Taft substituent constant

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)... 

RBR is the relative biological response, tt is defined as log Pg - log P where P is generally the water-octanol partition coefficent for the parent molecule and P is the partition coefficient for the molecule containing substituent S. The a s are standard Hammett or Taft substituent constants (26, 27). 

The asymmetric stretching frequency for 24 silver carboxylates [v COj)] was observed to correlate with the Taft substituent constant o. IR absorption data could be used to predict o values with a standard deviation of O.l.252... 

Further insights were obtained by analyzing the relative contributions of field, resonance, and polarizability effects to the barriers in a similar way as for the acidities, i.e., by correlating AAH = AH (CH3Y) - A//OCII4) with the respective Taft substituent constants according to Equation (29). The correlation is shown in Fig. 5 it yielded p =-22.6, = 9.81 and pj = 7.59 with... 

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. 

Also in 2-substituted ethanesulphonates,35 the 33S chemical shift has a reverse substituent effect and correlates with both Taft substituent constants and the chemical shift of the carboxylic carbon in related carboxylic acids. It seems that the substituent effect does not depend on conformation, but prevailingly on intramolecular electronic effects. Density functional theory (DFT) calculations of 33S nuclear shielding constants and natural bond orbital (NBO) analysis made it possible to conclude that substituents cause a variation in the polarization of the S-C and S-O bonds and of the oxygen lone pairs of the C — S03 moiety. This affects the electron density in the surroundings of the sulphur nucleus and consequently the expansion or contraction of 3p sulphur orbitals. 

In 1969, Clayton and Purcell (26) studied the structure-activity relationships of some l-decyl-3-carbamoylpiperidines against butyryl-cholinesterase. The equations in Figure 25 were found. In this case the activity is presented as the p/50, the negative logarithm of the concentration of derivative which inhibits butyrylcholinesterase 50%. The parameter o- is the Taft substituent constant which depends only on the net polar effect of the substituent. One compound of moderate activity was omitted from the calculations. The calculated equation was then used to predict the activity of this compound. The predicted p/50 was 5.00, that observed was 5.01 =t. 03. In fact, the activity of these compounds depends so strongly on the partition coefficient that the use of the much simpler equation (Equation 2) predicts the activity nearly as well. 

A number of early studies also found good linear relationships between the pK s of alcohols and the sum of their Hammett or Taft substituent constants, o or o [191,277,278]. Takahashi et al. found that the pK s of a large collection of alcohols could be estimated using either Taft substituent constants or the carbonyl frequencies of their esters [265]. Taft and co-workers [193] found a strong relationship (r = 0.999) between the gas-phase pKj,s ol a collection of OH acids and empirical substituent constants representing polarizability, field-inductive, and resonance effects. These workers also... 

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