POLAR AND STERIC EFFECTS IN ACYL PHOSPHATE MONOANION AND DIANION REACTIONS

The rate constants for hydrolysis of a series of aliphatic acyl phosphates have been determined at 60°. Complete pH-rate profiles for three of the derivatives, isobutyryl, trimethylacetyl, and 3,3-dimethylbutyryl phosphate, were obtained as well as the monoanion and dianion hydrolytic rate constants for isovaleryl phosphate. The values of ∆S* were uniformly close to zero, consistent with the postulated unimolecular mechanism of hydrolysis of acyl phosphates. A decrease in the rate of hydrolysis was observed for the monoanion and dianion reactions as steric bulk and electron donation in the acyl group, as measured by the Taft ó* constants, were increased. Second-order rate constants for reaction of pyridine with the monoanions and dianions were also correlated with ó* constants. The ρ* for kpyr (monoanion) was +2.0 and for kpyr (dianion) was +6.1. Imidazole and morpholine did not catalyze the hydrolysis of trimethylacetyl phosphate or 3,3-dimethylbutyryl phosphate. © 1969, American Chemical Society. All rights reserved.