Addition reactions of Hypohalous acids and halogens with halogen-substituted - olefins

Abstract

The reaction between hypochlorous acid and 2-chloropropene was studied. The products were analysed as chloroacetone (85^t) and 2,3-dicliloropropene (15%). The absence of any 2, 2-dich.loropropanol, formed by nucleophilic attack of water at the terminal carbon atom, is discussed in terms of polarisation of the molecule caused by electron-release from the 2-ciiloro substituent. The formation of 2,3-dichloropropene by proton-loss in the carbonium ion from C-3 (rather than of 1,2-dichloropropene formed by loss from C-l) is explained in terms of steric strain. The effect of silver ions on the course of the reaction was also studied. A product analysis revealed no change in the yield of chloroacetone, nor any sign of any unusual products. The kinetics were also unaffected by the presence of silver ions. This behaviour contrasts with that of 2,3-dichloropropene, where silver ions have been found to cause autocatalytic behaviour, and to be responsible for abnormal products. The kinetics of 2-chloroprop-2-en-l-ol were studied, and found to show characteristics midway between the other 2-chloro compounds. When radioactively labelled hypochlorous acid was used in the addition reaction, the chloroacetone produced was found to contain less than the full activity, showing that some chlorine exchange takes place. The extent of the exchange was found to be reduced by the presence of increasing concentrations of silver ions. In a series of experiments in aqueous acid solution chloride, bromide and iodide ions were added to hypochlorous acid in the presence of silver ions. Exchange was found to occur, to an increasing extent along the series chloride - bromide - iodide, and to a decreasing extent with increasing silver ion concentration. The rate of exchange between hypochlorous acid and silver chloride was found to be slow, of the same order as that between chloride ion and silver chloride. There was very little exchange between chloride ion and hypobromous acid. The 2,3-dichloropropene produced in the reaction between hypochlorous acid and 2-chloropropene was found to be of the same specific activity the chloroacetone, and to be exclusively 3-labelled, The conclusion was reached that the chlorine exchange in the addition reaction proceeded by an inter- rather than an intra-molecular mechanism. The addition of hypochlorous acid to 2-bromopropene resulted in the production of some broooacetone, as expected. The kinetics of the addition of iodine in aqueous solution t6 the series of terminally substituted 1- alkenes CH(_2) = CH(CH(_2)(_n)Br were followed, A rate maximum at n = 3 appeared to indicate bromine participation in a 5_membered ring, although calculations based on the fall-off of inductive effect with the progressive removal of the bromo substituent from the reaction centre showed the effect not to be very large.