Taft’s polar 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... 

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 effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

Taft s polar substituent constant, a, applicable to aliphatic systems is based on the notion that free energy relationships can exist between rate processes as well as between rate and equilibrium processes (Equations 3-5, Chapter 1). A rate process can thus be employed as a reference reaction. A simple example of such a Class II rate-rate free energy relationship is shown in Figure 3 where the reaction of phenoxide ion with 4-nitrophenyl-substituted benzoate esters is compared with the corresponding reaction of hydroxide ion, which therefore becomes the reference free energy change. 

Since the lipophilic character was found important for physiological dispositions of barbiturates, several workers investigated the relationship between partition coefficient and structural parameters (Taft s polar and steric substituents constants, number of C-atoms) of these compounds.548,549 Partitioning of barbiturates was also proved to be a significant factor for their... 

The Hammett-type correlation for the rate constants of 5-allyl-5-R-barbiturates has been reported by Carstensen et al. and suggested for use in stability predictions.569 Similar correlations were also found for the hydrolysis of 5-arylidenebarbituric acids.363,567 Linear free energy relationships have also been reported for dissociation constants,45,51 polarographic half-wave potentials,570 fluorescence70 and luminescence phenomena,71 and 13C-NMR chemical shifts129 for different classes of barbituric acid derivatives. Application of the dual substituent parameters method in LFER analysis of barbiturates, using Taft s polar and steric constants for various chemical and physicochemical properties, was also evaluated.571... 

Pentadiene and allylbenzene are about 10 times as reactive as 1-butene. The effect on reactivity of an alkyl group, relative to a methyl group at the same position, is nearly independent of whether the alkyl group is bonded to the 2, 3, or 4 position of the 1-alkene chain. The effects of unbranched alkyl substituents or reactivity correlate well with Taft s polar substituent constants , but isopropyl and tertiary butyl groups have much larger deactivating effects... 

Other workers [161] found that the distribution coefficients of the barbiturates are affected not only by the mass of the substituents, but by their inductive effects on the hydrophilic portion of the molecule, as defined by Taft s polar substituent constants. 

It is claimed that multiple regression analyses, involving Taft s polar substituent constant or and Dubois steric parameter E, can satisfactorily account for the properties of alkyl cobaloximes (R = alkyl) and alkyl cobalamines in terms... 

Substituents can be introduced not only into the betaine solute, but also into the surrounding solvent molecules. Let us consider a betaine dye solution in various alcohols R-OH with different alkyl groups R. An enhanced hypsochromic shift of the solvatochromic absorption band with increasing hydrogen-bond donor ability of the alcohols should be obtained. This is indeed the case(25). As Figure 9 shows, even exists a linear correlation between the modified E.p-values, measured in nine different alcohols, and Taft s polar substituents constants for the alkyl groups R of these alcohols(25). 

Clayton and Purcell have reported excellent correlations using dipole moments, amide group moments, and Taft s polar substituent constants a ) along with hydrophobic parameters when applied to an homologous series of molecules.A regression of the butyrylcholinesterase inhibitory potencies of these mono(carbamoylpiperidino)decanes gave eq 5 in which... 

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 . ... 

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... 

Charton used statistical methods to investigate the contributions to Taft s s values of the size of the groups concerned and their polar effects. As a measure of group size he took the van der Waal s radius, rv, and he used o-, and crR as measures of inductive and mesomeric effects. Correlations of steric substituent constants were then carried out250 using the equation (for group x)... 

Taft equation Various equations are associated with R.W. Taft, but the term is most often used to designate the family of equations that emerged from Taft s analysis of the reactivities of aliphatic esters and that involved the polar substituent constant a and the steric substituent constant Es... 

The homogeneous rate constant for the anodically-induced proton loss has been estimated and shown to correlate with the electron donor/acceptor ability of tfie trans isocyanide ligand, as indicated by P, the Taft a polar (for the alkyl isocyanides) or the Hammett s Op-i- constant (for the aromatic ones) [39]. 

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