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Chlorination: Free Radical Mechanism, Chlorination of Alkanes, Alkenes, Benzene, Toluene, Benzoic Acid, HVZ Reaction, Practice Problems & FAQs

Chlorination: Free Radical Mechanism, Chlorination of Alkanes, Alkenes, Benzene, Toluene, Benzoic Acid, HVZ Reaction, Practice Problems & FAQs

All of you must have seen plastic water pipes in your households. PVC is short for Poly Vinyl Chloride. It is prepared by the polymerization of vinyl chloride, which is a chloro derivative of ethylene, i.e., it is prepared by the chlorination of ethylene (an unsaturated hydrocarbon). Also, you must have heard about the anaesthetic "Chloroform", which was used in surgeries in the early 20th century. Chloroform has a chemical formula CHCl3. It is a trichloro derivative of the simplest saturated hydrocarbon (methane or CH4). It is prepared by the process of halogenation of alkanes, which we are going to discuss in this concept page.


  • Halogenation
  • Chlorination of alkane
  • Free Radical Mechanism
  • Important Points about Chlorination
  • Rate of reactivity
  • Chlorination of Alkenes
  • Allylic Chlorination
  • Chlorination of Alkynes
  • Chlorination of Benzene
  • Chlorination of Toluene
  • Chlorination of Benzoic Acid
  • Hell Volhard Zelinsky(HVZ) Reaction
  • Practice Problems
  • Frequently asked questions(FAQs)


When one or more hydrogen atoms of an alkane or any hydrocarbon are replaced by halogen(s) (F, Cl, Br, I), such reactions are known as halogenation reactions.

Condition of Halogenation Reactions:

It takes place at

  1. At High Temperature(573-773K)
  2. In the presence of ultraviolet light(UV)

Chlorination of Alkanes

When one or more hydrogen atoms of an alkane are replaced by chlorine ( Cl ), such reactions

are known as chlorination reactions. It takes place in the presence of sunlight, heat, or a suitable catalyst to form alkyl halides. It is a fast reaction.


Reaction Mechanism of Chlorination of Alkanes

Chlorination of alkanes proceeds through a free radical chain mechanism involving three steps i.e., initiation, propagation, and termination.

Free Radical Mechanism

It is an uncharged species that contains an unpaired electron in its atomic orbitals

and can exist independently.

Chlorination proceeds through a free radical chain mechanism involving the following three steps:

  1. Initiation

In the presence of heat or light the homolytic cleavage of Cl2 molecules take place. In the

homolytic cleavages, the two electrons of a covalent bond, are divided equally between the products.

Since the Cl-Cl bond is weaker than C-C and C-H bonds, So only Clis formed in the initiation step. It is easier to break the Cl-Cl bonds to form free radicals.

  1. Propagation

Step 1: Cl attacks CH4 , and the reaction proceeds by breaking the C−H bond to generate CH3 radicals along with HCl.

Step 2:

CH3 radical attacks the second molecule of Cl2 to form CH3-Cl and releases another Cl.

The CH3 formed in step 1 and the Cl in step 2 are repeated, thereby setting up a chain of


However, other steps are also possible to generate more halogen-substituted products.

  1. Termination

The reaction stops after some time due to the following events:

  1. Consumption of reactant
  2. Side Reactions

The possible termination steps are as follows:

Here, the product is formed, but the free radicals are consumed. Hence, the chain is terminated.

Important Points about Chlorination

  • The reactivity order of alkanes for chlorination reaction is as follows:

Tertiary alkanes > Secondary alkanes > Primary alkanes

  • The displacement of H atoms from hydrocarbons is in the order:

Benzylic > Allylic > 3 > 2> 1

  • Chlorination of alkanes with more than four carbons gives a mixture of optically active d- and l- isomers.

  • Alkanes can also react with sulphuryl chloride only in the presence of light and traces of organic peroxide.

Rate of reactivity

At Room temperature, the relative rates of formation of the alkyl radical by chlorine radical are as follows

Both probability (the number of hydrogens that can be removed that will lead to the formation of the particular product) and reactivity (the relative rate at which particular hydrogen is removed) must be taken into account when determining the relative amounts of different products obtained from radical chlorination of an alkane.

Chlorination of Alkenes

Chlorination of alkenes takes place at an ordinary temperature in the presence of inert solvent(CCl4) to form vicinal dichlorides. It is an anti-addition reaction.


It is a two-step process.

Step 1: Formation of cyclic chloronium ion takes place

Reaction of alkene with chlorine is a slow step process in which formation of non-classical carbocation takes place. Since non-classical carbocation is formed in this step, hence no rearrangement takes place.

Step 2: Formation of vicinal dichloride

In the second step, nucleophile(Cl-) attacks more electrophilic sites to form the final product.

Allylic Chlorination

When alkenes are heated with Cl2 at a high temperature of about 500C , the allylic hydrogen is substituted with chlorine atom without breaking the double bond and forms allyl halide.

Allylic substitution is also carried out by heating the alkene with N-chlorosuccinimide(NCS) at 200C in the presence of light, heat or peroxides.

Chlorination of Alkynes

Chlorination of alkynes gives trans alkene and further chlorination of trans alkene yields tetrachloroethane.

Chlorination of Benzene

For the chlorination of benzene and its derivatives, it requires a lewis acid because it is aromatic in nature, hence more stable than alkenes but less reactive, hence requires a better electrophile. Ferric chloride is used as a Lewis acid.

Chlorination of Toluene

  1. In the presence of Lewis Acid, the chlorination of toluene occurs. The -CH3 group attached to the benzene ring is an electron donating group i.e., ortho-para directing group, So it directs the incoming electrophile (Cl+) to the ortho and para position.

  1. In the presence of Sunlight

Chlorination of Benzoic Acid

In the chlorination of Benzoic Acid, -the COOH group attached to the benzene ring is an electron-withdrawing group i.e., meta directing group, So it directs the incoming electrophile (Cl+) to the meta position.

Hell Volhard Zelinsky(HVZ) Reaction

This reaction is used for the halogenation of carboxylic acids at the alpha position.

Practice Problems

Q.1. Calculate the percentage yield of the major product.


Multiplying the number of hydrogen that can be removed by the relative rates for the removal of that hydrogen will give the relative amount of each product formed.

The Major Product formed will be 2-chloro-3-methyl butane.

The percent yield of the primary product is calculated by dividing the relative amount of the major product by the relative quantities of all the reaction products. The main product would be obtained with a yield of 35%.

6.0 + 5.0 + 7.6 + 3.0 = 21.6

7.621.6 = 35%

Q.2. Which of the following orders is correct for the reactivities of the indicated hydrogen atoms on monochlorination?

  1. H1 > H2 > H3
  2. H3> H2 > H1
  3. H3> H1> H2
  4. H2 > H1 > H3


The monochlorination of alkanes proceeds through a free radical mechanism. The radical formed

by the removal of the H1 atom is stabilised by resonance with the phenyl ring.

The radical formed by the removal of the H2 atom is stabilised by both hyperconjugation and


The radical formed by the removal of the H3 atom is stabilised by hyperconjugation only.

The rate of monochlorination is directly proportional to the stability of the free radicals.

Therefore, the correct order of reactivity of hydrogen atoms is given as follows:

H2 > H1 > H3

Hence, Option D is correct.

Q.3. How many steps are involved in the free radical of halogenation of the alkanes?

Solution: Total three steps are involved in the free radical halogenation of alkanes.

Chain initiation, Chain propagation and Chain termination.

Q.4. Chlorination of CS2 gives:

  1. Chloroform
  2. Carbon tetrachloride
  3. Both A and B
  4. None


Hence, option (B) is the correct answer.

Frequently Asked Questions - FAQ

Q1. What are the favourable conditions for chlorination of alkanes?
The presence of sunlight, High temperature, and suitable catalysts are required for the chlorination of alkanes.

Q2. Which intermediate is formed during the chlorination of alkanes?
During the chlorination of alkanes, a free radical intermediate is formed.

Q3. What is the order of the rate of halogenation of alkanes?
The reactivity order for the halogenation of alkanes with different halogen is

Fluorination > Chlorination > Bromination > Iodination.

Q4. Which halogenation process is more selective?
Bromination is the most selective among all halogenation.

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