Hammett and Taft Constants

Hammett and Taft constants, S, 107 homolytic substitution, S, 5 hydrogen exchange, S, 57, 69-70 hydroxy... 

It is instructive to consider N-substituted azoles in reverse, i.e., the azole ring as the substituent linked to some other group. Hammett and Taft -constant values for azoles as substituents are given in Table 11. The values show that all A-azolyl groups are rather weak net resonance donors, imidazol-l-yl being the strongest. They are all rather strong inductive acceptors, with pyrazole considerably weaker in this respect than imidazole or the triazoles. 

Table 11 Hammett and Taft -constant values for azoles as substituents in a benzene ring <1981 JCR(S)364>...

In 1965 Jaffd and Jones [461] applied the Hammett equation to values of excited-state pK s and found a rough correlation with ground-state a values. That same year Wehry and Rogers [462] showed that ground-state Hammett and Taft constants could be applied to the excited singlet and triplet states of phenol and 16 monosubstituted phenols to produce a satisfactory correlation. 

Hammett and Taft substituent constants and, in particular. Tables 9.1 through 9.4 may also prove useful for estimating values. 

TABLE 9.1 Hammett and Taft Substituent Constants Continued)... 

Hammett and Taft Benzene Ring (sijcr(S)364)... 

The cyclic mechanism is probably seldom a fully concerted (E2) process, and the different timing of individual electron shifts results in a transition towards the El or ElcB mechanisms (cf. Sect. 2.1.1). The choice of the mechanism depends on the reactant structure as well as on the catalyst nature. As an indicator of the mechanism, either the degree of stereoselectivity (see refs. 68, 121, 132 and 141) or the value of the reaction parameter of a linear free energy relationship, e.g. p or p constants of the Hammett and Taft equations (cf. ref. 55), may be used. 

Table 12 Hammett and Taft cr-Constant values for Azoles as Substituents in a Benzene Ring"...

Another extension is in the field of alicyclic compounds (7, 112). In steroid compounds it has been proved that long-distance effects, e.g. from position 17 to 3 and vice versa, affect polarographic half-wave potentials. Finally it has been demonstrated (7, 113) that Hammett and Taft substituent constants can be used as a first approximation to express the substituent effects in numerous types of mono- and polycyclic heterocyclic compounds. 

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