Amide resonance energy

Resonance Energies of Aliphatic Acids, Esters, Amides, and Related Compounds... 

Although perhaps explicable, or at least precedented, in terms of the greater resonance energy of esters than corresponding amides, nonetheless, a further study is welcomed as this is the only thermochemical information we have on this class of compounds. 

The photoelectron spectrum of pyrrolidine in the low-energy (8-13 eV) region has been explored as part of a study of lone-pair ionization processes and an IP of 8.82 eV recorded for ionization from the N lone-pair. In A-methyl-3-pyrrolidinone the bands at 8.83, 9.53 and 12.24 eV have been assigned to ionization from the N lone-pair, the O lone-pair and the carbonyl 7r-system respectively. In 2-pyrrolidinone the first two bands are overlapped (at 9.53 and 9.76 eV), presumably a result of the amide resonance interaction, and on the basis of band appearance a reversed assignment to O lone-pair and N lone-pair ionization respectively has been proposed the 7rco ionization band is at 11.91 eV. Data are also available in the same paper for succinimide and its A-chloro and A-bromo derivatives (78MI30407). 

The keto tautomer of phenol does not retain any of the resonance energy associated with the aromatic ring, while the keto tautomer of 2-hydroxypyridine does (compare with amide resonance). 

Hydroxy derivatives. 2-Hydroxy derivatives usually exist as the oxo tautomers, unless the hydroxy tautomer is appreciably stabilized by electron-withdrawing or chelating substituents. The tendency for enolic hydroxy compounds to revert to the oxo form can be understood by reference to simple aliphatic ketones where the keto-enol equilibrium constants are of the order of 108. In the five-membered heterocycles under consideration, this tendency will be in opposition to the loss of aromatic resonance energy that increases in the order furan << thiophene < pyrrole. For the 2-hydroxy compounds 217 some extra stabilization of the oxo tautomers 218 and 219 is derived from the resonance energy of the X-C=0 group, which by analogy with open-chain compounds should increase in the sequence thiolester, ester << amide. 

Figure 2. Relationship between resonance energies and rotational barriers (AH ) of tertiary amides...

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