Properties, Purification, and Use of Organic Solvents

In Table A-2, a selection of chiral organic solvents is given, together with some physical data. Such chiral solvents have received much attention because of their use in determining optical purities [9, 134], as media for stereoselective syntheses [10-12, 135, 136], and as NMR shift reagents [13]. 

Another important property of organic solvents is their miscibility with other organic liquids cf. Fig. 2-2 in Chapter 2). The farther two solvents are located from each other in Hecker s mixotropic series of solvents [51], given in Table A-10, the less miscible they are. 

According to Hildebrand s solubility parameter approach [101], two liquids are miscible if their solubility parameters 8 differ by no more than 3.4 units [101, 102] cf. Eqs. (2-1) and (5-77) for the definition of 8. That is, mutual miscibility decreases as the 8 values of two solvents become farther apart. Higher mutual solubility will follow if the 8 values of the solvents are closer. A comprehensive collection of 8 values has been given by Barton [100]. 

An alternative, more empirical but more accurate method for predicting miscibility has been given by Godfrey [103], using so-called miscibility numbers (M-numbers). These are serial numbers of 31 classes of organic solvents, ordered empirically by means of their lipophilicity i.e. their affinity for oil-like substances), using simple test-tube miscibility experiments. All pairs of solvents whose Af-numbers differ by 15 units or less are miscible in all proportions at 25 °C a difference of 17 corresponds to immis-cibility, and an M-number difference of 16 units indicates borderline behaviour (limited mutual miscibility) [103]. The central class of solvents with Af-number equal to 16 e.g. 2-n-butoxyethanol) comprises universal solvents, which are miscible with less lipophilic as well as with more lipophilic solvents. 

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