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Nitrogen‐15 nuclear magnetic resonance spectroscopy. NH proton exchange reactions of urea and substituted ureas
Author(s) -
Yavari Issa,
Roberts John D.
Publication year - 1980
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270130114
Subject(s) - chemistry , urea , proton , base (topology) , steric effects , dimethyl sulfoxide , nitrogen , medicinal chemistry , nuclear magnetic resonance spectroscopy , catalysis , spectroscopy , stereochemistry , organic chemistry , mathematical analysis , physics , mathematics , quantum mechanics
Abstract Proton‐coupled nitrogen‐15 NMR spectra of urea, N ‐methylurea, N ,‐ N ′‐dimethylurea, N ‐methyl‐ N ′‐benzylurea and N ‐phenylurea have been obtained at natural abundance level in neutral, basic and acidic solutions at 25°C. Base‐catalyzed NH proton exchange of the NH 2 group of N ‐methylurea in water was found to be 1.5 times faster than that for the ‐NH‐ group, while the corresponding acid‐catalyzed exchange is 7.5 times faster. Comparison of urea and N ,‐ N ′‐dimethylurea in water shows urea to be 10 times faster in base but 2 times slower in acid. The ratio of the base‐catalyzed NH proton exchanges of the two ‐NH‐ groups of N ‐methyl‐ N ′‐benzylurea in dimethyl sulfoxide is close to unity, whereas the CH 3 NH‐ group exchanges 4 times faster in acid. Similarly, the C 6 H 5 NH‐ group of N ‐methyl‐ N ′‐phenylurea exchanges 50 times faster than the CH 3 NH‐ group in base and about 3 orders of magnitude slower in acid. The results are rationalized by consideration of steric and electronic effects.