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SolutionsChlorination of methane

1. Steps of the mechanism

Classify each step as initiation, propagation or termination:

  1. Cl2 → 2 Cl·
  2. Cl· + CH4 → CH3· + HCl
  3. CH3· + Cl2 → CH3Cl + Cl·
  4. CH3· + Cl· → CH3Cl
  5. 2 CH3· → CH3CH3
  1. Initiation.
  2. Propagation.
  3. Propagation.
  4. Termination.
  5. Termination.

In propagation a radical is consumed and another is formed, so the chain continues; in termination two radicals combine and the chain stops.

2. Calculate ΔH°

Using the bond energies C–H (105), Cl–Cl (58), C–Cl (85) and H–Cl (103 kcal/mol), calculate ΔH° for each propagation step and for the overall reaction.

Cl· + CH4 → CH3· + HCl: ΔH° = 105 − 103 = +2 kcal/mol.

CH3· + Cl2 → CH3Cl + Cl·: ΔH° = 58 − 85 = −27 kcal/mol.

Overall: ΔH° = +2 − 27 = −25 kcal/mol.

3. Exothermic but not spontaneous

If the reaction is exothermic, why can a mixture of methane and chlorine be kept for days without reacting?

Because the reaction must start by breaking the Cl–Cl bond (58 kcal/mol) to form the first radicals, and that only happens with light or heat. Without this initiation there are no radicals and the reaction does not start: the problem is kinetic, not thermodynamic.

4. Transition state and intermediate

In the energy diagram of the chlorination of methane, what is the difference between a transition state and a reactive intermediate? Give an example of each.

A transition state is an energy maximum in which bonds are breaking and forming; it cannot be isolated (for example, [H3C···H···Cl]‡). A reactive intermediate is a local minimum, a high-energy species with an existence of its own, such as the methyl radical, CH3·.

5. And with bromine?

The Br–Br, C–Br and H–Br bond energies are 46, 70 and 88 kcal/mol. Calculate ΔH° for the bromination of methane and for its first propagation step. What do you expect about its rate?

Overall: (105 + 46) − (70 + 88) = −7 kcal/mol, much less exothermic. First propagation: Br· + CH4 → CH3· + HBr, ΔH° = 105 − 88 = +17 kcal/mol.

This very endothermic step makes bromination much slower and more selective than chlorination.

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