Here's why:

* Free Radical Reactions: The reaction between methane and chlorine is a free radical chain reaction. It involves a series of steps: initiation, propagation, and termination.

* Initiation: This step involves the formation of chlorine radicals (Cl•) by homolytic cleavage of the Cl-Cl bond.

* Propagation: Chlorine radicals react with methane to form methyl radicals (CH₃•) and HCl. These methyl radicals then react with chlorine molecules to form chloromethane and another chlorine radical. This cycle continues, leading to a chain reaction.

* Termination: The chain reaction can be terminated when two radicals combine to form a stable molecule.

Lead compounds like tetramethyllead are known as "scavengers" of free radicals. This means they react with free radicals, removing them from the reaction mixture and preventing them from continuing the chain reaction.

How does tetramethyllead act as a scavenger?

* Lead-carbon bonds are relatively weak.

* When a chlorine radical encounters a tetramethyllead molecule, it can easily abstract a methyl radical, forming methyl chloride and a trimethyllead radical.

* The trimethyllead radical is less reactive and terminates the chain reaction.

Therefore, the presence of tetramethyllead would actually *decrease* the rate of the free radical substitution reaction between methane and chlorine.

Important Note: Tetramethyllead was previously used as an anti-knock agent in gasoline. However, it is now banned in most countries due to its toxicity and environmental concerns.