PROTON-TRANSFER CHEMISTRY OF URAZOLES AND RELATED IMIDES, AMIDES, AND DIACYL HYDRAZIDES

Equilibrium acidity constants have been determined for 1,2,4-triazolidine-3,5-dione (urazole), several substituted urazoles, and other related acids, in both dimethyl sulfoxide (DMSO) and aqueous solution. In DMSO, urazole has a pK(a) of 13.1. In water, urazole has a pK(a) of 5.8. In general, N-methyl and N-phenyl substituents are found to acidify the urazole moiety, in both DMSO and water. The acidifying effects of these substituents are attenuated by a factor of 3.3 in water. The solvent effects are ascribed to the aqueous stabilization of urazole anions via hydrogen-bonding interactions and the aqueous-promoted relief of lone pair-lone pair electronic interactions that manifest themselves upon deprotonation of a hydrazyl proton in 1 and related species. That a hydrazyl proton in 1 is at least as acidic as the imide proton in 1 is comparison of C-13 NMR spectra for the urazoles and related nitrogen acids with C-13 spectra for the conjugate bases derived from these species. Upon loss of an imide proton, in both DMSO-d6 and D2O solutions, carbonyl carbon atoms present in succinimide as well as appropriately substituted urazoles and hydantoins experience substantial (13-17 ppm) downfield shifts. In contrast, deprotonation of 4-substituted and 1,4-substituted urazoles, 4,4-dimethylpyrazolidine-3,5-dione, and diacetylhydrazine (species that contain hydrazyl acidic protons) results in shifts in the positions of the carbonyl resonances that range from 5 ppm upfield to 3 ppm downfield. Deprotonation of species containing both imide and hydrazyl protons (i.e., urazole and 1-substituted urazoles) results in shifts in the carbonyl carbon resonances consistent with hydrazyl proton removal. Comparison of DMSO-phase pK(a)'s for acetamide (25.5), diacetylhydrazine (16.7), 4,4-dimethylpyrazolidine-3,5-dione (13.5), and urazole (13.5), and urazole (13.1) suggest that the remarkable acidity of the hydrazyl proton in urazole and substituted urazoles is due mainly to its cyclic diacyl hydrazide structure.