ABSTRACTION OF HYDROGEN ATOMS FROM HALOMETHANES BY RECOIL TRITIUM ATOMS

Yields of the abstraction reaction by recoil tritium, T + RH → HT + R, have been measured for 14 halomethanes in excess perfluorocyclobutene. The yields per C-H bond, (HT)F, expressed relative to methane as 1.0, range from 1.34 (CHF3) to 14.6 (CHCl3). Progressive substitution of Cl or Br for H gives steadily higher values, while the fluoromethanes exhibit a maximum: CH3F, 1.70; CH2F2, 2.5; CHF3, 1.34. These trends are explained by the progressive weakening of C-H bonds with additional halogen substituents, coupled with a strengthening of the C-H bond by polar effects with multiple fluorine atom substituents. Comparison with hydrocarbon values leads to the postulate that residual radicals are more excited for RH = hydrocarbon than for RH = fluoromethane. The values of (HT)F are postulated to correlate with a combination of the bond dissociation energy of R-H and the excess internal excitation energy of the residual radical R formed by the abstraction reaction.