Reactions of trifluoromethyl radicals with organic halides. Part 5.—Fluoromethanes and fluoroethanes

Abstract

Hexafluoroacetone has been photolyzed in the presence of CH4, CHsF, CH2F2, CF3CH3, CF2HCF2H and CF3CF2H. Arrhenius parameters are obtained for the abstraction of H by CF3 radicals using the combination of CF3 radicals as a reference reaction. Results were checked by photolysis of hexafhoroacetone in the presence of CD4 plus each of the fluoroethanes and CH4.in turn and using the reaction, CF3+CD4 -+CF3D+CD3, as a reference. The activation energies obtained are compared with those for related series involving abstraction of H by CF3 and CH3 radicals and by bromine atoms.The secondary products of the system CF3+CH2F2 have been measured. For the combination and cross-combination reactions of CF3 and CHF2 radicals, we obtain 4 = 2-4 independent of temperature in the range 291-363°C. It is concluded that the second of the reactionsoccurs to a negligible extent.Previous papers have dealt with the reactions of CF3 radicals with halogenated methanes 1-3 and aromatic halides.4 None of these compounds contained fluorine so we have now studied the reactions of CF3 radicals with fluoromethanes and fluoroethanes ; the results are given in this paper. EXPERIMENTAL MATERIALSMost of these were prepared and purified as described e1sewhere.s-7 A sample of CF2= CHF was kindly donated by Prof. Haszeldine of the University of Manchester. C3F8 and n-C4Flo were from Peninsular ChemResearch, Florida, U.S.A. CD4 was made from D20+CCl4+Zn dust by the method of Leblanc et aZ. 8 It was passed through a trap at -80°C into a trap cooled in solid nitrogen. The CD4 was purified by several distillations from -183 to -210°C. Two samples were made which were analyzed by mass spectrometer to contain 96 % CD4, 4 % CD3H and 95 % CD4, 5 % CD3H respectively. Each reactant (except hexafluoroacetone) was freed of possible traces of acids by passing the gas through Carbosorb. All were better than 99 % pure as indicated by i.-r. and g.s.c.

Reactivities in Free-radical Polymerization

“…Hydrogen abstraction by # CF 3 from the compounds CH 4 , CH 3 C1, CH 2 C1 2 and CHC1 3 shows the expected decrease in activation energy, the values being 50 47.0, 44.5, 31.9 and 27.7 kJmol -1 , respectively. This may arise as a result of delocalization changes from one reaction to another, or of steric or polarity changes, i.e.…”

Some hydrogen atom abstraction reactions of CF2H and CFH2 radicals, and the CH bond dissociation energy in CF2H2

“…The comprehensive bromination work of Whittle [23] has established D(CF,--H) = 106.4 f 0.5 kcalmole-' at 298°K. Based on these bonddissociation energies we may calculate an activation energy for the process (4b) in excess of 20 kcal mole-l, using a value of E-4b = 11.2 kcal mole-' [24]. We had hoped to determine E,,, experimentally, as a further check on D (CF,H-H), but the high activation energy for the reaction prohibits the process in our accessible temperature range.…”

“…Also E-4a. -E-,& is only 1.2 kcal [21], and the reactivities of both CF, radicals [24] and Br atoms [25] …”

“…With CH, as the substrate, reaction (4) is ( 4 4 Reaction (-4a) has been investigated ( -4 4 and a n activation energy of 10.2 f 0.2 kcal mole-' was obtained [21]. in excess of 20 kcal mole-l, using a value of E-4b = 11.2 kcal mole-' [24]. We had hoped to determine E,,, experimentally, as a further check on D (CF,H-H), but the high activation energy for the reaction prohibits the process in our accessible temperature range.…”

Methyl and trifluoromethyl radical attack on ethers

“…One ethane determinatlo:~ (16) may be unreliable (19), tvhlle the use of hexafluo~oazo-n~ethaiie as a rad~cal source wltii n-butane (17, work, the data indicate that n-butane is more reactive than ethane to CF, attack by about a factor 6 at 182 "C. Also dimethylether is more reactive than ethane (14) (by a factor of about 3 at 182 "C) while diethylether is more reactive than n-butane (6) (by a factor of 2.6 at 182 "C).…”