Kamlet Taft Parameters

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, and tt 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. 

---

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.

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

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...

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

Kamlet-Taft parameters have also been obtained for some perfluorinated solvents.They are not hydrogen bond donors (a is typically 0.0) and are typically extremely poor hydrogen bond acceptors ( 3 is small and negative). They are extremely difficult to polarize, and this leads to a strongly negative tt parameter. This last property explains why they typically form biphasic systems with organic solvents. 

The activity coefficient at infinite dilution of a solute y°° is related to the relative strength of an intermolecular interaction with the ionic liquid. The choice of the model solutes was based on their Abbout-Kamlet-Taft parameters [4, 75, 76], to exhibit prevailing forces with regard to the hydrogen bond acceptor, donor and polarisability properties. Similar probe molecules have been used in computational studies [62, 63],... 

Predictions based on Abboud-Kamlet-Taft parameters lead to better results, since the anion s basicity (0-value) and the cation s hydrogen bond donor ability (a-value) are assessed separately. For example, in the nucleophilic substitution of methyl-p-nitrobenzene sulfonate and a halide (Fig. 11) the hydrogen bond donor ability of the cation should be reduced in order to eliminate strong interactions with the halide. This was impressively demonstrated for the rates of reaction, which decreased according to [C4mpyr][BTA] > [C4dmim][BTA] > [C4mim][BTA] [149,150],... 

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. 

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) ... 

With n and T(30) known, the Kamlet-Taft parameter a can be easily calculated according to... 

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] ...

The observed absorption maxima for the compounds 1-4 shown in Scheme 9 could be related to the Kamlet-Taft parameters as follows ... 

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... 

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. 

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]...

---