Effect of Poly(methacrylic acid) Hypercoils on the Neutral and Acid-Catalyzed Hydrolyses of 1-Acyl-1,2,4-triazoles in Aqueous Solution

Article abstract of DOI:10.1021/jo00209a023

Rates and thermodynamic activation parameters have been measured for the neutral and acid-catalyzed hydrolyses of the 1-acyl-1,2,4-triazoles 1-3 in the presence of atactic poly(methacrylic acid) (at-PMAA).Under the employed reaction conditions at-PMAA resides in a coiled, compact conformation.The rates of hydrolysis of the relatively hydrophilic 1-acetyl- (1) and 1-benzoyl-1,2,4-triazole (2) are only little affected by the presence of the polymer.By contrast, the hydrolysis of the hydrophobic 1-benzoyl-3-phenyl-1,2,4-triazole (3) is effectively inhibited as a result of binding of 3 to hydrophobic microdomains within the at-PMAA hypercoil.Only small rate retardations are found for the hydrolysis of 3 in the presence of poly(acrylic acid).The effect of at-PMAA concentration on the rates of hydrolysis of 3 can be described in terms of a kinetic scheme that is essentially a variant of Michaelis-Menten enzyme kinetic formalism.In the presence of at-PMAA, ΔH(excit.) and ΔS(excit.) for the neutral hydrolysis of 3 undergo large and partly compensatory changes, the retardations being dominated by the increase of ΔH(excit.).These findings are interpreted by assuming reduced hydration of the dipolar transition state for hydrolysis in the relatively "dry" hydrophobic microdomains.Comparable results were obtained for the HCl-catalyzed hydrolysis of 3.The inhibitory effect of at-PMAA on this reaction is attenuated in the presence of urea, presumably because of destabilization of the compact conformation of the polymer.