Addition of Other Halogens to Alkenes

The study of the addition of chlorine to alkenes is even older than the study of bromine addition. The synthesis of 1,2-dichloroethane was first reported by a group of Dutch chemists in 1795, and Michael Faraday investigated the addition of chlorine to ethene in 1821 In many ways the addition of chlorine to alkenes is similar to the addition of bromine to alkenes. For example, each additional alkyl group increases the relative reactivity of an olefin by a factor of 50 to 100 (Table 9.3). A Taft p a correlation is observed, and the p for chlorine addition is —2.9, which is similar to the value of —3.1 for addition of bromine. This result suggests that charges on the alkenyl carbon atoms in the transition structures for addition of chlorine are similar to those in the addition of bromine.  

The chloronium ion (the chlorine equivalent of the bromonium ion) was proposed by Lucas and Gould, and Olah reported evidence for a stable chloronium ion by NMR. ° Addition of chlorine to 2-butene was found to occur by an anti pathway, with trans-2-butene giving the meso 

TABLE 9.3 Reactivities of Aikenes and Yields of Substitution Products in Reactions of Ci2 with Neat Alkenes  

There are significant differences between the bromine addition and chlorine addition reactions, however. The addition reaction of ethene and chlorine is exothermic by 44 kcal/mol, which is 15 kcal/mol more exothermic than the addition of ethene and bromine. Poutsma noted that addition of chlorine to alkenes in nonpolar solvents can occur by either radical or ionic pathways, but that oxygen inhibits the radical reaction. For example, addition of chlorine to neat (i.e., not diluted by solvent) cyclohexene (19) gave frans-l,2-dichlorocyclohexane (20), 3-chlorocyclohexene (21), and 4-chloro-cyclohexene (22) in a 1.95 1.00 0.60 ratio when the reaction was carried out imder a nitrogen atmosphere. In the presence of oxygen, the ratio was 3-4 1.00 0.  

The addition of bromine to alkenes can also occur by a free radical pathway if the reaction is carried out in a nonpolar solvent such as CCI4 and if the reaction mixture is irradiated with light. Allylic substitution can be a competing reaction if the alkene contains allylic hydrogens. For a discussion of the competing substitution and addition reactions, see McMillen, D. W. Grutzner, J. B. /. Org. Chem. 1994,59,4516. 

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Because iodine is the best of the halogens at bridging, we might expect the iodonium ion to be the intermediate in the addition of iodine to alkenes. Indeed, an iodonium ion was isolated from the reaction of iodine with adamantylideneadamantane. The addition of iodine to alkenes is generally not synthetically useful, however, primarily because the formation of 1,2-diiodo compounds from I2 and alkenes is not as exothermic as are the additions of other halogens to alkenes. For example, the AH for addition of iodine to ethene is exothermic by only about 11 kcal/mol in the gas phase. Since AS is -31.5 eu, adduct stability decreases with increasing temperature. In one study it was found that a mixture of styrene and iodine produced styrene diiodide the product could be collected by filtration at 0°C, but it decomposed at room temperature to styrene and iodine. ... 

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