Taft’s equation

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... 

Decisive role of the VM factor during the adsorption process of the solvents by coal is in agreement with the determined in the work [25] proportionality for the alcohols between Igk and steric factor Es of the Hammet-Taft s equation. 

The rate constants of the reaction of 2,6-dimethyloct-7-en-2-ol separately with ozone and hydroxyl radical, in the gas phase, have been determined. The OH radical can either abstract hydrogen or add to the double bond. Ozone adds to the double bond. The formation of acetone, 2-methylpropanal, 2-methylbutanal, ethanedial, and 2-oxopropanal was discussed.191 The rate laws and activation parameters for the ozone oxidation of alcohols in aqueous solution have been determined and explained on the basis of formation of an ozone-alcohol complex.192 The reactivity of alkenes towards ozone, in aqueous solution, correlates well with Taft s equation.193... 

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). 

The participation of polar effects can be studied adequately by Taft s equation (14) ... 

Hammett s reaction constants using a values, unless otherwise indicated. By use of the Yukawa and Xsuno equation (see text), c p values by use of Taft s equation (see text). 

The fundamental assumption of SAR and QSAR (Structure-Activity Relationships and Quantitative Structure-Activity Relationships) is that the activity of a compound is related to its structural and/or physicochemical properties. In a classic article Corwin Hansch formulated Eq. (15) as a linear frcc-cncrgy related model for the biological activity (e.g.. toxicity) of a group of congeneric chemicals [37, in which the inverse of C, the concentration effect of the toxicant, is related to a hy-drophobidty term, FI, an electronic term, a (the Hammett substituent constant). Stcric terms can be added to this equation (typically Taft s steric parameter, E,). 

Tafel equation Tafel kinetics Tafel slope Taffy process Taft s SV function Tagamet [51481-61-9] d-Tagatose... 

The kinetics of the ethylene hydration reaction have been investigated for a tungstic oxide—siHca gel catalyst, and the energy of activation for the reaction deterrnined to be 125 kJ/mol (- 30 kcal/mol) (106,120). The kinetics over a phosphoric acid-siHca gel catalyst have been examined (121). By making some simplifying assumptions to Taft s mechanism, a rate equation was derived ... 

Then the AH values of sulphoxides and amides were correlated with Taft s a affording the following equation ... 

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 . ... 

To verify such a steric effect a quantitative structure-property relationship study (QSPR) on a series of distinct solute-selector pairs, namely various DNB-amino acid/quinine carbamate CSPpairs with different carbamate residues (Rso) and distinct amino acid residues (Rsa), has been set up [59], To provide a quantitative measure of the effect of the steric bulkiness on the separation factors within this solute-selector series, a-values were correlated by multiple linear and nonlinear regression analysis with the Taft s steric parameter Es that represents a quantitative estimation of the steric bulkiness of a substituent (Note s,sa indicates the independent variable describing the bulkiness of the amino acid residue and i s.so that of the carbamate residue). For example, the steric bulkiness increases in the order methyl < ethyl < n-propyl < n-butyl < i-propyl < cyclohexyl < -butyl < iec.-butyl < t-butyl < 1-adamantyl < phenyl < trityl and simultaneously, the s drops from -1.24 to -6.03. In other words, the smaller the Es, the more bulky is the substituent. The obtained QSPR equation reads as follows ... 

Ab initio SCRF/MO methods have been applied to the hydrolysis and methanol-ysis of methanesulfonyl chloride (334). ° The aminolysis by aromatic amines of sulfonyl and acyl chlorides has been examined in terms of solvent parameters, the former being the more solvent-dependent process.Solvent effects on the reactions of dansyl chloride (335) with substituted pyridines in MeOH-MeCN were studied using two parameters of Taft s solvatochromatic correlation and four parameters of the Kirkwood-Onsager, Parker, Marcus and Hildebrand equations. MeCN solvent molecules accelerate charge separation of the reactants and stabilize the transition... 

For the same reaction under similar conditions and on supported H3P04, another equation was applied [287]. It is based on Taft s mechanism for homogeneously catalysed olefin hydration [291,292] [see scheme (B), p. 327] according to which water enters into the reaction scheme in a fast step which follows the rate-determining step and therefore appears in the rate expression in a negative term, viz. 

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)... 

Data listed in Table 2 include the substituent constants R1 of trialkylchlorosilanes and the relative rate constants fc(R1Me2SiCl)/A (Me3SiCl) for the reactions of the two chlorides with lithium silanolates and isopropylate (equation 39)57. The reaction rates of silanes are influenced almost exclusively by the steric effects of the alkyl groups attached to the silicon atom. The log(A rei) values of the compounds with various R1 groups give a satisfactory correlation with Taft s Es values151. Thus the steric hindrance of silyl groups follows the order listed in entry 4457 of Table 1. 

In the oxidation of alkanethiols to disulfides with chloramine-T (CAT), in alkaline solution, the proposed reactive species are hypochlorous acid and TsNCl- anion. A correlation of reaction rate with Taft s dual substituent parameter equation yielded p = -5.28 and 5 = -2.0, indicating the rate-enhancing effect of electron-donating substituents.133 Michaelis-Menten-type kinetics have been observed in the oxidation of atenolol with CAT in alkaline solutions. TsNHCl is assumed to be reactive species. A mechanism has been suggested and the activation parameters for the rate-determining step were calculated.134 The Ru(III)-catalysed oxidation of diphenyl... 

The oxidation of substituted benzylamines by pyridinium hydrobromide perbromide is of first order with respect to both the oxidant and the amine. The oxidation of deuter-ated benzylamine exhibited a kinetic isotope effect (kH/kD = 3.20 at 303 K.). The rates showed excellent correlations in terms of Taft s and Charton s multiparametric equations. A mechanism has been proposed.135... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

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