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Preparation of Haloalkanes
from Hydrocarbons

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Preparation of Haloalkanes by Free Radical Halogenation of Alkanes

When the mixture of hydrocarbon and halogen is heated at 520-670 K in dark or is subjected to ultraviolet light at room temperature, the free radical substitution reaction takes place.

Preparation of haloalkanes from hydrocarbons by free radical halogenation

The reactivity of hydrogen towards free radical substitution is 3° > 2° > 1°. For example, when butane is used in free radical substitution reaction, the products we get are 1-Chlorobutane and 2-Chlorobutane. 2-Chlorobutane being 2° is the major product.

Preparation of 2-chlorobutane from hydrocarbons by free radical halogenation

Mechanism of free radical substitution reaction

Free radical substitution reaction is a chain reaction. Let's take the example of CH4 and Cl2 to understand the mechanism. The free radical substitution reaction consists of the following steps :

Initiation step

When a mixture of CH4 and Cl2 is heated at 520-670 K or is subjected to ultraviolet light at room temperature, Cl2 absorbs energy and undergoes homolytic fission.

Mechanism of free radical substitution : initiation step

Propagation step

Propagation step consists of two sub steps

  1. The free radical of chlorine (formed in initiation step) attacks the CH4 molecule and removes a hydrogen atom from CH4 forming CH3 and HCl.
    Mechanism of free radical substitution : Propagation step, substep 1
  2. CH3 thus produced, reacts with a molecule of Cl2 which results in the formation of methyl chloride.
    Mechanism of free radical substitution : Propagation step, substep 2
  3. This reaction continues until all the hydrogen atoms of methane are replaced by halogen atoms.
    Formation of Chain reaction in free radical substitution

Termination step

The chain reaction may terminate if two of the same or different free radicals combine among themselves without producing new free radicals.

Free radical substitution termination step : Cl radicals form Cl2
Free radical substitution termination step : CH3 radicals form CH3-CH3
Free radical substitution termination step : Cl and CH3 radicals form CH3Cl

Preparation of Haloalkanes from Alkenes

Alkenes react with halogen acids to form haloalkanes. Markovnikov's rule is applied in case of unsymmetrical alkenes.

Preparation of chloroethane from ethanol

Markovnikov's rule

Markovnikov's rule states that in the addition reactions of unsymmetrical alkenes, the negative part (e.g. halogen) of reagent attaches to the carbon having less number of hydrogen atoms. For example, prop-1-ene reacts with HBr to give 2-bromopropane as a major product.

Markovnikov's rule

Peroxide effect (Kharash effect)

If HBr is added to alkene in the presence of peroxide, the negative part (i.e. Br-) attaches to the carbon having more number of hydrogen atoms. Since the peroxide effect is contrary to Markovnikov's rule, it is also known as antimarkovnikov's rule. For example,
Prop-1-ene reacts with HBr in the presence of peroxide to give 1-bromopropane as a major product.

Peroxide effect

A hydrocarbon C5H12 gives only one monochlorination product. Identify the hydrocarbon.

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