The free radical chlorination of alkanes was examined in several solvents. The results of these experiments suggest that Cl• forms complexes with halogenated solvents and that these complexes exhibit higher selectivity in hydrogen abstractions than "free" chlorine atom. The observed selectivity decreases with increasing Cl-content of the solvent (paralleling the increase in the ionization potential of the solvent), and selectivity is greater in bromoalkane solvents compared to chloroalkanes, suggesting that these are donor/acceptor-type complexes.

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The relative rate constants were determined, and the absolute rate constants were estimated, for halogen transfer from N,N-dihaloarenesulfonamides, (dichloroiodo)arenes, benzyl bromides, and arenesulfonyl chlorides to cyclohexyl radical and from N,N-dihaloarenesulfonamides to benzyl radical. Polar effect of the substituent was found to be the main factor determining the rate of halogen transfer from benzyl bromides and arenesulfonyl chlorides; it increases with rise in the electrophilicity of the organohalogen substrate due mainly to charge distribution in the transition state.While considering the relations between the structure of organic molecules and the rate of radical transfer of hydrogen, the main factor is the heat effect of the hydrogen transfer stage, and its rate increases in parallel with the exothermic effect. In keeping with the Bell EvansPolanyi principle, in most cases the rate of transfer of an atom is linearly related to the heat effect. Exceptions are possible when a radical to which an atom is transferred possesses pronounced electrophilic or nucleophilic properties. In terms of the model based on the Marcus equation, increase in the electrophilicity or nucleophilicity of a radical leads to change of the internal reaction barrier as a result of charge separation in the transition state [1]. For example, the rate of abstraction of a hydrogen atom by chlorine and some other electrophilic radical species depends on polar effects of substituents in the substrate to a greater extent than on the heat effect. On the other hand, general relations holding in halogen transfer to alkyl radicals have been studied poorly. Therefore, we examined the effect of the structure of organic halogen compounds on the rate of halogen transfer therefrom to alkyl radicals.We determined the relative rate constants for abstraction of chlorine and bromine atoms from halogenating agents and arenesulfonyl chlorides by cyclohexyl and benzyl radical, following the procedure developed previDeceased.ously [2]. This procedure is based on competing abstraction of halogen from reagents XHlg 1 and YHlg 2 by alkyl radicals generated by radical chain halogenation of the corresponding alkane under conditions excluding formation of molecular halogen. Using the absolute rate constant for bromine transfer from N-bromosuccinimide (NBS) to benzyl radical, k(NBS) = (5.0 ± 0.3) 1 0 5 l mol 1 s 1 [3], we estimated the absolute rate constants of halogen transfer in the examined reactions. The relative and absolute rate constants are given in Table 1.

Cyclopropylbenzene radical chlorination leads to hydrogen atom abstraction and to ring opening. The contribution of the last process is increased in the presence of acids and in polar solvents. It is suggested that the ring opening process proceeds via radical cation formation by single electron transfer from cyclopropylbenzene to the chlorine atom in the first stage of reaction. Copyright © 2003 John Wiley & Sons, Ltd.

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