Kamlet-Taft parameters table

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

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

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...

Often the Kamlet-Taft parameter n is determined by measuring the wavelength of the absorption maximum vm ix of AOV-diethyl-4-nitroaniline in the respective solvent (Tables 2-5) ... 

Table 2 Kamlet-Taft Parameters obtained using Reichardt s dye, A,iV-diethyl-4-nitroaniIine as well as 4-nitroaniline...

Table 3 3 Kamlet-Taft parameter obtained with 3-(4-amino-3-methyl-phenyl)-7-pheyl-benzo[l,2-h 4,5-h ]-difuran-2,6-dione as a probe and n with 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)benzyhdene]A3-thiazohne (two evaluation methods, for details see [119])...

Table 4 Kamlet-Taft Parameters obtained Michler s ketone and tetracyanoethane [94] ...

Table 4.18. Comparison of Gutmann s donicity (DN) and acceptor-number (AN) scales with the Kamlet-Taft parameters tt, a, and p [Ta 81]...

Table 15.4 Acid/base ratios of the van Oss-Good parameters (+/-) from various sources as compared to the acid/ base ratios extracted from Kamlet-Taft parameters a/p (original values and normalized values from Lee)...

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.

The Lewis basicity or HB A ability of solvents is described by the Kamlet-Taft parameter, measured by means of the couples of solvatochromic probes 4-nitrophenol compared to 4-nitroanisole or 4-nitroaniline compared to 4-nitro-N, N-diethyaniline, the second probe of the couple serving to eliminate the effects of the solvent polarity and polarizability on its first probe. The resulting values of from the compilation of Jessop et al. [410], supplemented with data from Spange etal. [416] are shown inTable 6.16. An alternative measure of this property that has been applied by Schade et al. [413] to RTILs is Catalan s SB parameter, using the solvatochromic probe N.N-dimethyl-4-aminobenzodifuranone, and the resulting values are shown in Table 6.16. The SB values are linear with the values SB = 0.126 +1.056/. The basicity decreases in a series of RTILs with a common anion phosphonium > ammonium > pyrrolidinium > pyridinium > imidazolium, but more moderately than the acidity increases in the opposite direction shown above as shown by Spange et al. [416]. 

Table 1.8 Kamlet-Taft a, fi and tz parameters for selected solvents [1, 2]...

Table 9.1 Ej and Kamlet-Taft 7r polarity parameters for n-hexane and oxidized...

Hydrogen bond formation between dissimilar molecules is an example of adduct formation, since the hydrogen atom that is bonded to an electronegative atom, such as oxygen or nitrogen, is a typical acceptor atom. The ability of molecules to donate a hydrogen bond is measured by their Taft-Kamlet solvatochromic parameter, a, (or a . for the monomer of self-associating solutes) (see Table 2.3). This is also a measure of their acidity (in the Lewis sense, see later, or the Brpnsted sense, if pro tic). Acetic acid, for instance, has a = 1.12, compared with 0.61 for phenol. However, this parameter is not necessarily correlated with the acid dissociation constant in aqueous solutions. 

From the values of the Kamlet-Taft solvatochromic parameters (Table IV), R-134a is seen to behave as a moderately polar, weakly polarizable fluid with little or no basicity and weak acidity (of the order of dichloromethane). The negative value for P has been commented on previously (J) and may be an artifact of the original scale definition, the regression for which did not include heavily fluorinated species. In any case, the value suggests that R-134a is a poorer hydrogen bond acceptor than, for example, hydrocarbons. 

Solvent effects are not limited to polarity. Solvent molecules may act as electron pair donors or acceptors, as evidenced by the formation of charge transfer complexes or by the participation of solvent molecules as nucleophiles or electrophiles (Lewis bases or acids) in reactions. Solvent molecules may also behave as acids or bases in the Bronsted—Lowry sense, and they may play important roles in reactions by serving as hydrogen bond donors or acceptors. Kamlet, Taft, and co-workers also developed the parameter a as a measure of the ability of solvent to act as a proton donor in a solvent—solute hydrogen bond and the parameter j8 to describe the ability of solvent to act as a proton acceptor in a solvent-solute hydrogen bond. A compilation of e, IX, Z, Et(30), n, /3, and a values for selected solvents is given in Table 6.1. ... 

The three scales devised by Kamlet, Abboud and Taft have been used many times to formulate relationships between reaction rate constants and solvent polarity. These are known as linear solvation energy relationships (LSERs). The rate of amide formation for example, the most common single reaction in medicinal chemistry, is inversely proportional to jS for entropic reasons (Figure 3.4). Limonene and its derivative p-cymene were thus justified as excellent options for a renewable amidation solvent, not only in terms of performance but also because they are produced from a renewable feedstock. Other solvents are less suitable according to their solvatochromic polarity parameters (Table 3.3). As hydrocarbons, some aquatic toxicity concerns surround the use of limonene and p-cymene, but ideally these would be minimised with recycling. 

The polarizability of solvents, combined with their polarity, are measured by the Kamlet-Taft n values, obtained most profitably with the solvatochromic probe 4-nitroanisole and shown, as compiled by Jessop et al. [410] and supplemented with values from Kochly et al. [414] and Spange et al. [416] in Table 6.16. The jt values are in a narrow range, 0.95 0.15, i.e., the RTILs are all very polar as expected for ionic liquids. The disadvantage of having both these properties in raie parameter is removed by the separate Catalan parameters SP for polarizability and SdP for dipolarity. These use results from two solvatochromic probes a thiazoline one and 2-(4-(N,N-dimethylamino)benzylidene)malononitrile according to Schade et al. [413], and the values of SP and SdP are also shown in Table 6.16. The polarizability of RTILs is not affected much by the acidity or basicity of its constituent ions, but their polarity does depend on these properties. 

Physical organic chemists have tended to examine parameters based on shifts in the absorption peaks in the spectra of various dyes or indicator molecules. The a and P scales of Taft and Kamlet, the ET(30) scale of Dimroth and Reichardt, the 7t scale of Taft and co-workers and the Z value of Kosower are all examples of this type of parameter. The definitions and measurement means for these parameters, as well as important references, are shown in Table 5. An alternative definition of the Dimroth-Reichardt parameter is the dimensionless, ETN, which is now preferred by some organic chemists (for a discussion see Ref. 15). The Z value is important in that it led to the scale of Dimroth and Reichardt, which overcomes many of the limitations of the earlier scale. Several workers have shown that relationships exist, with good correlation coefficients, between similar parameters. Thus, DN is linearly related to p, both parameters being designed to measure the donor properties (or Lewis basicity) of solvent molecules. Also, Lr(30) is related to a as well as to AN all three parameters purport to measure the electron acceptor properties (or Lewis acidity) of solvent molecules. It has been found that different solvent types have different coefficients in linear relationships between n and the dipole moment. The Taft and Dimroth-Reichardt parameters, in particular, have been found to correlate with free energies and... 

A selection of Kamlet and Taft s solvatochromic parameters a, p, and n for 40 organic solvents, taken from a more recent comprehensive and improved collection [227, 294], is given in Table 7-4. 

Another important treatment of multiple interacting solvent effects, in principle analogous to Eq. (7-50) but more precisely elaborated and more generally applicable, has been proposed by Kamlet, Abboud, and Taft (KAT) [84a, 224, 226], Theirs and Koppel and Palm s approaches have much in common, i.e. that it is necessary to consider non-specific and specific solute/solvent interactions separately, and that the latter should be subdivided into solvent Lewis-acidity interactions (HBA solute/HBD solvent) and solvent Lewis-basicity interactions (HBD solute/HBA solvent). Using the solvato-chromic solvent parameters a, and n, which have already been introduced in Section 7.4 cf. Table 7-4), the multiparameter equation (7-53) has been proposed for use in so-called linear solvation energy relationships (LSER). 

The solvatochromic parameters are derived from spectroscopic and other measurements specifically designed to measure only a single interaction. In addition, the values are averages of the results from several solutes for each parameter and somewhat independent of solute identity. The most comprehensive solvatochromic treatment of solvent selectivity are the Tti, ai and Pi parameters of Kamlet and Taft, Table 4.15 [568-570, 578]. The rti value is an index of solvent dipolarity/polarizability, normalized to dimethyl sulfoxide = 1. The i scale of hydrogen-bond acidity measures the... 

Tfaeie have been a number of attempts to develop solvent parameter scales that could be used to correlate ttiermodynamic and kinetic results in terms of these patametois. Gutmann s Donor Numbers, discussed previously, are sometimes used as a solvent property scale. Kamlet and Taft and co-workers developed the solvatochromic parameters, Uj, B, and n that are related to the hydrogen bonding acidity, basicity and polarity, respectively, of the solvent. Correlations with these parameters also use the square of tte Hildebrand solubility parameter, (5, that gives the solvent cohesive energy density. Parameters for some common solvents are collected in Table 3.6. 

---