Taft scales

The 7t scale of solvents (Kamlet-Taft scale) is intended to represent the solute-solvent interactions in the absence of strong forces such as hydrogen-bonding or dipole-dipole interactions. It is based on the n to n absorption of a series of seven solutes (six uitroaromatics, such as 4-uitroanisole, A, A -diethyl-3-nitroaniline, 1-ethyl-4-uitrobeuzeue, etc., aud 4-dimethylamiuobenzophenone) [51-53]. 

Of the existing solvent scales, the Kamlet and Taft n scale ( D appears appropriate for use with supercritical fluid solvents. The basis of the Kamlet and Taft scale is the effect of solvent polarity and polarizability on the energy of the n - it electronic transition of a solute probe molecule. Solvent molecules in proximity to a solute (the cybotatic region) differentially affect the electronic energy levels of solutes (the solvatochromic effect)... 

Since its introduction the Kamlet-Taft it scale has been applied to a large number of diverse liquid solvents ( 5). Several studies have shown that the it solvent scale is consistent with other popular measures of solvent strength (6J. It has also been shown that there are good correlations between the empirical measurements of the Kamlet and Taft scale and several theoretical models of solvent behavior (J). Use of the solutes chosen by Kamlet and... 

In spite of this interest of physical and biochemists, prior to the work of Hansch and Fujita there appears to have been no eifort to devise a scale of hydrophobicity analogous to the Hammett scale for electronic effects, or the Taft scale for steric effects. Part of this reluctance was due to the selection of the relevant solvent system for such a scale. In addition, biochemists who were interested in hydrophobic stabilization of protein structures needed only to consider the few hydrophobic amino acid side chains. 

A study of the quaternization of 2-alkyl- and 2,4-dialkyl-thiazoles and 2-alkyl-l,3,4-thiadiazoles has served to elucidate the applicability of the Taft steric parameters, E,. It appears that E, values from the Taft scale will correlate structural or reactivity data, provided that the groups concerned are in a structural environment which does not induce a significant conformational preference of the group. ... 

The basic premise of Kamlet and Taft is that attractive solute—solvent interactions can be represented as a linear combination of a nonspecific dipolarity/polarizability effect and a specific H-bond formation effect, this latter being divisible into solute H-bond donor (HBD)-solvent H-bond acceptor (HB A) interactions and the converse possibility. To establish the dipolarity/polarizability scale, a solvent set was chosen with neither HBD nor HBA properties, and the spectral shifts of numerous solvatochromic dyes in these solvents were measured. These shifts, Av, were related to a dipolarity/polarizability parameter ir by Av = stt. The quantity ir was... 

In the form of treatment developed by Taft and his colleagues since 195660-62, the Hammett constants are analyzed into inductive and resonance parameters, and the sliding scale is then provided by multiple regression on these. Equations 5 and 6 show the basic relationships. 

This subject has been associated with the development of the or and type scales almost from the start74 (see Section II.B), but the first paper in which sulfinyl and sulfonyl groups played a part appears to have been one by Taft and coworkers in 196367. The main object of this paper was to study the effect of solvent on the inductive order by 19F NMR measurements on a large number of mcta-substituted fluorobenzenes in a great variety of solvents. The relationship between the NMR shielding parameter and selected systems as equation 10 ... 

By comparing pKa and pKm values, Taft predicted the preferred sites for the hydrogen bond when two different functional groups are present and both may act as an acceptor for the hydrogen bond. Other applications of the p/CHB scale have also been described81. 

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 . ... 

Although c is an e irical paraseter, it can be correlated to sore general solvent strength scales, such as the Kavlet-Taft constants [251] and Reichardt s E,(30) scale of solvent polarity [252]. 

More advanced scale was proposed by Kamlet and Taft [52], This phenomenological approach is very universal as may be successfully applied to the positions and intensities of maximal absorption in IR, NMR (nuclear magnetic resonance), ESR (electron spin resonance), and UV-VS absorption and fluorescence spectra, and to many other physical or chemical parameters (reaction rates, equilibrium constant, etc.). The scale is quite simple and may be presented as ... 

Kamlet MJ, Abboud J-L, Taft R (1977) The solvatochromic comparison method. 6. The it scale of solvent polarities. JACS 99 6027-6038... 

Kamlet, M. J., A. Solomonovici, and R. W. Taft. 1979. Linear Solvation Energy Relationships. 5. Correlations between Infrared Ar Values and the (3 Scale of Hydrogen Bond Acceptor Basicities. J. Am. Chem. Soc. 101, 3734. 

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,... 

The scale of hydrogen-bond basicity p hb, initially developed by Taft and coworkers14, has been extended by Abraham and coworkers81, and is based on the formation of hydrogen-bond complexes of a base B with a hydrogen-bond reference donor as 4-fluorophenol in CCI4 at 25 °C ... 

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