Solvent basicity the SB scale

5-Nitroindoline (Nl, 9) possesses the above-described electronic and structural features. It is an N-H acid with a single acid site borne by a donor group whose free rotation is hindered by an ethylene bridge on the ring. However, if charge transfer endows the compound with appropriate acid properties that increase with electronic excitation, the basicity of the acceptor (nitro) group will also increase and the polarity of the compound will be altered as a result. Both effects will influence the electronic transition of the probe that is to be used to evaluate basicity. 

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 

A brief analysis of these SB data allows one to draw several interesting conclusions as regards structural effects on solvent basicity. Thus  

The potential family-dependence of our SB scale was examined and discarded elsewhere.  

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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]. 

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 ... 

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). 

Reported scales for describing solvent polarity [f(8,n), Jt " and S ], basicity (DN and P ) and acidity (AN and a ) were previously analyzed against our SPP, SA and SB scales in the originating references, so no fiirther comment is made here. 

Obviously, the difference in solvatochromism between NI and MNI [Av(solvent) = Ni will cancel many spurious effects accompanying the basicity effect of the solvent. In addition, NI and MNI possess several advantageous spectral features thus, they exhibit a sharp first absorption band that overlaps with no higher-energy band in aity solvent, and both have the same spectral envelope in such a band (see Figure 1 in ref. 72), which facilitates comparison between the two spectra and the precise establishment of the basicity parameter, SB. The basicity of a solvent (SB) on a scale encompassing values... 

The general SPP scale of solvent dipolarity/polarizability and the specific SB and SA scales of solvent HBA basicity and HBD acidity, respectively, are orthogonal to one another and they can be used in the correlation analysis of solvent effects in single- or, in combination with the others, in two- or three-parameter correlation equations, depending on the solvent-influenced process under consideration see also Section 7.7. Examples of the correlation analysis of a variety of other solvent-dependent processes by means of SPP, SB, and SA values, including those used for the introduction of other solvent polarity parameters, can be found in references [335-337, 340-342]. In particular, comparisons with Kamlet and Taft s n scale [340] and Winstein and Grunwald s Y scale [341] have been made. 

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... 

Catalan J (2009) Toward a generalized treatment of the solvent effect based on four empirical scales dipolarity (SdP, anew scale), polarizability (SP), acidity (SA), and basicity (SB) of the medium. J Phys Chem B 113 5951-5960... 

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