Solvents basicity scale

As outlined in Section 1.3, the solvent acidity and basicity have a significant influence on the reactions and equilibria in solutions. In particular, differences in reactions or equilibria among the solvents of higher permittivities are often caused by differences in solvent acidity and/or basicity. Because of the importance of solvent acidity and basicity, various empirical parameters have been proposed in order to express them quantitatively [1, 2]. Examples of the solvent acidity scales are Kosower s Z-values [8], Dimroth and Reichard s Er scale [1, 9], Mayer, Gutmann and Gergefs acceptor number (AN) [10, 11], and Taft and Kalmefs a parameter [12]. On the other hand, examples of the solvent basicity scales are Gut-... 

For another approach to solvent basicity scales, see Catalan G6mez Couto Laynez J. Am. Chem. Soc. 1990, 112. 1678. 

A selection of SB values is collected in Table 7-5. Not unexpectedly, a satisfactory linear correlation exists between Catalan s SB values and Kamlet and Taft s y values (see Table 7-4) for 98 solvents (r = 0.928), with deviations for some aliphatic amines and ethers with long alkyl chains. For comparisons of the SB scale with further solvent basicity scales, see reference [336]. 

The averaged Kamlet-Taft solvent basicity scale / kt was established by averaging similar values of /J obtained within narrow limits for several homomorphic HBD and non-HBD... 

Replacement of the acid proton (N-H) by a methyl group in this molecular structure has the same side effects as a result, the compound will be similarly sensitive to the polarity and acidity of the medium, but not to its basicity owing to the absence of an acid site. Consequently, l-methyl-5-nitroindoline (MNI, 10) possesses the required properties for use as a homomorph of 5-nitroindoline in order to construct our solvent basicity scale (SB). The suitability of this probe/homomorph couple is consistent with theoretical MP2/6-31G data. Thus, both the probe and its homomorph exhibit the same sensitivity to solvent polar-ity/polarizability because of their similar dipole moments (p i = 7.13, = 7.31 D) and... 

Appropriate substitution in compound families such as amines and alcohols allows the entire range of the solvent basicity scale to be spanned with substances from sueh families. Thus, perfluorotriethylamine can be considered non-basic (SB=0.082), whereas N,N-dimethylcyclohexylamine is at the top of the scale (SB=0.998). Similarly, hexafluoro-2-propanol is non-basic (SB=0.014), whereas 2-octanol is very near the top (SB=0.963). 

Parameters describing and correlating the solvent abilities of liquids have been based on a great variety of chemical and physical properties. Some are measures of solvent basicity, and others are obtained from direct determinations of the solubility of a representative solute in a range of liquids. For example, the solubility of hydrogen chloride in liquids at 10 °C was used in this way by Gerrard and co-workers [69,70] and the resulting solvent basicity scale was compared with other scales by Arnett [71] and by Dack [72]. 

A few scales of Lewis affinity and some spectroscopic scales of Lewis basicity (see below) have been constructed by carrying out the reaction A - - B AB on a dilute solution of the acid in pure, liquid base as solvent. This pure base method will be studied in Chapter 4. It gives solvent basicity scales which are not strictly equivalent to solute basicity scales measured on a dilute solution of the acid and the base in an inert solvent. 

Catalan, J., Diaz, C., Lopez, V. et al. (1996) A generalized solvent basicity scale. The sol-vatochromism of 5-nitroindoline and its homomorph l-methyl-5-nitroindoline. Liebigs Ann., 1785-1794. 

Actually, many empirical parameters can be lumped into two broad classes, as judged from the rough interrelationships found between various scales. The one class is more concerned with cation (or positive dipole s end) solvation, with the most popular solvent basicity scales being the Gutmann DN, the Kamlet and Taft p, and the Koppel and Palm B. The other class is said to reflect anion (or negative dipole s end) solvation. This latter class includes the famous scales tt, a, Ej.(30), Z, and last but not least, the acceptor number AN. Summed up ... 

The a scale of solvent acidity (hydrogen-bond donor) and the (3 scale of solvent basicity (hydrogen-bond acceptor) are parameters derived from solvatochromic mea-siuements used in adsorption chromatography [51,54,55]. 

A third method of estimating solvent basicity is provided by the donor number concept 14 ). The donor number of a solvent is the enthalpy of reaction, measured in kcal per mole, between the solvent and a Lewis add such as antimony (V) chloride. (Other Lewis acids, such as iodine or trimethyltin chloride, may be used, but the scale most often reported is that for SbCl5.) Available values for the SbCls donor number have been included in Table 1. Plots of the Walden product versus solvent basicity (A//SbC1 ) for several solvents are shown for lithium, sodium, and potassium ions in Fig. 10 and for the tetraalkylammon-... 

The fact that equations (7) and (8) are related to the solvent, and indeed to the acid itself as the solvent, means that Xb or X -values apply to a particular solvent. It must therefore not be expected a priori that a basicity scale of unsaturated hydrocarbons formed by these values can immediately be transferred to another system. 

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. 

Figure 4.8 shows the potential windows obtained at a bright platinum electrode, based on the Fc+/Fc (solvent-independent) potential scale. Because of the overpotentials, the window in water is 3.9 V, which is much wider than the thermodynamic value (2.06 V). The windows for other solvents also contain some overpotentials for the reduction and the oxidation of solvents. However, the general tendency is that the negative potential limit expands to more negative values with the decrease in solvent acidity, while the positive potential limit expands to more positive values with the decrease in solvent basicity. This means that solvents of weak acidity are difficult to reduce, while those of weak basicity are difficult to oxidize. This is in accordance with the fact that the LUMO and HOMO of solvent molecules are linearly related with the AN and DN, respectively, of solvents [8]. 

It was later shown by Laurence and coworkers that there are significant systematic differences between P values of solvents obtained with indicators with an oxygen donor atom and those with a nitrogen donor atom (Nicolet and Laurence 1986). These authors recommended the use of a single indicator, preferably 4-nitrophenol relative to 4-nitroanisole or else 4-nitroaniline relative to 4-nitro-N,N-di/rae%>/aniline (rather than 4-nitro-N,N-die// y/aniline used by Kamlet and Taft 1976), to establish a basicity scale. The main point of difference is with respect to solvents that do not have an oxygen donor atom, such as amines, pyridines, and sulfides. In order to save the P scale, Kamlet and Taft proposed a family-dependent covalency parameter, equal to -0.20 for P=0 bases, 0.00 for C=0, S=0, and N=0 bases, 0.20 for -O-bases, 0.60 for pyridines, and 1.00 for amines, for use in linear free energy relationships (Kamlet etal. 1985). 

Iodoacetylenes as well as iodine cyanide are soft Lewis acids (Laurence etal. 1981), which interact with basic solvents yielding characteristic wavenumber shifts Av (C-I) (e g., for ICN relative to the wavenumber in CCI4 solutions). These shifts differ for soft solvents, with sulfur or selenium donor atoms or n systems, and hard solvents, with oxygen or nitrogen donor atoms. However, these authors have not converted this observation and their data to a solvent softness scale. In fact, if prorated values of A v (O-H), for phenol, relative to CCI4 solutions, see B0 H above, representing the hard basicity of the solvents, are subtracted, the remainder measures the solvent softness. Quantitatively,... 

Simplificaion of the acidity or basicity scale in low polar media consists of a series of equilibrium constants corresponding to the reaction above, with either B or HA being the reference compound. The acid-base association constants for the equilibria of substances interacting within these solvents, will be seen to fall mainly in the range logKBHA = 2 to 7 (Davies, 1968). 

Another remarkable Lewis basicity scale for 75 non-HBD solvents has been established by Gal and Maria [211, 212]. This involved very precise calorimetric measurements of the standard molar enthalpies of 1 1 adduct formation of EPD solvents with gaseous boron trifluoride, A//p gp, in dilute dichloromethane solution at 25 °C, according to Eq. (2-10a). 

Drago s EJC analysis and Gutmann s donor/acceptor approach [53, 67] have been compared [200, 217, 218], Eq. (2-12) has been extended for specific and nonspecific interactions between solutes and polar solvents [219]. Various Lewis acidity and basicity scales for polar solvents have been examined and compared by Fawcett, who concluded that the donor/acceptor scales of Gutmann seem to be the most appropriate [341],... 

Finally, an attempt was made to establish a measure of the electron-donating and electron-accepting power of organic solvents by means of infrared [72, 73] and F1 NMR measurements [73], Further empirical Lewis acid and base parameters will be discussed in Chapters 7.2... 7.5. A thorough and critical compilation of empirical solvent scales, including Lewis acidity and basicity scales, has recently been made for non-FlBD solvents [342],... 

Numerous acidity and basicity scales have been elaborated for water and other solvents. However, there is no one single scale of acidity and basicity, equally valid and useful for all types of solvents and applicable to both equilibrium and kinetic situations. Excellent reviews on different acidity functions are given by Boyd [60] and Bates [50]. 

Solution calorimetry of the molecular probes pyrrole, A-methylpyrrole, benzene, and toluene in 35 solvents has been used by Catalan el al. to determine a solvent HBA basicity scale, ranging from the gas phase to HMPT [31a]. Analogously, a solvent HBD acidity scale was derived calorimetrically using A-methylimidazole and A-methylpyrrole as probe molecules in 3 6 solvents, ranging from the gas phase to 2,2,2-trifluoroethanol [31b]. 

A compilation and critical comparison of various Lewis basicity scales for EPD solvents has been given by Persson et al. [144, 292]. 

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 ... 

The TLSER methodology has been successfully applied to develop correlation equations for a wide variety of solvent-dependent properties and processes [350, 364-369]. Some examples are the characterization of other solvent polarity, acidity, and basicity scales [364], the acidities of substituted acetic acids in various solvents [365], the basicities of substituted dimethylamines in various solvents [366], the decarboxylation kinetics of 3-carboxybenzisoxazole [367], the C=0 stretching frequencies of substituted pyrrolidin-2-ones [368], and gas-water distribution coefficients [369]. 

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