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Study of the mechanism of oxidative decarboxylation of [alpha]-hydroxy acids by bromine water Pink, Judith Margaret Osyany

Abstract

A kinetic study of the mechanism of oxidative decarboxylation of α-hydroxy acids by bromine water throughout the pH range revealed two distinct reactions paths. At low pH HOBr rather than Br₂ is the oxidant. The reaction kinetics is complex in bromine water but a second order rate law is observed when the formation of bromide is suppressed by the addition of silver ion,, On the basis of the pH - rate profiles, solvent isotope effect and the insensitivity of reaction rate to α-deuteration it is concluded that HOBr attacks the undissociated hydroxy acid but the rate controlling step also involves a base catalyzed proton loss from the hydroxyl group. Calculated pH - rate profiles, based on a steady state treatment, are in agreement with the observed values for all three model compounds studied in detail: benzilic, mandelic and 2-hydroxy-2-methylpropanoic acids. Rate equations are also derived for the complex reactions occurring in the presence of accumulating bromide. The mechanism of α-hydroxy acid oxidation is compared with the oxidation pattern of alcohols and of oxalic acid in the bromine water system. Above pH 6 molecular bromine is found to be the most important oxidant for α-hydroxy acid. Slow oxidation also occurs by HOBr but not by OBr¯. The reaction is general base catalyzed. A rate expression is derived, based on a steady state treatment, assuming carboxylate - Br₂ and carboxylate - HOBr reactions with concurrent base catalyzed loss of the hydroxylic proton. The calculated rate constants are in good agreement with the values observed both in the presence and absence of added bromide. Results are also presented of: 1) A preliminary investigation of oxidative decarboxylation of α-amino acids by bromine water. 2) A study of the oxidation of α-hydroxy and α-amino acids by permanganate throughout the pH range.

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