Basicity solvatochromic probes

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Acid-base interactions have been explored as a function of density in SCFs using solvatochromic probes. The basicity of SCF CO2 was found to be relatively constant with respect to density(13L although the polarizability/volume is a linear function of density. The acidity of SCF CHF3 was also observed to be density insensitive, over a range from 4 to 18 mol/L(S). Although it appears that hydrogen-bond interactions become fairly saturated at a relatively low density, further experimental and theoretical work is required to understand this behavior more quantitatively. 

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]. 

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. 

The primary standard betaine dye (44) is only sparingly soluble in water and less polar solvents it is insoluble in nonpolar solvents such as aliphatic hydrocarbons. In order to overcome the solubility problems in nonpolar solvents, the more lipophilic penta-t-butyl-substituted betaine dye (45) has additionally been used as a secondary reference probe [174]. The excellent Hnear correlation between the Ej values of the two dyes allows the calculation of t(30) values for solvents in which the solvatochromic indicator dye (44) is not soluble. Introduction of electron-withdrawing substituents e.g. Cl [323], F, CF3, C6F13 [324]) in the betaine molecule reduces the basicity of its phenolate moiety, which allows the direct determination of x(30) values for somewhat more acidic solvents. Moreover, the Hpophilic and fluorophilic penta(trifluoromethyl)-substituted betaine dye (46) is more soluble in nonpolar solvents e.g. hexafluoro-benzene) than the standard dye (44) [324]. Conversely, the solubility in aqueous media can be improved through replacement of some of the peripheral hydrophobic phenyl groups in (44) by more hydrophilic pyridyl groups, to yield the more water-soluble betaine dye (47) [325]. The Ej values of these new secondary standard betaine dyes correlate linearly with the x(30) values of (44), which allows the calculation of x(30) values for solvents in which only betaine dyes (45)-(47) are sufficiently stable and soluble for the UV/Vis spectroscopic measurements [324, 325]. 

Finally, the ambitious approach of Catalan et al. to introduce complete new comprehensive scales of solvent dipolarity/polarizabihty [SPP scale), solvent basicity SB scale), and solvent acidity SA scale) must be mentioned [296, 335-337]. These three UV/Vis spectroscopic scales are based on carefully selected positively solvatochromic and homomorphic pairs of probe dyes and include values for about 200 organic solvents for a recent review, see reference [296]. The molecular structures of the three pairs of homomorphic indicator dyes proposed are as follows ... 

A brief review has been presented of the correlation analysis of solvolysis rates 50 years later, i.e. since Grunwald and Winstein proposed their eponymous equation in 1948.111 -pije authors then propose a method of correlation analysis involving multiple regression on solvent scales SPP (polarity-polarizability), SA (acidity) and SB (basicity). These scales are based on the solvatochromism of suitable probes and were initially for pure (i.e. one-component) solvents, but have now been extended to binary solvent mixtures. This enabled the authors to present a correlation for the solvolysis rate constants of r-butyl chloride in 27 pure solvents and 147 binary solvent mixtures, having a correlation coefficient r = 0.990 and a standard error of the estimate s = 0.40. The most important term in the equation is that involving SPP next comes... 

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