3.2.4 Chlorination of Alkanes

The reaction of alkanes with chlorine is an example of a free radical substitution reaction, which occurs in the presence of UV light. This process, often referred to as photochemical chlorination, leads to the formation of halogenoalkanes, where hydrogen atoms in the alkane are substituted by chlorine atoms.

A well-known example is the chlorination of methane to form chloromethane. This reaction proceeds through a mechanism involving initiation, propagation, and termination steps.

Free Radical Substitution Mechanism

The chlorination of alkanes takes place in three stages:

1. Initiation

The initiation step requires UV light to provide enough energy to break the bond in a chlorine molecule ($Cl_2$) through photodissociation.

This is an example of homolytic fission, where the $Cl-Cl$ bond splits evenly, producing two highly reactive chlorine free radicals ($Cl•$), each with one unpaired electron.

Reaction:

$$Cl_2 \xrightarrow{UV} 2Cl•$$

2. Propagation

In the propagation stage, the reaction becomes a chain reaction as free radicals are both consumed and regenerated.

1. A chlorine free radical ($Cl•$) reacts with a methane molecule ($CH_4$) by abstracting a hydrogen atom, forming hydrogen chloride ($HCl$) and a methyl free radical ($CH_3•$). Reaction:

$$Cl• + CH_4 \rightarrow HCl + CH_3•$$

2. The newly formed methyl free radical ($CH_3•$) then reacts with another chlorine molecule ($Cl_2$), producing chloromethane ($CH_3Cl$) and regenerating the chlorine free radical ($Cl•$). Reaction:

$$CH_3• + Cl_2 \rightarrow CH_3Cl + Cl•$$

This regenerated $Cl•$ can go on to react with more methane molecules, perpetuating the chain reaction.

3. Termination

In the termination step, the reaction comes to an end when two free radicals combine to form a stable molecule. There are several ways this can happen:

3. Two chlorine radicals combine to form a chlorine molecule:

$$Cl• + Cl• \rightarrow Cl_2$$

4. A chlorine radical combines with a methyl radical to form dichloromethane:

$$Cl• + CH_3• \rightarrow CH_3Cl$$

5. Two methyl radicals combine to form ethane ($C_2H_6$):

$$CH_3• + CH_3• \rightarrow C_2H_6$$