Taft parameters, correlations

The observed deceleration (Table 20) is then essentially attributed to steric interaction in the transition state between the 2- or 4-alkyl group and the entering methylating agent. A good correlation between log k/kMe and the Taft parameter Es has been found for both series (equation 4). 

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. 

For attempts to improve the Px(30)/ r correlations using modified Kamlet-Taft parameters [nf and an), see references [234, 235]. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

The Ei parameters correlate with the Taft s inductive or constants of the substituents (Table 44) [347]. As a consequence, one can observe a correlation between the redox potential E values and the sum of the inductive constants for all the substituents (Figs. 44-51). 

Hence, the anion affects both the equilibrium position and the rate of reaction. This latter aspect has been previously correlated to the Abboud-Kamlet-Taft parameters of ionic liquids and organic solvents [176], where more basic anions gave lower rates of reaction. 

FIGURE 10. Correlation of the fluorescence spectra of 8-methoxy-l,3,6-tris(Af,Af-dimethylsutfona-mido)pyrene (MPTA) measured in pure solvents with Kamlet-Taft parameters. MPTA has similar electronic stmcture to HPTA but is much less affected by hydrogen-bonding interactions (from... 

FIGURE 26. Correlation of the peaks of Pekarian functions (energy scale) used to approximate the UV-vis absorption spectra of 2-naphthol (a) blue band and (b) red band with Kamlet-Taft parameters . See Figure 24 for details of the absorption spectra... 

Other parameters (molar refractivity of the molecules/substituents, molar volume, Taft s steric constant, and Verio op s sterimol parameters) also appear in several QSAR. In some cases, these parameters correlate all of the observed variations in activity, but they do not seem to play as important a role as hydrophobicity for the data sets that we have examined. 

The rates of epoxidation of cyclododecene with a series of aliphatic peroxy-acids have been correlated, using the Taft equation. The reaction constant (p ) was + 2.0 and the steric constant (6) was found to be essentially zero. A two-parameter correlation has been found for the effect of basicity and polarity of the solvent on the rate of epoxidation of propene with peracetic acid. Rate constants and activation parameters for the epoxidation of a number of cycloalkenes, including (11 R = H or COOMe), (12 R = H, Ph, or 2-furyl), (13), (14), and cyclo-octa-l,5-diene, have been measured. An isokinetic relationship was demonstrated, with the isokinetic temperature of 3 C. There was only a weak dependence of the rate on the structure of the alkene. 

A subsequent multiple linear regression analysis [138] focused on the Kamlet-Taft solvatochromic parameters, employing transfer Gibbs energies and enthalpies AG°t and AH°r) for 26 solvents. Standard molar Gibbs energies of transfer for nine univalent and six divalent small cations correlated well with the Kamlet-Taft parameters via linear solvation energy relationships of the form... 

This graph gives a selection of 14 (out of approximately 360) of the usual solvents above the baseline and seven more exotic solvents (supercritical CO2 and ionic liquids included) below. The 14 compormds, from left to the right with increasing solvent polarity, include apolar, aprotic (such as TMS, cyclohexene, or benzene), bipolar (such as acetone or DMF), and eventually bipolar, protic solvents (cyclo-hexanol, ethanol, phenol). Using the values, numerous solvent-dependent processes may be correlated with each other. Other measures that can be used for the estimation of miscibility/solvent power are the cohesive pressures, solubility parameters, dispersive forces, Kamlet-Taft parameters, etc. [6a,b]. Solvent combinations of exotic members and systems with more than two members are known and have been recommended, but their application has been concentrated in the lab because of economic disdavantages with their handling and recyclability/ separability [6b-e]. 

Cu was shown to agree well with the Kamlet-Taft parameter p, as it does for molecular solvents. This has important consequences for the idea of solvent design. Since pis correlated with gas-phase acidities of the conjugate acid of the anion and has been found to be an important influence in the rates of catalytic reactions, it is possible to design an IL with some idea of its likely effect on a catalytic reaction. This is the first time that a real possibility of designing the solvent for a reaction has arisen and it could prove to be a very powerful tool in the future. 

It is known that carbonyl oxides are predominantly formed at carbon atoms with electron-donating substituents [19]. Excellent correlations of the regioselectivities of MO fiiagmentation with electron donation by substituents (as measured by Hammett and Taft parameters) have been obtained, consistent with the effects expected for stabilization of a zwite-rionic carbonyl oxide [20], According to Ref. [23], for polyisoprenes the ratio between the two intermediates, DCI and MCI, is 64 36. 

Figure 7 Correlation between substituent effects (approximated by the Taft parameter ecrm. and the reversible potentials for reduction of nickel(II) dithioacetylacetonate complexes Ni(R-Sac R -Sac)2 (reproduced by courtesy Inorg. Chem. 1976, 75, 1118 American Chemical Society).

None of these scales has received universal acceptance by inorganic chemists, and it may be that the heterogeneity of the field will defy anyone to establish a truly general scale. As yet, there seems to be nothing as widely applicable as the Hammet and Taft parameters in organic chemistry. Within certain areas and types of applications, one finds one of these scales more often used than others, presumably because it has proven more successful in correlating information. 

An Example of a Change in Mechanism in a Solvolysis Reaction Studied Using cr" 452 A Swain-Lupton Correlation for Tungsten-Bipyridine-Catalyzed Allylic Alkylation 453 Using Taft Parameters to Understand the Structures of Cobaloximes Vitamin 612 Mimics 455 The Use of the Schleyer Method to Determine the Extent of Nucleophilic Assistance in the Solvolysis of Aryl vinyl Tosylates 459... 

Figure 9.2 Correlation of transition state bond order (n ) and the Kamlet-Taft parameter for the Menschutkin reaction (data from Table 9.2). The intercept has a value = 0.62.

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