POTENTIOMETRIC AND SPECTROSCOPIC INVESTIGATIONS OF THE AQUEOUS PHASE ACID-BASE CHEMISTRY OF URAZOLES AND SUBSTITUTED URAZOLES

Aqueous-phase equilibrium acidity constants have been determined for urazole (1,2,4-triazolidine-3,5-dione), several substituted urazoles and other selected imides. Aqueous phase pK(a) values for urazole (5.8), 1-methylurazole (5.3), 4-methylurazole (5.7) and 1,4-dimethylurazole (5.3) indicate that (in water) the methyl group acidifies the urazole moiety when bonded to N-1 but has little effect on acidity when bounded to N-4. Aqueous phase pK(a) values for hydantoin (9.0) and 1-methylhydantoin (9.1) suggest that a 1-methyl substituent has little effect on the acidity of the imide proton present in hydantoin. These data, combined with aqueous phase pK(a) values for 1,2-dimethylurazole (7.5), a substituted urazole lacking amide protons, indicate that an amide proton in urazole (i.e. the proton bound to either N-1 or N-2 in urazole) is ca 1.5 pK(a) units more acidic than the imide proton (i.e. the proton bound to N-4). Changes in C-13 NMR chemical shifts for the carbonyl carbon atoms present in variously substituted D2O phase urazoles, hydantoins, succinimide and their conjugate bases also confirm that amide N-H bonds found in urazole and 1-methylurazole are weaker, in a heterolytic acid-base sense, than imide N-H bonds.