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.

The kinetics of the thermal bromination reaction Brz + CFaI --+ IBr + CF& For the step have been studied in the range of 173-321°C.( 1 1) we obtainwhere 0 = 2.303RT cal/mole. that From the activation energy for reaction (1 11, we calculate D(CF3-I) = 52.6 + 1.1 kcal/mole at 25°C This is compared with previously published values of D(CF3-I). result to published work on k, for a combination of CF3 radicals is discussed.The relevance of the

The overall photobromination reactions Brz + RFI -+ IBr + RFBr have been studied using a competitive technique. obtained for the rate-determining step The D ( R r 1 ) are compared with related D(R-I) and it is concluded that for a given alkyl group R H and the corresponding perfluoroalkyl group RF, D ( R r 1 ) > D ( R r 1 ) whereas it has previously been found that D ( R g X ) < D ( R r X ) where X is not iodine.

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The overall reaction(1) occurs readily in the gas phase, even at room temperature in the dark. much faster than the corresponding processThe reaction is Brp + CFBI -+ IBr + CF3Br and does not involve the normal bromination mechanism for gas phase reactions. Reaction (1) is probably heterogeneous although other mechanisms cannot be excluded.The overall reactions proceed, for all practical purposes, completely to the right-hand side in the vapor phase. The expected mechanism iswhere reaction (3) is initiated thermally or photochemically. Reaction (4) is of interest because little kinetic data are available on reactions involving abstraction of halogen by halogen and also because an accurate determination of the activation energy E4 would prmit us to calculate an acccurate value of the bond dissocia tion energy D( CH3--I). I047