Water catalyzed hydrolysis of p-nitrotrifluoroacetanilide and trifluoroacetanilide. Carbonyl O-18-exchange does not accompany the water reaction

The water catalyzed hydrolyses of p-nitrotrifluoroacetanilide (3) and trifluoroacetanilide (8) were studied at various pH's and temperatures. The activation parameters for the water reactions of 3 and 8 are: Delta H double dagger = 14.4 +/- 0.6 and 11.7 +/- 0.3 kcal/mol and Delta S double dagger = -36.1 +/- 1.8 and -52.3 +/- 0.7 cal/mol.K, respectively. These are consistent with reactions that involve considerable restriction of degrees of freedom of the solvent/substrate in the transition state, A proton inventory analysis of the rate constants for hydrolysis of 3 in media of different mole fraction D2O indicates the process involves two or more protons in flight or undergoing loosening of their bonding in the transition state. O-18-Labeled amides were subjected to the hydrolytic conditions for various times up to 3 half-times of hydrolysis and recovered. Mass analysis showed that the O-18 content in the recovered amide did not change during the course of the reaction. All the data support a process where the rate-limiting step for the water reaction involves a concerted or nearly concerted formation of a diol which undergoes subsequent C-N cleavage in preference to OH expulsion.