Aspects of the hydrolysis of formamide: revisitation of the water reaction and determination of the solvent deuterium kinetic isotope effect in base

A study of the hydrolysis of formamide is reported with the aims of isolating the water reaction for hydrolysis from the acid and base hydrolysis terms and determining the solvent deuterium kinetic isotope effect (dkie) on base-catalyzed hydrolysis. Respective activation parameters (DeltaH(double dagger) and DeltaS(double dagger)) of (17.0 +/- 0.4) kcal mol(-1) and (-18.8 +/- 0.3) cal mol(-1) K-1 for the acid reaction and (17.9 +/- 0.2) kcal mol(-1) and (-11.1 +/- 0.5) cal mol(-1) K-1 for the base reaction were determined from Eyring plots of the second-order rate constants over the range of 27-120degreesC. Kinetic studies at the minima of the pH/rate profiles in the pH range from 5.6 to 6.2 in MES buffers at 56degreesC, and in the pH range of 4.25-6.87 in acetate and phosphate buffers at 120degreesC are reported. At 56degreesC the available data fit the expression k(obs)(56) = 0.00303[H3O+] + 0.032[HO-] + (3.6 +/- 0.1) x 10(-9), while at 120degreesC the data fit k(obs)(120) = (0.15 +/- 0.02)[H3O+] + (3.20 +/-0.24)[HO-] + (1.09 +/- 0.29) x 10(-6). Preliminary experimental estimates of E-a (ln A) of 22.5 kcal mol(-1) (15.03) for the water rate constant (k(w)) are calculated from an Arrhenius plot of the 56 and 120degreesC data giving an estimated k(w) of 1.1 x 10(-10) s(-1) (t(1/2) = 199 years) at 25degreesC. Solvent dkie values of k(OH)/k(OD) = 1.15 and 0.77 +/- 0.06 were determined at [OL-] = 0.075 and 1.47 M, respectively. The inverse value is determined under conditions where the the first step of the reaction dominates and is analyzed in terms of a rate-limiting attack of OL-.