Stereochemistry of radical halogenation reactions. An ab initio molecular orbital study

Abstract

The stereochemistry of radical halogenation of alkyl halides has been studied by ab initio molecular orbital theory. Two key elementary reactions, hydrogen abstraction reaction [XCH 2CH 3 + Y . → XCH 2CH 2 . + HY (R1)] and halogen abstraction reaction [XCH 2CH 2 . + Y 2 → XCH 2CH 2Y + Y . (R2)], as well as rotational barrier of XCH 2CH 2 . radical, with X = H, F, Cl, and Br and Y = F, Cl, and Br, were studied using the G2(MP2,SVP) theory. Reactions R1 and R2 with X = F, Cl, and Br were found to be stereoselective. For X = F, both reactions prefer a gauche abstraction, whereas for X = Cl and Br, both reactions prefer a trans pathway. The high rotational barriers of ClCH 2CH 2 . and BrCH 2CH 2 . radicals and the low abstraction barriers of their reactions with Cl 2 and Br 2 are the two main factors that guarantee the retention of their radical configuration during the abstraction reactions. Thus, radical chlorination and bromination of alkenes and chlorine- and bromine-substituted alkanes are predicted to be stereospecific, in good accord with experimental results. We show that the stereochemical control observed in radical halogenation reactions can be explained without the use of Skell hypothesis. The trends of the calculated energy differences between the gauche and trans transition states of reactions R1 and R2, the rotational barriers of XCH 2CH 2 . radicals, and the gauche effect of XCH 2CH 3 can be rationalized in a uniform manner in terms of hyperconjugation interaction.