Ester resonance energy

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

It is to be expected from chemical evidence that the replacement of hydrogen by an aliphatic radical would have some further inhibiting effect on the carboxyl resonance. It is found, however, that to within the experimental error of about 0.2 v.e. the resonance energy is the same for methyl and ethyl esters as for carboxylic acids. 

Following previous work, Hagiwara and collaborators [71] recently prepared 5 -terminal acridone-labeled DNAs, using the succinimidyl ester 24 of the acridone acetic acid 23 reported before [69], and evaluated their use as donors for a fluorescence resonance energy transfer (FRET) system in combination with 3 -dabcyl-tagged DNA... 

A. Hydroxyfuran (also analogous thiophene, pyrrole) - Tautomer non aromatic but some additional resonance energy arises from ester etc. group... 

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 lack of equivalence of structures A and B does not inhibit their resonance very thoroughly, however, since the corresponding resonance energy is still large, having the value of 28 kcal/mole for acids and 24 kcal/mole for esters. 

The value 42 kcal/mole foi fhe resonance energy is given by the thermochemical data for the dialkyl carbonates. This value for resonance of the double bond among three positions is not unreasonable when it is compared with the corresponding value of 24 kcal/mole for esters of the fatty acids, in which the bond resonates between two positions. 

This low thiolester resonance energy has profound biochemical consequences. In particular, assumption of the low resonance energy provides much of the folklore explanation for why the biological acyl coupling system employs sulfur in Coenzyme A rather than oxygen and the biochemically more conventional ester functionality. (See, for example, T. C. Bruice and S. Benkovic, Bioorganic Mechanisms, Vol. 1, Chapter 3, W. A. Benjamin, New York, 1966.)... 

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. 

We thus conclude that tropolone has a greater degree of stabilization than 2-amino-tropone, a rather surprising conclusion perhaps, until it is realized that esters (and thus presumably carboxylic acids) have higher resonance energies than amides55. 

By the use of the bond energies in Table VII it is possible to derive satisfactory heats of formation and reaction in many cases, provided the substances involved do not contain certain double-bonded compounds. Whenever wave-mechanical resonance is possible, the energy required to dissociate the molecule, as calculated from the results given above, is too small. It is necessary in such instances to make allowance for the resonance energy. Although this varies from one compound to another, its value is approximately 38 kcal. mole for benzene and its simple derivatives, 75 kcal. mole for naphthalene compounds, 28 kcal. per mole for carboxylic acids, and 24 kcal. mole for esters. 

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