Problems and exercisesChlorination 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

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.

3. Exothermic but not spontaneous

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

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.

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?