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

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