Taft polar constants

Taft polar constant Taft a constant -> electronic substituent constants (O inductive electronic constants)... 

Kinetics of the reaction of diazodiphenylmethane (92) in a wide range of alcohols with pyridine and pyridine-A -oxide 3- and 4-carboxylic acids (84)-(87), 4-substituted benzoic acids (88)," cw -substituted cinnamic acids (89), 2-(4-phenyl-substituted)cyclohex-l-enyl carboxylic acids (90), and 4 -substituted-biphenyl-2-carboxylic acids (91)" have been reported. Comparison of the new results for 4-substituted benzoic acids with the published results of data for 3-substituted benzoic acids was made, " and it was concluded that the most important solvent property influencing the rate of reaction appears to be the polarity of the alkyl group expressed as Taft s polar constant a. Transmission coefficients in the cinnamic acids (89) were compared with those in the bicyclic acids (90) and... 

The structure of the reactant also influences the orientation, i.e. the ratio of 1- to 2-alkenes in the dehydration of 2-alkanols and the ratio of cis to trans alkenes. Table 4 shows that these ratios can also be correlated by the Taft equation. For the cis/trans ratio, a better fit was obtained with steric Es constants of substituents than with polar constants [127]. 

Px is the partition coefficient of a derivative and PH is that for the parent compound.) Also used were Hammett s o- constant, Taft s polar constant, steric parameter, Es. In a few examples (Equations 17, 21, 24, and 30), P values from oleyl alcohol/water have been used. In one instance (Equation 69) the chemical shift of a phenolic proton has been used for comparison with the a constant. Where possible, the experimentally measured partition coefficients for all members of the series have been used. In other instances only one member of a set has been measured. Values for the other members were obtained by taking advantage of the additivity principles of log P and tr. Details are given elsewhere (4, 7, and 8). For the new work of Table II, log P values for the parent compounds are given in the footnotes. 

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

In particular, called resonance polar effect [Taft, 1956] is defined for any benzene derivative where there is no direct conjugation between substituent and reactive it can be considered constant for a particular solvent, therefore expressing resonance interactions between substituent and skeletal group. 6r is usually referred to as the effective resonance constant and of hold for electrophilic and nucleophilic reaction series, respectively. 

Oj electronic constants of Dewar-Grisdale (37-40) represent o values determined from j-XCioHgCOOH substituted compounds (1-naphtoic acids) withj=3, 4, 5, 6, 7 the corresponding percentages of resonance (D%) are 29, 57, 38, 43, and 48. values (27) were estimated from the Cl quadrupole resonance frequency/in ortho-substituted chlorobenzenes it is related to the Taft O polar constant, aj values (34) were estimated from the integrated intensity A of the Vi6 band in IR spectra. 

The use of computer techniques in the correlation of biological activity with substrate physical-chemical properties has received much attention in the area of medicinal chemistry. The use of these techniques, denoted Quantitative Structure Activity Relationships (QSAR), were developed mostly by Hansch and his coworkers eind have been reviewed by Tute te), Purcell et. al. (9) and Dunn (10). These techniques were utilized by Greenberg (1 l) ln the correlation of odor threshold and suprathreshold data with Log P, the log (n-octanol/water partition coefficient). In the same study it was reported that steric and polar effects as measured by the Taft Steric and Polar Constants poorly correlated with odor intensity data. 

Linear Free Energy Relationships The observation that familiar polar effects influence the orientation ratios for radical addition to olefins prompts the application of the well established free-energy relationships. The orientation data can be accommodated in a Hammett plot by assigning a polar constant, a, to each radical (14). This is defined as the sum of the Hammett sigma constants for the groups attached to the radical centre. In practice, Taft a constants have been used (45). 

It was found (32) that in the acid range (pH 4—6) the alkyl group does not influence the rate of decomposition, which is similar for all xanthates. In the alkaline range the rates are markedly influenced by the substitutional group, and the rates could be correlated with the Taft polar substituent constants estabhshed for the various groups. 

Solvent effects on chemical equilibria and reactions have been an important issue in physical organic chemistry. Several empirical relationships have been proposed to characterize systematically the various types of properties in protic and aprotic solvents. One of the simplest models is the continuum reaction field characterized by the dielectric constant, e, of the solvent, which is still widely used. Taft and coworkers [30] presented more sophisticated solvent parameters that can take solute-solvent hydrogen bonding and polarity into account. Although this parameter has been successfully applied to rationalize experimentally observed solvent effects, it seems still far from satisfactory to interpret solvent effects on the basis of microscopic infomation of the solute-solvent interaction and solvation free energy. 

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. 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

Although c is an e irical paraseter, it can be correlated to sore general solvent strength scales, such as the Kavlet-Taft constants [251] and Reichardt s E,(30) scale of solvent polarity [252]. 

In the nineteen sixties, there was some confusion about the value of p, the reaction constant for polar effects of alkyl groups on bromination rates, as calculated according to Taft s equation, log (k/k0) = p a. For 22 alkenes substituted by one, two or three linear alkyl groups, there is a satisfactory relation (16) between their reactivities, log k, in methanol and the sum of their... 

A multi-parameter approach is preferable and the re scale of Kamlet and Taft (Kamlet et al., 1977) deserves special recognition because it has been successfully applied to the positions or intensities of maximal absorption in IR, NMR, ESR and UV-visible absorption and fluorescence spectra, and many other physical or chemical parameters (reaction rate, equilibrium constant, etc.). Such observables are denoted XYZ and their variations as a function of solvent polarity can be expressed by the generalized equation... 

---