Taft parameters

This approach to separating the different types of interaetions eontributing to a net solvent effeet has elieited much interest. Tests of the tt, a, and p seales on other solvatochromie or related proeesses have been made, an alternative tt seale based on ehemieally different solvatochromie dyes has been proposed, and the contribution of solvent polarizability to ir has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system eonstitutes the best or ultimate extrathermodynamie approaeh to the study of solvent effeets. There are two objections One of these is to the averaging process by which many model phenomena are eombined to yield a single best-fit value. We eneountered this problem in Section 7.2 when we eonsidered alternative definitions of the Hammett substituent eonstant, and similar eomments apply here Reiehardt has diseussed this in the eontext of the Kamlet-Taft parameters. - The seeond objeetion is to the elaim of generality for the parameters and the eorrelation equation we will return to this eontroversy later. 

Table 20. Kamlet-Taft parameters and corresponding ET(30) values.

Figure 4. Proton affinities (PA) of carbenes from Table 7 (circles) and Table 8 (squares) as a function of Taft parameters for proton transfer equilibria.110,150 The coefficients for Ecra, Ectf, and Ectr+ were optimized, with PA( CH2) = 206 kcal/mol constant (r2 = 0.897).

A direct comparison of the stereochemical efficiency of the fragmentation reaction versus the tandem reaction (Scheme 53) was studied by Porter et al. as a function of the steric effect based on the Taft parameters for different substituents [146]. In general, the tandem reactions perform better and provide higher levels of ee s than the fragmentation reactions. This effect could be due to the tinbromide by-product catalyzing a non-stereoselective process as has been uncovered by the same authors (vide supra) and by Sibi and Ji in their diastereoselective studies [147]. 

From a practical viewpoint, Ej values are quickly and easily obtained, giving a very useful and convenient scale. However, a general polarity scale based on a single probe molecule has its limitations because a single compound cannot experience the diversity of interactions that the whole range of solvents can offer. The Kamlet-Taft parameters a, /3 and n tackle this problem by using a series of seven dyes to produce a scale for specific and nonspecific polarity of liquids [23], Whilst it undoubtedly gives a more detailed description of the solvents properties,... 

Table 3.2 Kamlet-Taft parameters for three perfluorinated...

Here, p measures the sensitivity of the redox potential to substitution at a ligand L, and Eu is the sum of the Hammett, or Taft, parameters of the substituents attached to L. The symbols R, T, n, F have their usual meaning. 

While the development of the Taft parameter is similar to that of Hammett and Hansch, / )-val ucs are based on rate constants instead of equilibrium constants. The Taft parameter is a measure of changes in activation energy, not standard free energy. Of the Hammett, Hansch, and Taft parameters, the Taft parameter is utilized the least in QSAR studies. Other steric parameters have been developed over time, and like the Taft parameter, all have shortcomings. One alternative steric parameter was developed by Marvin Charton of Pratt Institute in New York. Charton s parameter is based on the van der Waal radius of a substituent.6 Another alternative steric model is the STERIMOL parameter set developed by Arie Verloop of Philips-Duphar in Holland.7 Unlike Taft and Charton, Verloop... 

SOLUTION Ethyl formate (12.12, R = H) should hydrolyze faster than ethyl acetate (12.12, R = CH3) because steric hindrance about the carbonyl is lower in ethyl formate. The question is How much faster The only equation provided in this section on Taft parameters is Equation 12.17, and that is where we will begin. In this case, we are solving for the ratio of the hydrolysis rate constants, kR/kcu, . / ( n is known and equals +1.24 according to Table 12.4. Ethyl formate hydrolyzes 17.4 times faster than ethyl acetate. 

With Equation 12.20 along with the Hammett constants and Taft parameters in Table 12.5, one can predict... 

Hansch analysis Hansch analysis is a common quantitative structure-activity relationship approach in which a Hansch equation predicting biological activity is constructed. The equation arises from a multiple linear regression analysis of both observed biological activities and various molecular property parameters (Hammett, Hansch, and Taft parameters). 

Taft steric parameter A Taft steric parameter describes the physical size of a substituent on a parent molecule. Taft parameters, denoted as Es, are often used in Hansch analyses to link lipophilicity changes to biological activity, tag A tag is a signal molecule that is attached to a bead to facilitate deconvolution in molecular libraries prepared by split... 

Kamlet/Taft Parameters for a Selection of Ionic Liquids and Molecular... 

Ionic liquids (Coni.) cations used for, 253 characteristics of, 252-253 chemical and physical properties, 251 green credentials, 253 hydroarylations of alkynes in, 273 Kamlet/Taft parameters for, 255 oxidation, 278-279... 

Kamlet/Taft parameters for ionic liquids, 255 for molecular solvents, 255... 

Kamlet-Taft parameters are known to express three distinct measures of the solvent polarity such as dipolarity/polarizability hydrogen-bond acidity (a) and hydrogen-bond basicity (/S). These parameters have been determined by absorption measurements for individual or pairs of the following dye molecules N,N-diethyl-4-nitroaniline, 4-netroaniline and Reichardt s dye, as seen in Figure 3.4 [81-83]. 

Welton reported the effect of cations and anions on the Kamlet-Taft Parameters [78]. The Kamlet-Taft parameters for some ILs are summarized in Table 3.9. As seen, n values for these ILs are high, 0.9-1.3, in comparison with those for protic molecular solvents as shown in the same table. Both cation and anion affect the n value. For anions, the n value for ILs having TFSI anion is low due to weakened coulombic interaction caused by delocalized anionic charge. 

Table 3.9 Kamlet-Taft parameters for typical ionic liquids...

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). 

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