Alkenes polar addition

There have also been relatively few mechanistic studies of the addition of iodine. One significant feature of iodination is that it is easily reversible, even in the presence of excess alkene. The addition is stereospecifically anti, but it is not entirely clear whether a polar or a radical mechanism is involved. ... 

To account for this very high TRANS stereoselectivity, it has been suggested that addition proceeds via a cyclic bromonium radical (71), analogous to the cyclic bromonium cations involved in the polar addition of bromine to alkenes (p. 180) ... 

There is a further aspect of polar additions to alkenes that we should consider, namely, that electrophilic reagents form loose complexes with the 77 electrons of the double bonds of alkenes prior to reaction by addition. Complexes of this type are called charge-transfer complexes (or ir complexes). Formation of a complex between iodine and cyclohexene is demonstrated by the fact that iodine dissolves in cyclohexene to give a brown solution, whereas its solutions in cyclohexane are violet. The brown solution of iodine in cyclohexene slowly fades as addition occurs to give colorless trims-1,2-diiodocyclohexane. 

The reactions of 1,3-butadiene are reasonably typical of conjugated dienes. The compound undergoes the usual reactions of alkenes, such as catalytic hydrogenation or radical and polar additions, but it does so more readily than most alkenes or dienes that have isolated double bonds. Furthermore, the products frequently are those of 1,2 and 1,4 addition ... 

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