Electronic Effects of Substituted Heteroaromatic Groups

Electronic effects of substituted six-membered heteroaromatic groups have been little studied. 

As for those with one heteroatom, there are data available only on the 2,6-diphenyl-4-pyridyl group, on its charged forms, and some substituted pyridinio groups (Table XIV). But the bulk concerns the substituted pyrimi-dinyl and s-triazinyl groups. This section also contains evidence on some fused and other heterocyclic groups. 

The T constants for pyrimidinyl groups were determined by the NMR method from the values of the shift of meta and para carbon atoms of the benzene ring in the spectra of substituted phenylpyrimidines (series 8). For some substituted pyrimidinyl groups the constants were also determined by NMR from the shifts of meta and para fluorine atoms in the spectra of fluorophenylpyrimidines (series 9). Either method makes it possible to determine only inductive and mesomeric constants for substituted groups using Eqs. (10)-(13). Using the two methods separately to determine these constants enables one to judge more confidently the reliability of the values (Table XII). 

Group X Substituent Y o, Method Structural series (solvent) Footnotes 

Attempts to calculate anisotropic corrections more accurately can hardly justify the effort, bearing in mind that the other factors (medium, concentration, temperature, detection technique), combined with the errors due to the calculation equations used, cause a similar dispersion of values. 

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Evidence on the electronic effects of substituted heteroaromatic groups is, in our opinion, of greatest interest for researchers engaged in the field of synthetic organic chemistry, pharmaceutical and applied chemistry, and molecular spectroscopy. This evidence is therefore discussed separately (Section III). Another separate section (I V,B) deals with the peculiarities of the effect of substituents in the heteroaromatic ring and with the approaches to the a priori evaluation of inductive and resonance effects of such composite substituents. 

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