Taft constants, inductive

The Hammett equation has been further extended to include substituents of aliphatic systems too. In this case the constants that quantify the inductive effects, which are represented as o, have different values compared to those concerned with aromatic systems and are defined as Taft constants. Some values of these latter are also reported in Table 1. 

The rate of acid-catalyzed degradation of the penicillins also depends largely on the nature of their acylamido side chain. Structure-activity-relationship studies undertaken for the rational design of orally active penicillins have shown that the stability in gastric juice increases with the sum of Taft s inductive substituent constants (of values) of the 6-amino side chain [95]. 

Thus we could use the charges given by any LCAO basis of our choice to create an arbitrary set of scahng parameters (as a replacement for the cr constants) and proceed with Eqs. (5.1) and (5.2) without invoking Taft s inductive effects. For convenience, however, we shall go along with the a constants of Table 5.1. 

A crude justification for such a study can be found in the work of Widing and Levitt [167], who described numerous correlations between adiabatic ionization potentials and Taft s inductive substituent constants (an indirect way of correlating IPs with charges) and in a note by Streitwieser [168], who justified to some extent a dependence of IPs on local charge densities. 

A study of the selectivity of lariat aza[15] crown-5 (3.133) and BiBLE (bi-brachial lariat ether) diaza[18] crown-6 derived ligands (3.134) containing variously substituted arene rings showed that the Ag+ complex stability decreased as a function of the electron-withdrawing ability of the substituents, R, as measured by the substituents Taft o° inductive constant (a parameter measuring the electron-withdrawing... 

Substituent Hammett constants Inductive constants Taft constants a Swain-Lupton constants ... 

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. 

The Ei parameters correlate with the Taft s inductive or constants of the substituents (Table 44) [347]. As a consequence, one can observe a correlation between the redox potential E values and the sum of the inductive constants for all the substituents (Figs. 44-51). 

Taft s inductive constants for substituents are rather obvious forms of the correlation constants for macrobicyclic systems, since they are determined for structurally similar organic compounds, namely, bicyclic esters of the type... 

The Taft-Lewis inductive constant Oj was proposed [Taft and Lewis, 1958] to measure the inductive effect in aliphatic series on a scale for direct comparison with aromatic o values, derived from the o constant as ... 

Taft-Lewis inductive constant - electronic substituent constants (O inductive electronic constants)... 

The force constant (K) for t(COXY), v for the carbonyl halides, shows a good correlation with a linear expression involving Taft s inductive (ff]) and resonance (ag) parameters for X and Y [604] K = 0.233 + 0.199J(Ti - 0.268J(Tr. More remarkable is the linear correiation found, for the same molecules, between < ... 

For meta- and para-substituted phenols, log values spread over 2 log units from 3-dimethylaminophenol to 4-nitro-3-trifluoromethylphenol. Their order is well explained by classical electronic effects. A dual-substituent parameter analysis gives equations 16 and 17, where crp and ctr are the Taft field-inductive and resonance substituent constants, respectively. 

Taft constant (or electronic constant, Taft polar constant) was proposed by Taft [Taft, 1956] to measure the inductive effect in the aliphatic series ... 

LINEAR FREE ENERGY RELATIONSHIPS Hammett substituent constants Taft substituent constants Inductive and resonance substituent constants Steric substituent constants Homolytic substituent constants Field and resonance components Aqueous solubility The parachor... 

R = naphthalene, quinoline, or pyridine, have allowed examination of the inductive effect on electron transfer reactivity. Where the substituent R is methyl or a fluorinated methyl derivative, estimated precursor association constants vary little and thus provide a direct comparison A linear dependence of et on the Taft CT inductive parameter was found for acetato and formato complexes. Although redox potentials for these systems are not readily available, it is thought that they may be correlated with the inductive parameter. Thus —CF3, the most electron-withdrawing of the groups, increases the potential most. 

A comparison of these scales with Taft s induction constants a and Taft s steric constants for different alkyl groups leads to the conclusion that both electronic and steric factors influence the doublebond activity in stereospecific catalysis but that steric factors seem to be more important. An especially significant decrease in the olefin activity was found for olefins branched vicinally to the double bond (3-methyl-pentene-1,3-methylbutene-l, vinylcyclohexane, styrene). This is protebly connected with the space limitations for monomer coordination in the stereospecific active sites (IS, 16). 

As suggested by Roberts and Moreland many years ago (1953), the acidity constants of 4-substituted bicyclooctane-l-carboxylic acids provide a very suitable system for defining a field/induction parameter. In this rigid system the substituent X is held firmly in place and there is little possibility for mesomeric delocalization or polarization interactions between X and COOH (or COO-). Therefore, it can be assumed that X influences the deprotonation of COOH only through space (the field effect) and through intervening o-bonds. On this basis Taft (1956, p. 595) and Swain and Lupton (1968) were able to calculate values for o and crR. 

In the form of treatment developed by Taft and his colleagues since 195660-62, the Hammett constants are analyzed into inductive and resonance parameters, and the sliding scale is then provided by multiple regression on these. Equations 5 and 6 show the basic relationships. 

Oj and aR are respectively the inductive and resonance constants of Taft s analysis of ordinary composite Hammett constants (values obtained by Charton were used) and v is the steric substituent constant developed by Charton161,162. The intercept term, h, notionally corresponds to log k for H as an orfho-substituent, but is not found in practice always to agree closely with the observed value of log k for the parent system. 

Taft (21) has suggested that the electrical effect of a substituent is composed of localized (inductive and/or field) and delocalized (resonance) factors. Thus we may write the substituent constant of the group X as... 

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