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 Incoming links: Ethane

Methane chlorination chain Created 2026-10-13 Updated 2026-10-20

Light initiates methane chlorination by photodissociation of

C l_{2}

through homolysis. The elementary bookkeeping is

initiation: propagation: C l_{2} h ν 2 Cl^{∙}, Cl^{∙} + C H_{4} ⟶ HCl + C H_{3}^{∙}, C H_{3}^{∙} + C l_{2} ⟶ C H_{3} Cl + Cl^{∙} .

(1)

Here

h ν

denotes the photon energy. The first propagation step replaces a chlorine radical with a methyl radical; the second regenerates a chlorine radical. Adding the two propagation equations cancels both free radicals and gives

C H_{4} + C l_{2} ⟶ C H_{3} Cl + HCl

: chloromethane and hydrogen chloride are made while the chain carrier is regenerated.^[ref]

Figure 1.
Methane chlorination: initiation supplies carriers, propagation replaces them, termination removes them
. 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. **Details**.

For example, recombination

2 C H_{3}^{∙} ⟶ C_{2} H_{6}

of methyl radicals makes ethane and a covalent bond from two unpaired electrons, removing two carriers. Recombination of two

Cl^{∙}

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.^[ref]

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Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 1 / Solution 2026-10-20

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The isolobal analogy compares fragments through their frontier molecular orbitals: their number, approximate energies, shapes, symmetries and electron occupancies should correspond. It does not require the fragments to have the same total number of electrons or the same constituent elements. If the orbitals available for forming new bonds correspond, analogous ways of joining the fragments become plausible.

For example, $C H_{3}^{∙}$ and $Mn (CO)_{5}^{∙}$ are isolobal. The methyl radical has one singly occupied approximately outward-pointing orbital; removing one carbonyl ligand from an octahedral molecular geometry leaves an analogous frontier orbital on the 17-electron

Mn (CO)_{5}

fragment. Joining two such fragments pairs the two electrons: the organic example is the C–C bond of ethane, and the organometallic example is the Mn–Mn bond of

M n_{2} (CO)_{10}

. The analogy predicts a bonding pattern, rather than equality of bond energies or reaction rates.^[ref]

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