David S. Ashton

Free radical addition to olefins. Part 11.—Telomerization of tetrafluoroethylene with dibromodifluoromethane and trifluoroiodomethane

The telomerization of tetrafluoroethylene in the gas phase by CF2Br2 results in the formation of products of general formula Br[CF2]nBr n= 2–11. When CF3I was used the products were found to be CF3[CF2CF2]nI where n= 1 to 5. Transfer constants have been determined for radicals containing various numbers of tetrafluoroethylene units, and Arrhenius parameters from temperature variation data. Trends in the transfer constants are compared with previous work in solution. The activation energies from the transfer constants are combined with known values of the activation energies of the addition steps to give estimates of the activation energies of the halogen abstraction steps.

Free radical addition to olefins. Part XV. Addition of bromoform and carbon tetrabromide to fluoroethylenes

The peroxide-initiated addition of carbon tetrabromide and bromoform to ethylene, vinyl fluoride, 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene has been investigated in sealed tube experiments at 150°. With carbon tetrabromide the major products were adducts involving tribromomethyl radicals while in experiments with bromoform both dibromomethyl and tribromomethyl radicals were found to be important chain-carrying species. Telomers were formed, and the extent of telomer formation increased with increasing fluorine content of the alkene. The relative rates of addition of tribromomethyl and dibromomethyl radicals to the alkenes were estimated, and the orientation ratio for addition of each radical to vinyl fluoride and trifluoroethylene was obtained. The orientation ratios of radicals with any unsymmetrical alkene show significant correlations with the size and polarity of the radicals so that the orientation of radical addition appears to be governed by a complex interplay of factors in which steric and polar effects are of major importance.

Free radical substitution in aliphatic compounds. Part XXVI. The gas-phase bromination of halogenocycloalkanes

The gas-phase bromination of halogenocyclohexanes gave the expected isomeric bromohalogenocyclohexanes, together with cyclohexene, bromocyclohexane, and traces of trans-1,2-dibromocyclohexane. These latter products were formed as a result of elimination of hydrogen halide from the starting material. The initial halogenocyclohexanes were shown to undergo elimination of hydrogen halide in the presence of hydrogen bromide. Variation of the surface: volume ratio had a pronounced effect on the elimination which suggested that this reaction had a large heterogeneous component. Bromination of chloro- and fluoro-cyclopentane also produced elimination products which were formed as a result of the reaction of hydrogen bromide with the halogenocyclopentane.

Free-radical substitution in aliphatic compounds. Part XXVIII. The gas-phase bromination of halogenocyclohexanes and halogenocyclopentanes in a fast flow reactor

The bromination of fluoro-, chloro-, and bromo-cyclohexane and chloro- and bromo-cyclopentane has been investigated in the gas phase using a flow system. Unlike previous work in solution and in a static gas phase apparatus, normal directive effects were observed, and no olefinic products were obtained. These results suggest that the high yields of 1,2-dibromoalkanes reported previously in solution phase experiments are not due to ‘anchimeric assistance’ although ‘bridging’ by the substituent may be an important factor.

Free-radical substitution in aliphatic compounds. Part XXVII. Chlorination and bromination of 1-cyanobutane and cyanocyclobutane

1-Cyanobutane and cyanocyclobutane have been chlorinated and brominated in the gas phase over a wide range of temperatures. The results for 1-cyanobutane are compared with previous halogenations of 1-substituted n-butanes. The chlorination of cyanocyclobutane gives a very high trans/cis ratio for the 1-cyano-2-chlorocyclobutane and quite a high trans/cis ratio for the 1,3-product. This is interpreted in terms of an interaction between the π-orbitals of the cyano-group and the half-filled 2p2 atomic orbital of the radical site. The bromination of cyanocyclobutane is accompanied by a small amount of elimination of HCN; this reaction is increased by addition of HBr or by an increase in the surface area.

The effect of radical size and structure on the orientation of addition of perfluoroalkyl radicals to fluoro-alkenes

The orientation of the addition of CF3·, C2F5·, n-C3F7·, n-C4F9·, n-C7F15·, n-C7F18·, and (CF3)2CF· radicals to CFHCH2, CF2CH2, and CFHCF2 has been determined in the gas phase.

FREE-RADICAL SUBSTITUTION IN ALIPHATIC COMPOUNDS PART 28, THE GAS-PHASE BROMINATION OF HALOGENOCYCLOHEXANES AND HALOGENOCYCLOPENTANES IN FAST FLOW REACTOR

Fluor-, Chlor- und Bromcyclohexan sowie Chlor- und Bromcyclopentan werden in der Gasphase in einem Stromungsreaktor bromiert.