The thermal broniinations of CF3H and CzFsH have been stiidied in the ranges 361-431" and 323-458°C respectively. The kinetics agree with the reactions Brz +2BrBr+ CF3H%HBr+ CF3(3, -3)} or (sr + C2FsH+HBr+ C2F5
Hexafluoroacetone has been photolyzed with HC1 and with mixtures of H a + C12, HCl+Br2, Br2+Clz, and ClZ+HBr. Absolute Arrhenius parameters have been obtained for the reaction, CF3 + HCl-tCF3H + C1(4) using the combination of CF3 radicals as a reference reaction. The values of A4 and E4 have been combined with the results obtained from competitive experiments on the above pairs of reactants and from previous work to give other Arrhenius parameters as follows :CF3+HBr +CF3H+Br CF3 + C l 2 +CF3Cl+ C1 CF3 + Br2 -tCFsBr + Br C F~+ I~ +CF~I+I k = 1 . 7~ 1011 exp (-5100/RT) IC = 6.0 x 1011 exp (-2900/RT) k = 7.7 x 1012 exp (-36OO/RT) k = 2.3 x 1012 exp (-700/RT) k = 2.6 x 1012 exp (O/RT) units ; 1x13 mole-1 sec-1 A study of the reactions Br + CF3H+HBr + CF3(1, -1) is important in the determination of the bond dissociation energy D(CF3-H).To obtain information on reaction (-I), Tucker and Whittle 1 studied the competitive reactions of CF3 radicals with HBr, I2 and Br2, CF3 + 12 -tCF31 +I CF3 + Br2 +CF3Br + Br.Reactions (-l), (2) and (3) were described as " fast " because each removes CF3 radicals from the system so rapidly that the stationary concentration of CF3 radicals is very low and consequently the amount of C2F6 formed is too small to measure. Thus, the combination of CF3 radicals cannot be used as a reference reaction. The above work yielded ratios of A factors and differences in activation energies but not absolute Arrhenius parameters. We have therefore studied the reactions of CF3 radicals with HC1 and with mixtures of HCl + Cla and HCl + Br2 to determine absolute Arrhenius parameters for one of the reactions which could then be used to obtain the Arrhenius parameters of the others. The source of CF3 radicals was hexafluoroacetone, HFA. EXPERIMENTAL HCl gas from a cylinder (I.C.I. Ltd.) was purified by several bulb-to-bulb distillations. Other materials were as described before.l.2 Details of apparatus and procedure have been given elsewhere.29 3 For the photolyses, a mercury lamp was used with either of the following filters : filter solution A (ref.(2)) or filter 33 which consisted of 40 ml of a solution of 1662
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
The equilibriumBr2 + CH4 + HBr + CH,Br(2) has been studied in the range 347-477"CY with equilibrium being approached from both sides. Products additional to those required by eqn (2) were formed but it is believed that reliable values of the equilibrium constant Kz have been measured. Third-law calculations lead to AH; = -26.4 k 0.7 kJ mol-' at 298 K from which AHfO(CH3Bry g) = -34.3 f 0.8 kJ mol-I.Studies of equilibria of the type X,+RH + HX+RX, where X = halogen, provide an important route to determination of APZ," which in turn can be used to calculate the difference in bond dissociation energies D(R-H) - D(R-X).The method has been extensively applied by Benson and co-workers to iodination reactions which have the advantage that the equilibrium generally lies far to the left hence secondary iodination of the RI formed by reaction (1) is unimportant.A few equilibria involving Br, + RH have been studied but here complications are much more likely since Kl is usually > 1 and the product RBr is usually brominated significantly faster than is RH. Hence further bromination of RBr can occur. This was avoided by Coomber and Whittle 2 p who studied equilibria where the group R was fully fluorinated so that further bromination of RBr was prevented.
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