[RedsAreRaw]

Came across this question: When a small amount (0.02%) of tetraethyllead, (C₂H₅)₄Pb, is added to a mixture of methane and chlorine, chlorination takes place at only 140° instead of the usual minimum of 250°. How does this fact strengthen the mechanism of just methane and chlorine?

Is it because the E_act is lower for CH₃CH₂· + Cl-Cl  CH₃CH₂Cl + Cl·
So this in turn increases the concentration of Cl· at a lower energy?

[Hunter2]

I think that is the decomposition of the lead compound to Ethyl-Radicals and lead metal.

[pgk]

Both of you are right.
Radical reactions start by initiation step, go on by propagation step and finally finish by termination step.
Initiation step with chlorine alone, starts by thermal degradation of the chlorine atom to chlorine radicals.
Cl-Cl  → 2 Cl·
In contrast to the thermal degradation of Et4Pb to ethyl radicals that is the initiation step in that case and which has a lower activation energy and therefore, it occurs at lower temperature.
CH3CH2Pb(CH3CH2)3  → CH3CH2· + ·Pb(CH3CH2)3 →→ 4 CH3CH2· + Pb4.
Pb4. + 2 Cl-Cl  → PbCl4
CH3CH2· + Cl-Cl  →  CH3CH2Cl + Cl·
PS: So this in turn increases the concentration of Cl· at a lower temperature? (it’s more descriptive.)