The chemical reactions in which a hydrogen atom of an alkane is replaced by a halogen atom are known as halogenation. 

RH + X2 → RX + HX

The reactivity of halogens is in the order of

F2 > Cl2 > Br2 > I2

Fluorine is the most reactive among halogens in halogenation reactions, and iodine is the least reactive.

Free Radical Substitution

Chlorination of methane takes place via the free radical mechanism.  There are three main steps in the free radical mechanism:

  1. Initiation
  2. Propagation
  3. Termination

When the reaction mixture is exposed to sunlight, chlorine molecules absorb energy from sunlight and get converted to free radicals - chlorine atoms with an unpaired electron. The chlorine radicals then combine with methane and form methyl radical. The methyl radical. further  reacts  with  chlorine  molecule  and produces chloromethane. 

This reaction continuously takes place till it is stopped or the reactants completely react to form the products. The free radical mechanism involves the following three steps:

1. Chain Initiation Step:

It involves the formation of free radicals. This step involves the dissociation of the halogen molecule (chlorine molecule) into two chlorine atoms. Even though the total reaction is exothermic, initially energy should be supplied for the reaction to proceed.

Cl2 + hν → 2 Cl•

2. Chain Propagation Step:

The free radicals give rise to the formation of more free radicals. During the propagation step, the hydrogen atom is abstracted from methane by a chlorine atom. This is followed by the reaction between the methyl radical and the chlorine molecule.

CH4 + Cl• → •CH3 + HCl

•CH3 + Cl2 → CH3Cl + Cl•

3. Chain Termination Step:

In this step, free radicals combine with one another and the further reaction stops.

•CH3 + •Cl → CH3Cl

•Cl + •Cl → Cl2

•CH3 + •CH3 → CH3–CH3

Bromination

Bromo derivatives of alkanes are prepared by direct bromination.

The reactivity order of abstraction of H atoms towards bromination of alkane is 3°H (tertiary) > 2°H (secondary) > 1°H (primary).

Iodination and Fluorination

Direct iodination is not possible with iodine as the reaction is reversible. Direct fluorination is also not possible because due to the high reactivity of the fluorine, the reaction cannot be controlled.