Abboud-Kamlet-Taft solvent parameters

More recent solvatochromic studies [14-20] employ specialised dyes reflecting specific microscopic molecular interactions. For example, the Abboud-Kamlet-Taft solvent parameters [21-23] a, 3 and n give information on hydrogen bond donor and acceptor properties and the polarisability of a compound, respectively. For ionic liquids with [C4mim]-cations, the following order of [3- values was estab-... 

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. 

Palm s group has continued to develop statistical procedures for treating solvent effects. In a previous paper, a set of nine basic solvent parameter scales was proposed. Six of them were then purifled via subtraction of contributions dependent on other scales. This set of solvent parameters has now been applied to an extended compilation of experimental data for solvent effects on individual processes. Overall, the new procedure gives a signiflcantly better flt than the well-known equations of Kamlet, Abboud, and Taft, or Koppel and Palm. 

As is shown below, the polarity measured by A T(30) for a protic solvent shows its ability to donate a hydrogen bond to a solute in addition to its polarity per se. A different solvatochromic polarity parameter, that is devoid of this complication (but has others), is Kamlet and Taft s 7t (Kamlet, Abboud and Taft 1977). This is based on the average of values of the 7C —> transition energies for several... 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

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). 

Both Kamlet, Abboud, and Taft et al. s [224, 226] and Swain et aVs [265] multiparameter solvent effect treatments have an inherent weakness in so far as the solvent parameters a, and n as well as and are averaged and statistically optimized parameters the former are derived from various types of solvatochromic indicator dyes. 

Many different solvent parameters and multiparameter equations have been introduced in this Chapter 7. Certainly, only a few of them will survive the test of applicabihty and aeeeptanee by organic chemists. Indeed, the preference for certain time-tested solvent seales and multiparameter treatments is already clearly discernible. Amongst the one-parameter seales, the t(30) or Ej seale and the DN scale have frequently been used, while the Kamlet-Abboud-Taft (KAT) LSER approach seems to be the most widely applied multiparameter approach. 

Over the last few years, the development of solvents of desired properties with a particular use in mind has been challenging. To evaluate the behaviour of a liquid as solvent, it is necessary to understand the solvation interactions at molecular level. In this vein, it is of interest to quantify its most relevant molecular-microscopic solvent properties, which determine how it will interact with potential solutes. An appropriate method to study solute-solvent interactions is the use of solvatochromic indicators that reflect the specific and non-specific solute-solvent interactions on the UV-Vis spectral band shifts. In this sense, a number of empirical solvatochromic parameters have been proposed to quantify molecular-microscopic solvent properties. In most cases, only one indicator is used to build the respective scale. Among these, the E (30) parameter proposed by Dimroth and Reichardt [23] to measure solvent dipolarity/polarisability which is also sensitive to the solvent s hydrogen-bond donor capability. On the other hand, the n, a and P (Kamlet, Abboud and Taft)... 

Here, n is the dipolarity and polarizability of the solvent (so-called Kamlet-Abboud-Taft solvatochromic parameter), a the hydrogen-bond donor (HBD) acidity, and P the hydrogen-bond acceptor (HBA) basicity of the solvent, whereas s, a, and h are the (so-called) susceptibilities of the solute with reference to tt, a, and / , respectively. In the absence of hydrogen bonds between the solvent and the... 

Figure 19. Solvatochromic parameters of five supercritical fluids from measurements of Maiwald compared with literature data for some selected liquid solvents according to Kamlet, Abboud, and Taft. Filled symbols refer to the probe molecules 4-nitroaniline / N,N-dimethyl-4-nitroaniline (NH-hydrogen bonding), open symbols to 4-nitrophenol / 4-nitroanisole probes (OH-hydrogen bonding) adapted from [75].

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. 

Abraham, in collaboration with Taft, Kamlet, and Abboud, has made an interesting attempt to characterize the nature of a solvent in terms of its H-bond donor ability (HBD or), H-bond acceptor ability (HBA jS), and its specific dipolarity/polarizabiUty (jt ). He has used these parameters in multiparameter relationships to analyze medium effects on a variety of chemical processes. Briefly, the HBD propensities (a) were obtained from the enhanced sol-vatochromism of the t(30) probe relative to 4-nitroanisole, the HBA propensities (j8) from the enhanced solvatochromism of 4-nitroaniline relative to N,N-diethyl-4-nitroaniline in HBA solvents, and the rt values from solvent effects on the n n transition of nitro-substituted aromatic compounds. For a more detailed discussion of these parameters and their apphcations, the reader is referred to the original literature. ... 

In most instances, the components of a reaction will be soluble in a large number of solvents, and the main concern is usually not in what solvent to dissolve the reactants, but rather what solvent will promote the reaction and provide the most product in the shortest period of time. Polarity parameters based on the UV absorbance of solvatochromic dyes are suitable for correlations with kinetics and equilibria. Because the wavelength of the dye absorption is determined by the relative energy difference between the ground state and the excited electronic state of the probe molecule, it mirrors the variable energy levels of reaction components and intermediates in solution (Figure 3.3). Although several polarity scales of this type exist, the most reliable and broadly applicable are the Kamlet-Abboud-Taft... 

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