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 pKa 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 bonded to N‐4. Aqueous phase pKa 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 pKa 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 pKa units more acidic than the imide proton (i.e. the proton bound to N‐4). Changes in 13C 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.