Abstract'The photolysis of azocyclohexane, carbon tetrachloride, and cyclohexane at 360 nm has been investigated over a wide temperature range. At moderate temperatures a chain reaction ensues from which the following approximate rate constants could be determined assuming 2CcI3. + CZC16, k5 = (303-673K):(1 1,800 ;f3000 calThe really striking feature of the results is that they show that termination in bicyclohex$ [reaction ( 7 ) ] is extremely slow:The root-mean-square rule for estimating the cross-combination rate c c l 3 -+ c-C6HI1--% C -C G H~~-C C I~ is also followed. The photolysis of carbon tetrachloride and cyclohexane at 250 n m has also been investigated. The reaction is complicated by the occurrence of two concurrent photolytic processes, the main one yielding trichloromethyl radicals and chlorine atoms, and the subsidiary one yielding dichlorocarbene and molecular chlorine. Nonetheless the results from this reaction can be interpreted in the medium temperature range 36@430K, where long chains are present, in terms of the rate constants derived from the azocyclohexane svs te m .

The photolysis of carbon tetrachloride in the presence of a number of alkanes has been investigated in the gas phase. The products obtained from the photolysis experiments were those expected from a chain reaction in which trichloromethyl radicals abstract hydrogen atoms from the alkane. The data have been used to determine Arrhenius parameters for hydrogen abstraction from the series of alkanes CH4, CzH6, CzH8, and i-C4H1O by trichloromethyl radicals, ( 4 )

The effect of the addition of hydrogen chloride on the photolysis of carbon tetrachloride in the presence of cyclohexane has been investigated in a companion paper. The data enable the rate constant ratio k8/(k5)1/2 to be determined. Since k−8 is well established, k5 can be estimated from known thermochemical data. The validity of the thermochemical derivation is checked by applying it to trifluoromethyl radicals. The photolysis of bromotrichloromethane and carbon tetrachloride in the presence of hydrogen chloride has been investigated over a range of temperatures. From these results and assuming reaction (5) has no activation energy, Arrhenius parameters for reaction (8) have been determined: The activation energies for the reaction of methyl, trichloromethyl, and trifluoromethyl radicals with hydrogen chloride are compared, and at first sight surprising results are rationalized in terms of relative electronegativity.

The photolysis of azocyclopentane in the presence of cyclopentane-carbon tetrachloride mixtures has been investigated in the gas phase. Product analysis data have been used to determine the Arrhenius parameters for the reactionsThe rate data for chlorine atom abstraction from CC4 by the cyclopentyl radical were compared with available data for other alkyl radicals in both the gas and the solution phases. The results indicate that the rate constant for chlorine atom abstraction in the gas phase is fairly insensitive to the nature of the attacking alkyl radical and that the activation energy for a secondary radical is about 4 kcal/mol higher than the corresponding reaction in the solution phase.c-CsHg. + C c 4 -c-CsHgCl+ CC13. cc13. t c-C~HIO -CC13H + c-CsHg.

The effect of the addition of hydrogen chloride on the photolysis of carbon tetrachloride in the presence of cyclohexane has been investigated in a companion paper. The data enable the rate constant ratio kg/(kJ1'2 to be determined. Since k-8 is well established, k5 can be estimated from known thermochemical data. The validity of the thermochemical derivation is checked by applying it to trifluoromethyl radicals. The photolysis of bromotrichloromethane and carbon tetrachloride in the presence of hydrogen chloride has been investigated over a range of temperatures. From these results and assuming reaction (5) has no activation energy, Arrhenius parameters for reaction (8) have been determined:The activation energies for the reaction of methyl, trichloromethyl, and trifluoromethyl radicals with hydrogen chloride are compared, and at first sight surprising results are rationalized in terms of relative electronegativity.(1 954).