Pyridyl groups, substituent constants

Some values for the same substituent constant estimated by different methods are found to be rather scattered (see Table I). The reason for the wide scatter in a constants for pyridyl groups is that they were obtained via correlations of low sensitivity or statistical unreliability and that the measurements were made with various solvents having strong solvation effects (83KGS66). 

To estimate the and constants for unsubstituted pyridyl groups, Pasternak and Tomasik attempted to use the dependence of the frequencies of symmetric (vj and asymmetric (VaJ NO2 stretching vibrations in substituted nitrobenzenes (2) on the Hammett substituent constants (75BAP923). The pyridyl group constants obtained from the data of the IR spectra of the nitrophenyl pyridines in CHClj and CHBrj have considerable scatter in their values depending on the solvent. Most of the and values estimated from Vs-NOa Jiff widely from the respective values obtained from v noj even for the same solvent. Four of the seven Hammett-type relationships used by the above authors to calculate the a values have a low correlation coefficient (r = 0.949-0.965). The method in question, just as polarography, is suitable only for a rough estimation of constants. 

Pasternak and Tomasik attempted to calculate theoretically the constants for pyridyl and N-protonated pyridyl groups by using the relationship between the ffp substituent constants and the Dp index [Eq. (4)], which... 

The a values estimated from Eq. (7) for 2-, 3-, and 4-pyridyl groups agree well with the mean values for these groups (cf. Table VI, Section II,B,4). A set of a (contiguous) constants was proposed to account for contiguous delocalization interactions between adjacent functionalities X and G in series (5) and (6). This set was shown to overlap with Mine s a set for the majority of substituents, but it provides new values for substituents, including 4-pyridyl, in the Hammett and iso-Hammett series. A duality of the CTr and... 

Deady and Zoltewicz studied the influence of a larger set of substituents on the kinetics of quaternization of derivatives 20 and 21 including alkyl and functional groups R in position-2 and/or R-6 (72JOC603) (Scheme 15). As expected, the kinetic results are poorly correlated with pK values. Separate correlation lines were required for 2-substituted and 2,6-disubstituted pyridines. It was suggested that steric effects are superimposed on electronic effects, and that steric effects are nearly constant for ortho substituents like NH2, Me, Cl, Br, and CN, but are more important for Et, CH2Ph, 2-pyridyl, and C02Me. 

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