Chlorination of Alkanes

Homolytic Fission: The reaction begins when UV light causes the covalent bond in a halogen molecule (e.g., $Cl-Cl$) to break. Each atom takes one of the shared electrons from the bond.

Radical Formation: This process results in the formation of two highly reactive chlorine radicals. The equation is represented as: $Cl_2 \xrightarrow{UV} 2Cl^{\bullet}$

Energy Source: Without UV light, the $Cl-Cl$ bond does not break at room temperature, which is why the reaction will not occur in the dark.

Chain Reaction: This stage involves two main steps that repeat in a cycle, regenerating the radical needed to keep the reaction going.

Step 1: Hydrogen Abstraction: A chlorine radical attacks an alkane molecule, removing a hydrogen atom to form hydrogen chloride and an alkyl radical: $CH_4 + Cl^{\bullet} \rightarrow \cdot CH_3 + HCl$

Step 2: Halogenation: The newly formed alkyl radical then attacks a chlorine molecule, forming the halogenoalkane and regenerating a chlorine radical: $\cdot CH_3 + Cl_2 \rightarrow CH_3Cl + Cl^{\bullet}$

Radical Collision: The reaction ends when two free radicals collide and react with each other to form a stable, non-radical molecule.

Product Variety: Because any two radicals in the mixture can collide, multiple termination products are possible. For example, two methyl radicals can combine to form ethane: $\cdot CH_3 + \cdot CH_3 \rightarrow C_2H_6$

Stopping the Chain: Once the radicals are consumed in these collisions, the chain reaction stops because no new radicals are generated to continue the propagation cycle.

Mixture of Products: A major disadvantage of this method is that it produces a mixture of substitution products. If excess chlorine is present, further propagation steps occur where $CH_3Cl$ is further chlorinated to $CH_2Cl_2$, $CHCl_3$, and eventually $CCl_4$.

Structural Isomers: For longer alkanes, the chlorine radical can attack different carbon atoms, leading to a mixture of structural isomers (e.g., 1-chloropropane and 2-chloropropane).

Radical Notation: Always ensure the radical dot is clearly visible. In equations, the dot should be placed on the atom that carries the unpaired electron (e.g., the Carbon in a methyl radical).

Identifying Steps: If an equation has a radical on the right but not the left, it is Initiation. If it has radicals on both sides, it is Propagation. If it has radicals on the left but not the right, it is Termination.

Termination Pitfall: When asked for a termination step, do not simply write the reverse of the initiation step ($2Cl^{\bullet} \rightarrow Cl_2$). While technically correct, examiners usually look for steps that form the organic product or side products like $C_2H_6$.