Organophosphorus chemistry. Part XV. Kinetics and mechanism of the gas-phase thermal reaction of dimethylphosphine with tetrafluoroethylene

Abstract

Reaction of tetrafluoroethylene with an excess of dimethylphosphine (mole ratio 1 : 2 to 1 : 9) in the gas phase at 212–288 °C and total pressures of 67–337 Torr leads to near-quantitative formation (i) of the 1 : 1 adduct, dimethyl-1,1,2,2-tetrafluoroethylphosphine, with a rate expression (ii) where log₁₀[K/(dm³ Mol^–1)^1·1 s^–1]=(8·63 ± 0·06)–(88·4 ± 0·6) kJ mol^–1/2·303RT. These and other results are consistent with a homogeneous Me₂PH + C₂F₄→ Me₂P·CF₂·CHF₂(i), d[Me₂P·CF₂·CHF₂]/dt=k[Me₂PH]^0·5[C₂F₄]^1·6(ii), Me₂PH + C₂F₄→ Me₂P˙+˙CF₂·CHF₂(iii) free-radical chain mechanism initiated by an unusual type of reaction, the bimolecular abstraction (iii) of H from dimethylphosphine by tetrafluoroethylene for which the approximate rate equation log₁₀k/dm³ mol^–1 s^–1= 11·0 – 106 kJ mol^–1/2·303RT is derived. The chain is propagated by dimethylphosphino and dimethylphosphinotetrafluoroethyl radicals, and terminated by combination of dimethylphosphino radicals. Extensions of this mechanism account satisfactorily for the formation of the other products (predominantly at low phosphine:olefin ratios).