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Studies in nuclear magnetic resonance—IX. Rotational barriers in substituted N,N-dimethylbenzamides

✍ Scribed by L. M. Jackman; T. E. Kavanagh; R. C. Haddon

Publisher
John Wiley and Sons
Year
1969
Tongue
English
Weight
780 KB
Volume
1
Category
Article
ISSN
0749-1581

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✦ Synopsis

The barriers to rotation about the C-N bond in eighteen substituted N,N-dimethylbenzamides have been determined by complete line shape analysis of the N M R spectra of the N,Ndimethyl protons. The barriers have been correlated with the substituent constants cr and u b . It has been shown that polar solvents increase the barrier in N,N-dimethylbenzamide.

Acid catalysis of rotation about the amide C-N bond in N-@-N,N-dimethylcarboxamidobenzy1)pyridinium bromide has been investigated. l80 exchange studies show that catalysis is due to Nprotonation rather than the formation of a tetrahedral intermediate. The rate of rotation is a function of the Hammett acidity function, H,, and the water activity, and it is shown that proton exchange between the N-and 0-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.

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