Free-radical addition to olefins. Part 26. - Kinetics of the addition of trifluoromethyl radicals to acetylene and substituted acetylenes

Absolute Arrhenius parameters have been determined for the addition of trifluoromethyl radicals to acetylene, propyne, but-2-yne, 1,1,1-trifluoropropyne and hexafluorobut-2-yne by a competitive method using the previously determined Arrhenius parameters for the addition of trifluoromethyl radicals to ethylene as standard. X^α k_X/k_C2H2 (164 °C) E_act/kcal mol^-1 logA/dm³ mol^-1 s^-1 ↓HC≡CH 1 5.00±0.30 8.70 ±0.25 ↓HC≡CH₃ 5.2^b 3.45±0.28^b 8.63±0.08^b ↓HC≡CCF³ 0.49 5.10±0.26 8.44±0.13 ↓CH₃C≡CCH₃ 2.0 4.36±0.30 8.68±0.16 ↓CF₃C≡CH 0.043 6.61±0.33 8.13±0.18 ↓CF₃C≡CCF₃ 0.068 5.86±0.38 7.95±0.06 1 cal ≡ 4.1845 J. a Arrow shows site of attack; b sum of attack at both sites in propyne. Both the activation energies and the A-factors are larger than those for the addition of trifluoromethyl radicals to similarly substituted olefins. As a result of these opposing effects the rates of addition are similar.