Halonium ions, structure calculations

The transition state structure for the addition of molecular fluorine to ethylene VIIc was found by the 3-2IG calculations [17] to closely resemble that for the addition of HCl (IV). Of special interest are the computational results obtained on transition state structures Vlld, Vile for gas phase electrophilic addition reactions of molecular chlorine and bromine to ethylene. Whereas the four-centered structure VIIc for the fluorination of ethylene indicates concerted c/s-addition, which is in accord with experimental finding for this reaction [18], the transition state geometries for chlorination and bromination may be regarded as cyclic halonium ions backed by halogenide counter-ions. Noteworthy is that the calculations [17] predict the heterolysis to occur intrinsically without any assistance from polar solvents. The three-centered structures Vlld, Vile help to clarify the reason for trans-stereoselectivity of the chlorination and bromination reactions of ethylene [1, 19]. 

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