Effects of Substituents on the Radical Exchange Reaction between Benzyl Iodide and Iodine

Abstract

The previously unknown compounds p-nitro, p-methyl, and p-methoxybenzyl iodide were prepared. The p-methoxy compound decomposes rapidly in the presence of iodine, but the exchange of elementary iodine with the other two compounds and with unsubstituted benzyl iodide was studied photochemically in ethylene dichloride. The exchange mechanisms are entirely free radical and apparently involve comparable contributions from a direct substitution by iodine atoms and from the abstraction of iodine to form benzyl radicals, although the direct substitution reaction is not established with certainty. Electron-withdrawing substituents accelerate the radical abstraction of iodine, but they slow the abatraction of benzylic hydrogen. Apparently electron withdrawal facilitates radical abstraction of a group that would tend to ionize as a negative entity and depresses radical abstraction of a group that would tend to ionize as a positive ertity. In air saturated ethylene dichloride at 30 deg , oxygen, and 1.4 x 10/ sup -//sup 4/ M iodine compete for benzyl radicals with about equal efficiency. (auth)