The Hammett Equation—the Present Position

Reinvestigation of the tautomerism of some substituted 2-hydroxypyridines

“…For all solvents, the best fit of data of Table 1 in the Hammett treatment of the electronic effect of substituents, is given by using the substituent constant σ I . 16 ρ Values for the solvents considered are high and negative (see Table 3). In principle, the use of σ I values and the calculated ρ values may be explained in two main ways.…”

Multiparameter Extensions of the Hammett Equation

“…The importance of the separation of sigma parameters into (TI and (Twtype contributions is that it suggests the possibility of a "dual substituent-parameter" treatment for reaction series through an equation of the form (4.7).17 Provided that the various (TR -type scales distinguished (4,7) above are linearly related to each other, it should be satisfactory to characterize each substituent by (TI and, say, (T~, and apply equation (4.7) to meta and para reaction series separately; i.e., with the Hammett equation we have, for each substituent, position-dependent sigma values (Trn and (Tp (arbitrarily becoming (T + or (T -on occasion) but a single P value for each reaction series, while in Taft's treatment each substituent is characterized by position-independent (TI and (T~ values, but the susceptibility to inductive and to resonance effects (variability of cross-conjugation) is to be expressed separately through position-dependent PI and PR values.…”

“…We have deliberately excluded from the present chapter extensions of the Hammett equation designed to deal with the effects of multiple substitution 4 or the ortho-effect (with slight exceptions);21 nor have we paid much attention to extensions to polycyclic or heterocyclic systems (see Chapter 5). We are thus concerned almost entirely with the area for which the Hammett equation was originally devised, i.e., the reactions of meta-and para-substituted benzene derivatives.…”

“…Exner has discussed these in detail. 4 Change of mechanism or of rate-controlling step (cf. above) can give Hammett plots showing minima or maxima, or, less extremely, curved plots without turning points.…”

Heuristic molecular lipophilicity potential (HMLP): A 2D‐QSAR study to LADH of molecular family pyrazole and derivatives

“…Traditionally, the 2D structural descriptors include hydrophobicity constants, Hammett electronic constants, and steric factors. [15][16][17] HMLP provides a structure-based computational method for the 2D QSAR descriptors. The HMLP indices have clear physical meaning and contain all information in the traditional 2D QSAR descriptors.…”