Taft analysis

The kinetics of the oxidation of a series of para- and meto-substituted benzaldehydes by quinolinium chlorochromate are first order in substrate, oxidant, and hydronium ion the results were subjected to a Taft analysis. Oxidation of 2-pyridinecarboxaldehyde to the acid by dichromate follows an unusual mixed fourth-order rate law it is first order in hydronium ion and Cr(VI), and second order in aldehyde. 

The Hammett-Taft analysis of the redox potentials gave a good correlation using parameters. Also, the dependence was additive for the disubstituted ferrocenes,... 

FIGURE 19. The good correlation found between the fluorescence maximum of 2N and solvent polarity using Kamlet-Taft analysis in 22 solvents. In this case no level crossing is evident and the emitting state is assumed to be Z-i, in all solvents (from Reference 91)... 

This problem may be tackled by a more systematic analysis of the Stokes shift. Pines and coworkers assumed that the first absorption transition of 1-naphthol and the two absorption transitions of 2-naphthol may be described by Pekarian functions. These functions were analyzed by the Kamlet-Taft analysis (Figures 25 and 26). 

The Taft analysis has the working disadvantage that it requires two measurements to define a a value, and it also suffers from the problem that the parameters of some substituents cannot be obtained either because the ester decomposes too quickly for measurements to be made or because it would not decompose by ester hydrolysis. Such restrictions apply to the halogen substituents, nitrile or the nitro group, which would require study of such compounds as Hal-CO-OR, NC-CO-OR and O2N-CO-OR. This problem can be solved by use of Taft analysis of esters of the type X-CH2-CO-OR instead of X-CO-OR. In this analysis the similarity coefficient, p, for the substituted acetic acid derivatives is attenuated by 0.41 from the set value of p = 2.48 for the formic acid derivatives (Equations 8 and 9). The a constants based on formic acid derivatives are recorded in Table 1 in Appendix 3. ... 

The reader should be aware that a set of a constants has been defined from Taft analysis of acetic acid derivatives with p set at 2.48. There is an obvious problem with comparing the definitions as the formate standard refers to as zero (Oh = 0,49) and the acetate standard refers to a H as zero (o Me = -0.11). Indeed the simplest way of deciding which ct is tabulated is to check its magnitude for hydrogen as a substituent. 

A note of caution must be sounded with regard to a values for alkyl groups. When CT is substantial there is little doubt that it represents the inductive effect of the group small values might easily result from neglect of the very small steric effect difference in the Taft analysis [60,63,64]. In the event of the use of a in correlations the small but different values for alkyl substituents probably do not make a significant difference. 

As mentioned in Section 1, the occurrence of steric as well as polar effects in aliphatic systems and orr/io-substituted aromatic systems complicates correlation analysis. Little progress in developing LFER in this area was made until R. W. Taft in 1952 devised a procedure for separating polar, steric, and resonance effects, based on an analysis of the rate coefficients of basic and acidic hydrolysis of esters. Considerable use has been made of this Taft analysis and of substituent parameters developed from it. 

We now summarize a few aspects of later developments of the Taft analysis and parameters. Equations (14) and (16) have sometimes been applied in forms modified to include a role for the effect of C-H and C-C hyperconjugation on reactivityEssentially the same matter has been addressed in coimection with the Es scale itself For instance, as long ago as 1961 Hancock introduced a scale, in which a correction for the supposed role of hyperconjugation in the standard reaction had been incorporated. The subsequent applications of such a scale necessarily involved consideration of the role... 

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