Three mechanisms (A–C) have been proposed for the chlorination of methane at 400 K.
Mechanism A (radical chain):
Step 1 Cl₂ → 2 Cl·
Step 2 Cl· + CH₄ → HCl + CH₃·
Step 3 CH₃· + Cl₂ → CH₃Cl + Cl·
Mechanism B (polar):
Single step CH₄ + Cl₂ → CH₃⁺ + Cl⁻ + HCl, followed by CH₃⁺ + Cl⁻ → CH₃Cl
Mechanism C (concerted):
Single step CH₄ + Cl₂ → CH₃Cl + HCl with a cyclic transition state and no discrete intermediates.
Laser flash photolysis detects transient Cl· and CH₃· radicals during the reaction but finds no evidence for CH₃⁺ ions. Rank Mechanisms A, B, and C from most likely to least likely based on these observations.