Q.Explain the free-radical substitution mechanism of the following reaction: CH4 + Cl2 -> CH3Cl + HCl

Free Radical Halogenation

Imagine you have a row of identical, perfectly content people sitting in chairs — that's an alkane. Every carbon is holding onto its hydrogens tightly, and nothing much happens. Now, you bring in a highly reactive, aggressive molecule like chlorine or bromine. But here's the catch: these molecules won't attack on their own. They need a trigger — a flash of UV light or heat — to break them into something far more dangerous.

That trigger is the entire point.

The Intuition: Why Light?

A chlorine molecule ($Cl_2$) is perfectly happy as a pair. The bond between the two chlorine atoms is strong, but not unbreakable. UV light carries enough energy to snap that bond cleanly, producing two free radicals — atoms with an unpaired electron. That unpaired electron is desperate to find a partner, making the radical violently reactive. It will rip a hydrogen off the nearest alkane to pair up, leaving behind a carbon radical. That carbon radical then grabs a chlorine from another $Cl_2$ molecule, producing the final product and regenerating a chlorine radical to start the cycle again.

This is a chain reaction — one photon of light can lead to thousands of substitution events.

The Precise Mechanism: Three Phases

Free radical halogenation proceeds through three distinct stages:

1. Initiation — The trigger. UV light (or heat, around 250-400°C) homolytically cleaves the halogen-halogen bond:

Each dot represents the unpaired electron. This is the only step that requires external energy.

2. Propagation — The chain. Two steps repeat in a loop:

Step 1: A chlorine radical abstracts a hydrogen from the alkane:

$Cl^\bullet + CH_4 \longrightarrow HCl + CH_3^\bullet$

Step 2: The methyl radical reacts with a chlorine molecule:

$CH_3^\bullet + Cl_2 \longrightarrow CH_3Cl + Cl^\bullet$

The chlorine radical is regenerated in Step 2, so it can attack another methane molecule. This cycle can repeat tens of thousands of times before termination.

3. Termination — The chain ends when two radicals meet and combine:

The last reaction is important — it explains why you sometimes get trace amounts of ethane when chlorinating methane.

Selectivity: Which Hydrogen Gets Replaced?

If the alkane has different types of hydrogens (primary, secondary, tertiary), the reaction is not random. The radical intermediate that forms is more stable when the unpaired electron is on a more substituted carbon.