The halogenation mechanism of substituted ethylenes in acetic acid solution

Abstract

The olefinic double bond in a molecule has the characteristic property that it causes the addition of halogens to form addition compounds. This property has long been known but only in recent years; when the factors governing it have received more particular attention, has it been recognised as a problem of gent complexity. Though fundamentally on easy reaction to formulate, surely in terms of valency, difficulties have been encountered in explaining the extreme varations in reactivity displayed by different olefin derivatives towards halogens. Earlier kinetic studies were successful in roughly classifying some compounds using promine as halogen and green tetrachloride as solvent but only recently (Robertson and co-workers J. C. S. 1937 onwards) have the studies been extended to inculde chlorine, promine and iodine in the solvent water, acetic acid, chlorobonzene and carbon tetrachloride. The first clue to the mechanism of the process was give by Francis (J. A. C. S. 1945; 47, 340) in experiments prominating ethylene in aoueous solution. He found that in the pressence of salts such as sodium chloride, a chloro-prome derivative was formed whereas in water alone bromohydrins resulted. These experiments (followed and extended by Meinel - Annalen 1935; 516, 321 - ) showed suite conclusively that promine addition was at least a two stage process.