**Homolysis: The Initiation Step of Photochemical Chlorination**

The photochemical chlorination of alkane is initiated by a process called homolysis. Homolysis refers to the breaking of a covalent bond, resulting in the formation of two radicals, each carrying one electron. In the case of photochemical chlorination, the bond that undergoes homolysis is the carbon-hydrogen (C-H) bond in the alkane.

**The Process of Photochemical Chlorination**

Photochemical chlorination is a reaction that involves the substitution of a hydrogen atom in an alkane with a chlorine atom. It occurs under the influence of sunlight or ultraviolet (UV) radiation. The reaction proceeds through a series of steps, including initiation, propagation, and termination.

**Initiation: Homolysis of Chlorine**

The initiation step involves the homolysis of a chlorine molecule (Cl2) to generate chlorine radicals (Cl•). The homolytic cleavage of the Cl-Cl bond occurs when the molecule absorbs energy from sunlight or UV radiation. The absorbed energy promotes one of the chlorine atoms to a higher energy level, resulting in the formation of Cl• radicals.

Cl2 → 2 Cl•

**Propagation: Radical Chain Reaction**

Once the chlorine radicals are formed, they react with the alkane molecule to initiate a radical chain reaction. The reaction proceeds through the following steps:

1. Chlorine radical abstraction: A chlorine radical reacts with the alkane, abstracting a hydrogen atom from one of the carbon atoms. This process forms a hydrogen chloride (HCl) molecule and an alkyl radical.

Cl• + RH → HCl + R•

2. Chlorine radical regeneration: The alkyl radical formed in the previous step then reacts with another chlorine molecule, regenerating the chlorine radical and forming a chlorinated alkane.

R• + Cl2 → RCl + Cl•

**Termination: Radical Combination or Disproportionation**

The radical chain reaction can continue until all the hydrogen atoms in the alkane are replaced by chlorine atoms. However, it can also be terminated through two processes:

1. Radical combination: Two alkyl radicals can combine to form a covalent bond, eliminating the unpaired electrons and producing a stable molecule.

R• + R• → R-R

2. Radical disproportionation: A radical can react with a molecule of the same kind, resulting in the transfer of a hydrogen atom and electron to form a stable molecule and a new radical species.

R• + R' • → R-R' + H•

**In conclusion**, the photochemical chlorination of alkane is initiated by the process of homolysis. This process involves the homolytic cleavage of a chlorine molecule to generate chlorine radicals, which then react with the alkane to initiate a radical chain reaction. The reaction proceeds through initiation, propagation, and termination steps, ultimately resulting in the substitution of hydrogen atoms with chlorine atoms in the alkane.