Phosphate monoester monoanions, hydrolysis mechanism

Converging lines of evidence have led to a general acceptance of the monomeric metaphosphate mechanism for the hydrolysis of phosphate monoester monoanions. The pH rate profile for aryl and alkyl phosphate monoester hydrolyses commonly exhibits a rate maximum near pH 4. where the concentration of the monoanion is at a maximum. The proposed mechanism is based on these principal points of evidence (a) a general observation of P-O bond cleavage (b) the entropies of activation for a series of monoester monoanions are all close to zero, which is consistent with a unimolecular rather than a bi-molecular solvolysis where entropies of activation are usually more negative by 20 eu7 (c) the molar product composition (methyl phosphate inorganic phosphate) arising from the solvolysis of the monoester monoanion in a mixed methanol-water solvent usually approximates the molar ratio of methanol ... 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

The spontaneous hydrolysis of 2-pyridyl phosphate (PP) is a good model for the special mechanism (Scheme 15) for the hydrolysis of phosphate monoester monoanions (M ), which are believed to react via an initial proton transfer equilibrium to form the highly reactive species the proton transfer occurring from a hydroxyl group attached... 

The pH-rate profile for unbuffered hydrolysis of glyceraldehyde-3-phosphate (6-3-P) has been attributed to hydrolysis of the monoanion of the phosphate monoester at pH < 4, spontaneous formation of glyceraldehyde from the phosphate dianion at pH 7-8, and, at higher pH, hydroxide-catalysed methylglyoxal formation. Reaction of the dianion is not subject to a solvent isotope effect and is believed to occur by the irreversible ElcB mechanism whereby an enediolate intermediate, formed on rate-determining C(2) deprotonation, subsequently expels phosphate trianion by C—0 bond breaking. The diethylacetal and 2-methyl-G-3-P do not hydrolyse under the same conditions.5... 

The dissociative mechanism (1) for the hydrolysis of phosphate monoesters was originally invoked to explain the enhanced reactivity of the monoanion-monoacid form of methyl phosphate (Bunton et al., 1958 Butcher and Westheimer, 1955). The facts that were cited in support of the applicability... 

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