The Resonance Contribution to Ketone and Amide Tautomerism

37b set against that present in the functional group R itself. Any other form of competitive conjugation is likely to behave in the same way. 

Acetaldehyde, logfCj -6.23 [36] has been omitted from this series since out of line with the remainder, probably because of the different hybridization of C-H and C-C bonds and the difference that results in their electronic effects. 

With an unbalanced data set such as that of Table 5.5 no great reliance can be placed on the coefficients of Eq. (5.20) but the conclusions which matter, that both (7[ and (jg are important but the second more so, are probably valid nevertheless. The evidence regarding amide tautomerism is of a different sort. While R for 2-pyridones 39 is dominated by ff[, as is the 2(6)-position of 4-pyridone, it is generally true for any other type of position in any other oxoheterocycle, which has been examined that, where there are enough data to work on, will be the a-constant that gives the best fit [1]. A particularly good example Is provided by R 

Note that the corresponding data for R = H and Me are incommensurate since the defining equation [1] contains a small difference term between two large quantities which, for these substituents, has the misfortune to be negative. Nevertheless, these data are not only best fit by cr,, but this possesses a value close to about -3 as seems to be typical of most such cases we have examined [1]. It has been demonstrated [38] that cr contains about 78% T[ to 22% ffg, which will illustrate the dominance of the field effect in dictating the behavior of amides. Thioamides might be expected to behave like amides and thioketones like ketones in their sensitivity to electronic factors, but there are few if any data on which any such argument can be based. 

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