Search

Mixtures of bromotrichloromethane and the alkanes at circa 60 mm pressure have been irradiated in a Pyrex vessel with light from a medium pressure mercury lamp. The products, which consist solely of chloroform and monobromoalkanes, have been analysed. The reaction has been shown to be a chain process involving attack on the alkane by trichloromethyl radicals which abstract hydrogen atoms from the primary, secondary and tertiary positions in the order 1 : 80 : 2,300 at 190°.kk2/kp2 = 2.68 ± 1.48 exp (3.08 ± 0.46 k. cal/RT); kt2/k82 ≈ 5.25 × exp (5.5 k. cal/RT).

The light-induced reaction between bromotrichloromethane and n-butane has been studied in a series of gas-phase experiments. Taking a literature value for the recombination of trichloromethyl radicals, the value k2= 7.6 × 106 exp (–7530/RT) l. mole–1 sec–1 was determined: CCl3˙+ RH→CCl3H + R˙.(2) The difference in the rates of abstraction of primary and secondary hydrogen atoms is given by ks2/kp2= 0.67 exp(3640/RT). From experiments in which hydrogen bromide was added a value of k46305= 108.44±0.63 was determined: CCl3˙+ HBr→CCl3H+Br. (5)

The light-induced chlorination of butane and 2-methylpropane by t-butyl hypochlorite has been been studied from 10 to 70°C in the gas phase. Light intensity, pressure, reactant concentration and temperature have been varied and the results have been rationalized in terms of chlorination proceeding by way of a t-butoxy radical chain carrier. The relative rates of hydrogen abstraction by the t-butoxy radical, [graphic ommitted] have been obtained per H atom for the primary, secondary and tertiary positions of the two hydrocarbons studied. kS2/kP2= 1.63±0.01 exp (3.81±0.1 kJ/RT); kT2/kP2= 1.73±0.01 exp (4.94±0.1 kJ/RT). It was also possible to relate the rate of hydrogen abstraction k2 to the rate of decomposition of the t-butoxy radical (k4), [graphic ommitted] kP2/k4= 2.7±1.1 × 10–5 exp (38.1±1.7 kJ)/RT. These results are consistent with earlier results for alkyl and alkoxy radicals.

Trichloromethyl radicals have been allowed to react with 1-fluorobutane and 1,1,1-trifluoropentane in the gas phase. Arrhenius parameters have been obtained for the attack at each individual site in these molecules.

The chlorination of n-butyl formate, acetate, and trifluoroacetate, and the bromination of n-butyl acetate and trifluoroacetate have been studied in the gas phase by use of a static system. Chlorination is appreciably deactivated at the 1-position and slightly in the 2-position. Bromination is activated at the 1-position and slightly deactivated at the 2-position. From halogenations at a range of temperatures approximate Arrhenius parameters have been calculated for attack at each site. The difference in reactivity of different positions is principally determined by differences in activation energy although the pre-exponential factors for the substituted position are invariably lower than for the 4-position. The results are considered in terms of current theories.

Methyl radicals generated by the photolysis of azomethane have been allowed to react with n-butane in the presence of methyl iodide. The butyl iodides formed have been analysed and the reaction carried out over a wide temperature range. From these results the rates of attack CH3·+ C4H10→ C4H9·+ CH4 by methyl radicals at the primary and secondary positions in n-butane have been determined: ks/kp= 1·21 ± 0·16 exp (1·65 ± 0·10 kcal./RT)

The chlorination of chlorocyclobutane and methylcyclobutane has been studied in the gas phase over a wide temperature range. Competitive experiments with 1-chlorobutane have shown that unlike unsubstituted cyclobutane, chloro- and methyl-cyclobutane are more reactive than the corresponding cyclopentyl and cyclohexyl derivatives. This difference between substituted and unsubstituted cyclobutanes is attributed to release of strain in the formation of the substituted cyclobutyl radical. The trans to cis ratio of the 1,2-dichlorocyclobutane and the 2-chloro-1-methylcyclobutane products at 100° were 6·8:1 and 1·2:1 respectively. These results are consistent with ideas concerning the trans:cis ratios discussed in previous papers.

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 chlorination of 1-phenylpropane has been studied in the gas phase from 90–200°. Attack is favoured at the 1-position, while the 2-position is deactivated in comparison to the secondary positions in butane. Appreciable yields of 1-phenylprop-1-ene are formed. The amount of olefinic product was reduced by the addition of an inert gas (SF6). It is suggested that when an alkyl radical reacts with molecular chlorine a thermally excited alkyl chloride is formed which, in cases where elimination is favoured, may lose hydrogen chloride.

The chlorination of chlorocyclopentane and methylcyclopentane has been studied in the gas phase over the temperature range 70–200°. The trans to cis ratio of the products derived from chlorine atom attack at the 2-position was 16:1 for 1,2-dichlorocyclopentane but only 1·5:1 for 1-methyl-2-chlorocyclopentane. This different trans/cis ratio is consistent with the idea of an interaction between the half-filled π-orbital of the 2-chlorocyclo-pentylradical and the 3pz orbital of its substituent chlorine atom. Alternative explanations are considered and found to be less satisfactory.

The chlorination of chlorocycloheptane and fluorocyclohexane have been studied in the gas phase over the temperature range 70–200°. The trans to cis ratio of the products derived from chlorine atom attack at the 2-position was 41:1 for 1,2-chlorofluorocyclohexane and 5:1 for 1,2-dichlorocycloheptane. These results are considered in the light of current theories relating to the high trans/cis ratios observed.

Pentanoyl, hexanoyl, and heptanoyl fluorides have been chlorinated at two different temperatures in the gas phase. The corresponding chlorides have also been chlorinated in acetonitrile. The results confirm that in the gas phase a substituent in an aliphatic hydrocarbon chain has no effect on chlorination beyond the β-position. In the solution phase the substituent has a far longer-range effect which seems to be manifest as least to the Iµ-position for chlorination in acetonitrile. The effect of changes in solvent (from acetonitrile to carbon tetrachloride) has been investigated for esters in addition to acid chloride and found to be comparatively slight.

The photochemical chlorination of ethers is shown to result in predominant attack at the carbon atoms to which the ether oxygen is attached. The resultant radical is unstable and in the gas phase breaks down to yield an alkyl radical and an aldehyde. Assuming that attack at the α-positions yields the aldehyde quantitatively, we calculated values for relative rates of attack RSpx in butyl methyl ether.

n-Hexane has been chlorinated in the gas phase, with nitrogen as diluent, and in the liquid phase with carbon tetrachloride as a solvent. The relative rates of chlorination at the primary and secondary positions are given by the expressions (ks/kp)gas≈ 2·2 ± 0·6 exp (214 ± 127 cal./RT) and (ks/kp)liquid= 0·8 ± 0·2 exp (597 ± 20 cal./RT). Possible explanations of this difference are discussed.

The gas-phase chlorination of 2,2-dimethylhexane has been studied in a static system, in the temperature range 70–200°. No evidence was observed for ‘neopentyl’ type rearrangement during halogenation. Evidence was obtained, however, for intramolecular hydrogen atom transfer from C(5) to C(1) in the appropriate radical.