Conformational Properties of Carboxylic Acids and Amides

The H NMR spectra of acetic acid and acetamide are quite different. The OH proton generates a single sharp peak at room temperature, while the NHi protons generate a broad, double-humped peak that turns into two sharp peaks at lower temperatures. This suggests that the NH, protons occupy different chemical environments, while the OH proton occupies a single environment. 

examine the equilibrium structure of acetamide (see also Chapter 16, Problem 8). Are the two NH protons in different chemical environments If so, would you expect interconversion to be easy or difficult Calculate the barrier to interconversion (via acetamide rotation transition state). Rationalize your result. Hint Examine the highest-occupied molecular orbital (HOMO) for both acetamide and its rotation transition state. Does the molecule incorporate a n bond. If so, is it disrupted upon rotation  

148 Chapter 10 Carboxylic Acid Derivatives. Nucleophilic Substitution 

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Idoux, J.P., Scandrett, J.M. and Sikorski, J.A. (1977). Conformational Influence of Nonacyl Groups on Acyl Group Properties in N-Monosubstituted Amides and in Other Carboxylic Acid Derivatives a 7-Position Proximity Effect. J.Am.Chem.Soc., 99,4577-4583. 

Dichroic Behavior of Carboxyl and Amide Chromophores in Polysaccharide It is evident that the configurational differences of the uronic acid moiety are well reflected in the dichroic behavior of the polymer, and the carboxyl chromophore thus plays a significant role in the chiroptical properties of the molecules. Certain facts regarding carboxyl chromophores are apparent from this study a) monomeric CD properties are well reflected in the Cotton effect of the polymer b) similarities in A0 values (Table 11) between monomers and polymers containing similar uronic acids indicate the additivity of monomer contributions to polymer CD c) iduronic acid shows considerably larger CD than glucuronic acid, which may have some origin in the observed difference in the NMR behavior (29) of methyl a-D-idopyranosiduronic acid from uronic acid with normal C-1 conformation. This has been interpreted in terms of either an equilibrium between the C-1 and 1-C chair forms or adoption of a hybrid skew boat structure. 

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