Hydroxylation by potassium permanganate

Olefins can be easily oxidized to cis- 1,2-glycols by dilute potassium permanganate solutions.110 This reaction has been used for hydroxylation of numerous unsaturated compounds of the most diverse types.111 

It is very important to choose the reaction conditions so as to avoid further oxidation of the glycols formed if, for instance, the permanganate is added too rapidly to a neutral solution of olefin, the main product is an acyloin RCH(OH)—COR. 112 Hydroxylation is usually effected in the cold with neutral or alkaline potassium permanganate solution water, acetone, alcohols, methylcyclohexane, and mixtures of alcohol and water can serve as solvent. Magnesium sulfate is often added for reactions in a neutral medium.113 

Oxidation of olefins in alkaline solution (pH 12) in mixed solvents (tert-butyl alcohol/water) gives higher yields of purer glycols and proceeds more readily than does reaction in neutral solution.114 115 

Preparation of erythro-9,10-dihydroxystearic acid from oleic acid 114 A solution obtained by heating a mixture of oleic acid (1.00 g, 3.5 mmoles), sodium hydroxide (l.O g, 25 mmoles), and water (100 ml) is cooled and diluted with ice-water (800 ml) and crushed ice (100 g). To this cold solution is added, as fast as possible and with vigorous stirring, a solution of potassium permanganate (0.80 g, 5.1 mmoles) in water (80 ml). After 5 min the mixture is decolorized by S02, concentrated hydrochloric acid (30 ml) is added, and the whole is cooled for 1 h in ice. The precipitate is then filtered off and dried in a drying pistol at room temperature, affording crude er /zra-9,10-dihydroxystearic acid (1.05 g, 94%). Recrystallization from 95 % ethanol gives a pure product (0.90 g, 81 %), m.p. 130-131°C. 

If more potassium permanganate (1.10 g, 7 mmoles) in water (110 ml) is used and de-colorization is effected after reaction for 1 min, the yield of crude diol is 98% (1.10 g) and of pure product 79 % (0.88 g). 

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