Free Radical Chain Reactions of Alkanes

Reactions of Alkanes

All the carbon atoms in alkanes are sp³ hybridized. Therefore, this carbon only forms sigma bonds (single bonds) between carbon-carbon. There are only C-H and C-C bonds in an alkane. The polarity of the C-H bond and C-C bond is very low.

So, there is no significant positive or negative charge throughout the alkane molecule. Therefore, alkanes do not react with electrophiles (electron deficient) and nucleophiles (electron-rich) under normal conditions.

The reactivity of alkanes is very low. At high temperatures, alkanes react with oxygen and liberate carbon dioxide (CO₂) and water (combustion reaction of alkanes). These reactions are highly exothermic. So, it releases high energy. That is why alkanes are commonly used as fuels.

Except that alkanes react with chlorine (Cl₂). Bromine (Br₂) in the presence of radiation or catalysts. Alkanes are involved in this reaction by homolytic cleavage of bonds.

Chlorination of alkanes

In the presence of light, alkanes undergo a free radical chain reaction with Cl₂ or Br₂ and result in a mixture of alkyl halides. As an example, methane (CH₄) reacts with Cl₂ and results in a mixture of chloromethane as follows.

The reaction takes place through a free radical chain reaction. Free radicals are atoms or substances that have an unpaired electron. Free radicals are neutral but highly reactive.

Reaction mechanism of methane chlorination

The chlorination reaction of methane occurs in a free radical chain reaction. The reaction starts from a starting material and undergoes subsequent steps to give the final product. This reaction occurs in three steps.

1. Initiation
2. Chain Propagation
3. Termination

1. Initiation

In the initiation step, free radicals are generated. In the presence of light Cl₂ results *Cl free radicals from the homolytic cleavage of the Cl-Cl bond. These free radicals are highly reactive, so they will break a C-H bond symmetrically (homolytic cleavage) and form *CH₃ free radical.

2. Chain propagation

The generated CH₃* free radicals are also highly reactive. Therefore, it will break the Cl-Cl bond symmetrically and form the CH₃Cl and Cl* free radical. The generated Cl* free radicals will break another C-H bond. The reaction takes place until all the hydrogen is replaced with chlorine.

The reaction takes place until the reactants run out.

3. Termination reactions

The free radical chain reaction is totally a random process. At the propagation step, free radicals are generated. Sometimes, two free radicals react to each other and result in a particular product. Thus, the chain propagation ends. These reactions involve the termination step. A few reactions in the termination step are given below.