Photolysis of mixtures of acetone and hexafluoroacetone. Combination reactions of methyl and trifluoromethyl radicals

Abstract

The photolysis of mixtures of acetone and hexafluoroacetone has been studied in the range 23-240°C. In addition to C2H6, C2F6 and CH3CF3, the products also contained CH2 = CF2.The following reactions are therefore proposed :Reaction (5) as not previously been observed. Defining $* by q5* = kla/k$k$, $* = (2.0f0.2) exp (170f80)/RT. These results indicate that Ela = E2 = E3 4.The product of reaction (la) is a "hot" molecule which can be stabilized to CH3CF3 by collisions. The ratio R C H~C F~/ R C H~= C F~ is dependent on both pressure and temperature. The relative efficiencies of deactivation of CH3CF; by acetone, hexafluoroacetone, nitrogen and perfluorocyclohexane were measured. The Arrhenius parameters have been measured of the various hydrogen abstraction reactions and also of the reaction, CH3 + CF3COCF3 -tCF&OCFs. The I CH3 importance of the reaction, CF3 + CH2=CF;! +CF3-(CH;!-CFz)-is discussed in an appendix. The reactionsCR3 + CF3+CH3CF3 C H~+ C H~+ C Z H~ CF3 + CF3-+CzF6 have been studied by Pritchard and Dacey,1 who photolyzed mixtures of acetone and hexafluoroacetone, HFA, to generate CH3 and CF3 radicals in the same system. They found the # factor given by to be temperature dependent, and suggested that reaction (3) had an activation energy of 2.14-&0.20 kcal mole-1, assuming that El = Ez = 0. This result was unexpected since for simple radical combinations # is usually independent of temperature 2 and approximately equal to 2 ; Pritchard and Dacey's values of 4 varied in the range 1.8-3.5.Alcock and Whittle 3 studied the reactionThis is so fast that a significant concentration of CH3 radicals was present, consequently the products contained CH3CF3 as well as C2F6 (but not C& because

Vibrational energy distribution in HF formed by elimination from activated CH3CF3 and CH2CF2

“…The combining radicals in this case were 2CH2F. Giles and Whittle (2) demonstrated that such a process also occurs in the combination of methyl and trifluoromethyl radicals, and showed that the combination occurs with zero activation energy to give an activated trifluoroethane molecule (CH,CF,?) which may decompose to give difluoroethylene and H F or may be stabilized by collision…”

Some Hydrogen Abstraction Reactions of Perfluoroethyl Radicals

“…The respective collision numbers at 400 ' I< for CF3, C2F5, and C3F7 are 7.5 X 1013, 1.1 X 1014, and 1.3 X 1014 rnole-l cc s-l (u,,, = 4.0 A, uc2,: = 5.5 A, ucari = 6.5 A). Very recent work (11) indicates that Ez = 0 for CF3 con~bination, so that we may equate El -$E2 with El as an absolute value. In the C3F7 + ethane system, allowance was made for this from a knowledge of the C4H10 (R + R R2) and the C3F7C2H5 formed (12), but the value of El was not affected whether the correction was made or not, although the reason for this is not apparent (13).…”

PHOTOLYSIS OF MIXES OF PERFLUOROACETONE AND PERFLUORODIETHYL KETONE REACTIONS OF CF3 AND C2F5 RADICALS

“…I n combination reactions of unlike radicals, ratio [I] would be temperature independent even if the same barrier existed for the three reactions. A small barrier for such reactions (not over 1.5 kcal/mole) has been suggested for such combinations (7,8) but very recent worl; on cross colnbination of CI-13 and CF3 (9,10) indicates no temperature dependence.…”