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Potentiometric and spectroscopic investigations of the aqueous phase acid–base chemistry of urazoles and substituted urazoles
Author(s) -
Bausch Mark,
Selmarten Don,
Gostowski Rudy,
Dobrowolski Piotr
Publication year - 1991
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610040111
Subject(s) - chemistry , substituent , amide , aqueous solution , imide , moiety , medicinal chemistry , zwitterion , succinimide , chemical shift , organic chemistry , stereochemistry , molecule
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 p K 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 bonded to N‐4. Aqueous phase p K 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 p K 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 p K a units more acidic than the imide proton (i.e. the proton bound to N‐4). Changes in 13 C NMR chemical shifts for the carbonyl carbon atoms present in variously substituted D 2 O phase urazoles, hydantoins, succinimide and their conjugate bases also confirm that amide NH bonds found in urazole and 1‐methylurazole are weaker, in a heterolytic acid—base sense, than imide NH bonds.