Electronic Effects of Unsubstituted Heteroaromatic Groups

Approaches and Methods for Evaluating Electronic Effects of Heteroaromatic Groups 

To determine a constants for heteroaromatic substituents, use is made of various physical parameters, as well as quantitative data on the reactivity of hetaryl, aliphatic, and aromatic compounds. Wide use has been made of NMR a constants for heteroaromatic groups are often determined by using and F chemical shifts in spectra of substituted benzenes (63JA709 72BCJ1519 79ZOR1737 80AJC1763 82MI1). 

Comparison of a constants calculated independently from the data on substituted fluorobenzenes, anilines, phenols, and monosubstituted benzenes in the same solvent makes it possible in most cases to conclude that they are in fairly good agreement. 

An advantage of NMR is that it is necessary to obtain only one phenyl derivative, whereas in using F(or H) NMR one has to synthesize meta and para isomers of the fluorophenyl (or the aminophenyl and hydroxyphenyl) derivatives. 

From the F spectra of fluorophenyl derivatives and spectra of phenyl derivatives, one can determine the r, and (Tr constants for heteroaromatic groups from the chemical shift values of amino and hydroxy group protons in the H-NMR spectra of aminophenyl and hydroxyphenyl derivatives, one can obtain other a constants for heteroaromatic groups, in particular Tr constants. 

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