Hydride ions chlorohydrin

The Hydrolysis of cis and trans Cyclohexene Chlorohydrin. When trans cyclohexene chlorohydrin is treated with strong alkali, trans cyclohcxene glycol is obtained. Thus inversion does not appear to have occurred during the displacement. When the reaction is carried out with the cis isomer, cyclohexanone is formed.27 These facts can be explained if it is assumed that the first step in the reaction is the removal of a proton from the hydroxyl group (XXI to XXIII and XXII to XXIV), followed by the displacement of chloride ion by the alkoxide ion of the trans form (XXII1 to XXV) and by a hydride ion in the cis form (XXIV to XXVII). Displacement by alkoxide leads to cyclohexene oxide (XXV), and this intermediate can be further attacked by hydroxide ion to give the dl trans glycol (XXVI). Cyclohexanone is produced by the hydride displacement (XXIV to XXVII) ... 

Since the action of dilute alkali on trans cyclohexene clilorohydrin constitutes a synthetic preparation for cyclohcxene oxide,28 there seems to be little doubt that the epoxide is an intermediate in the reaction of the trans chlorohydrin. There appears to be no evidence for the hydride ion displacement shown for the cis isomer. 

Although the limited conversion of pinol chlorohydrin VI to ketone XIV by treatment with only one equivalent of LAH lends support to this idea, VI may not be the most appropriate model, because it embodies a more favorable orbital alignment for a concerted 1,2-hydride shift and ejection of the chloride ion. One could argue that compound I might isomerize first to XVI. However, this proposition would force into existence the rather odd intermediate XV (see Scheme 47.3). ... 

Further evidence for heterolytic decomposition is obtained from the effect of olefin structure on product distribution. Table II shows the ratio of carbonyl to glycol product for three olefins. Listed for comparison is the carbonyl/glycol ratio for the chlorohydrin, which corresponds to the structure of the oxythallation adduct from ethylene and propylene. The effect of structure on the ratio is qualitatively the same for the thallic ion oxidation and the hydrolysis of the corresponding chlorohydrin. Since the product distributions for both are inconsistent with neighboring hydroxyl participation (Reaction 33) the carbonyl/glycol ratio is a measure of the competition between hydride shift vs. water attack in... 

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