Bond-Cleavage and Addition Reactions

WJiile the linear approach of a radical to the bond being attacked is allowed by the spin symmetry (there is no intersection between the doublet electron states of the reactants and the products), the angular approach is, on the contrary, forbidden (the terms intersect)—see Refs. [3, 4]. From this it follows that an attack upon the a-bond R—X is, in reactions of the cleavage of a hydrogen atom by the radical, developing in a strictly collinear configuration I  

Indeed, the experimental and calculation data on the simplest reaction of the hydrogen atom abstraction H + H2- H corroborate the conclusions as to the preferability of the approach path of the type I [5]. 

The linear transition state structure II was revealed in the ab initio calculations [6], employing the IRC technique, on the reaction of the hydrogen atom abstraction from a methane molecule by the tritium atom  

In accordance with the theoretical rules, there occurs a collinear (Ci, S, R ) cleavage of R from a sulfur atom in the intramolecular cyclization of the radical III  

Another direction of the reaction, i.e., the formation of jS-arylethyl radicals that would require angular approach of the radical center to the SC2 bond does not materialize [7]. 

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