Solvents, acceptor number choice

Although the success of the empirical solvent parameters has tended to downgrade the usefid-ness of the dielectric approach, there are correlations that have succeeded as exemphfied by Figure 13.1.1. It is commonly held that the empirical solvent parameters are superior to dielectric estimates because they are sensitive to short-range phenomena not captured in dielectric measurements. This statement may not be generahzed, however, since it depends strongly on the chemical reaction investigated and the choice of solvents. For instance, the rate of the Menschutkin reaction between tripropylamine and methyl iodide in select solvents correlates better with the log e function than with the solvent acceptor number. ... 

The structures of the glycosyl donors and acceptors and the choice of the promoter and solvent are the major factors influencing the stereoselectivity of glycosylations. Additionally, a number of other parameters including temperature, pressure, concentration and even the sequence of addition of the reactants also exert effects. A detailed discussion of the effects of all these parameters are beyond the scope of this chapter and can be found in various reviews [11,38,39]. 

Solvent Regression Equations. The selection of the appropriate solvent regression equation sometimes depends upon the nature of the solute. Table 1-8 lists a number of solute classes in two basic groups A (hydrogen donors) and B (hydrogen acceptors). Table 1-9 provides values of a and b for the basic set of solvent regression equations (Eq. 1-7 to -37), all of which are of the form shown in Eq. 1-6. If the solute (the chemical for which Kow is to be calculated) is listed under Group A or B in Table 1-8 and if the solvent (associated with the available KSw value) is one of those listed in the first two sections of Table 1-9, then a choice between two equations must be made. For example, if a value of Ksw is available from the xylene/water system, one must choose between Eqs. 1-10 and 1-21. The choice depends on where the solute is listed in Table 1-8 — e.g., Eq. 1-10 would be used if the solute were an alcohol, and Eq. 1-21 would be used if it were an ether. 

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