for the petrochemical industry

Properties of alkanes

• Saturated hydrocarbons

• Contains only carbon-carbon single bond

• C-H bond- non-polar covalent bond

• Alkanes- non-polar compounds

• Quite unreactive- contain only strong sigma bonds

• A compound containing a halogen atom covalently bonded to
an sp3 hybridized carbon atom. Given the symbol R-X

• Called as haloalkane or alkyl halide 

Preparation of haloalkanes

• Halogenation of alkanes

• Can also be prepared by addition of X2 to
alkenes

• Halogenation of alkanes is common with Br2 and
Cl2

• Mixture of methane and chlorine gas- in dark and at room
temperature- no detectable change

• On heating or exposed to light- reaction occurs

• If allowed to react with more amount of chlorine

• Regioselectivity

• Treating propane with bromine gives a mixture consisting
of approximately 92% of 2-bromopropane and 8% of 1-bromopropane

Problem-01

       Bromination

Free
Radical Substitution Reaction of Alkanes

• Chlorination of methane

Step 1

Mechanism of Halogenation of Alkanes

• Occurs as radical chain mechanism

• Involve three types of steps

1)      Chain
initiation

2)      Chain
propagation

3)      Chain
termination  

Mechanism of Free Radical Substitution Reaction

Step 1

   Chain Initiation step

Step 2

• Chain propagation
step

Step 3

• Chain propagation
step

Step 4

• Chain termination
step

– coupling of 2 free radicals

• Chain propagation
step

– Formation of CH2Cl2

Summary

• Alkanes are also known as saturated hydrocarbons and
sometimes as paraffins

• The name “paraffin” is based on the Latin words
parum + affinis = “little affinity”.

• The “little affinity” behind their Latin name referring to
their relatively low reactivity.

• They have “little affinity” for other elements
or compounds

• Methane. The largest amount of methane produced is used
for heating purposes

• Ethane is being used as heating fuel

• Propane is an important raw material in petrochemistry

• Butane is an important feedstock for the petrochemical
industry

• Free radicals – highly reactive reaction intermediates
with odd electron formed by homolysis of covalent bond

• Free radicals – generated by heat or irradiation of light
or initiators

• Halogenation of alkanes takes place by free radical chain
reactions

• Free radical mechanism involves 3 steps – chain
initiation, propagation and termination

• Free radicals – generated in the initiation step

• One radical generates another radical with the formation
of product in the propagation step

• Chain termination takes place by coupling of 2 free
radicals

• Butane is an important feedstock for the petrochemical
industry