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Studies in nuclear magnetic resonance—IX . Rotational barriers in substituted N , N ‐dimethylbenzamides
Author(s) -
Jackman L. M.,
Kavanagh T. E.,
Haddon R. C.
Publication year - 1969
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.1270010204
Subject(s) - chemistry , substituent , protonation , pyridinium , amide , chemical shift , medicinal chemistry , reaction rate constant , bromide , catalysis , crystallography , molecule , proton , resonance (particle physics) , stereochemistry , inorganic chemistry , organic chemistry , kinetics , ion , physics , quantum mechanics , particle physics
The barriers to rotation about the CN bond in eighteen substituted N , N ‐dimethylbenzamides have been determined by complete line shape analysis of the NMR spectra of the N , N ‐dimethyl protons. The barriers have been correlated with the substituent constants σ and σ + . It has been shown that polar solvents increase the barrier in N , N ‐dimethylbenzamide. Acid catalysis of rotation about the amide CN bond in N ‐( p ‐ N , N ‐dimethylcarboxamidobenzyl)‐pyridinium bromide has been investigated. 18 O exchange studies show that catalysis is due to N ‐protonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H 0 , and the water activity, and it is shown that proton exchange between the N ‐ and O ‐protonated species involves the intermediacy of a water molecule. The differences in chemical shifts for the non‐equivalent N , N ‐dimethyl groups of the benzamides are also a function of the substituents. Possible explanations of this phenomenon are discussed.