Light initiates <methane> chlorination by <photodissociation> of $\mathrm{Cl_2}$ through <homolysis>. The elementary bookkeeping is
$$
\begin{aligned}
\text{initiation:}\quad &\mathrm{Cl_2}\xrightarrow{h\nu}2\mathrm{Cl}^{\bullet},\\
\text{propagation:}\quad &\mathrm{Cl}^{\bullet}+\mathrm{CH_4}\longrightarrow\mathrm{HCl}+\mathrm{CH_3}^{\bullet},\\
&\mathrm{CH_3}^{\bullet}+\mathrm{Cl_2}\longrightarrow\mathrm{CH_3Cl}+\mathrm{Cl}^{\bullet}.
\end{aligned}
$$
Here $h\nu$ denotes the <photon> <energy>. The first propagation step replaces a <chlorine> <radical (chemistry)> with a <methyl radical>; the second regenerates a <chlorine> <radical (chemistry)>. Adding the two propagation equations cancels both <free radicals> and gives $\mathrm{CH_4+Cl_2\longrightarrow CH_3Cl+HCl}$: <chloromethane> and <hydrogen chloride> are made while the chain carrier is regenerated.<Organic Chemistry by John McMurry/6.6 Radical Reactions>[Organic Chemistry by John McMurry, §6.6: chlorine photolysis and the two-step methane chlorination cycle]{ref}

\Image[/methane-chlorination-radical-bookkeeping.png]
{title=<Methane> chlorination: initiation supplies carriers, propagation replaces them, termination removes them}
{id=image-methane-chlorination-radical-bookkeeping}
{description=The left panel follows the reacting <electron pair> in <homolysis>, then counts chain carriers through two propagation steps. The right panel shows recombination termination. Every <chemical species> in this example is neutral. Electron marks are bookkeeping, not a measured electronic <wavefunction> or a reaction-rate plot.}

For example, recombination $2\mathrm{CH_3}^{\bullet}\longrightarrow\mathrm{C_2H_6}$ of <methyl radicals> makes <ethane> and a <covalent bond> from two unpaired <Electrons>, removing two carriers. Recombination of two $\mathrm{Cl}^{\bullet}$ also terminates carriers; in the gas phase, stabilization of the product requires disposal of released <energy>, for example to a third collision partner. The figure omits that partner. Termination competes with propagation, and further chlorination can consume <chloromethane>, so this cycle alone does not predict a pure product or a measured <reaction rate>.<Organic Chemistry by John McMurry/10.2 Preparing Alkyl Halides from Alkanes: Radical Halogenation>[Organic Chemistry by John McMurry, §10.2, Figure 10.2 and following paragraphs: radical combination terminates the chain; further substitution gives product mixtures]{ref}


Methane chlorination chain

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