ABSTRACT
In the presence of solute-solute and solute-solvent intermolecular interactions, it is known that specic optical rotation (SOR) may no longer be independent of the concentration of solution. In 1968, it was demonstrated that [α]D for (α-methyl-α-ethyl)succinic acid changes from being positive at low concentrations to negative at higher concentrations (Krow and Hill 1968) (see Figure 9.1). Such drastic concentration dependence, attributed to sample aggregation at higher concentrations, has consequences in the way SOR can be used. For example, the enantiomeric composition (i.e., composition of enantiomers) of a synthesized sample used to be estimated in the literature by comparing its [α]D with that of enantiomerically pure substance and designated as optical purity (op); op is the ratio of SOR measured for an enantiomeric mixture to that of pure enantiomer. In the presence of aggregation effects, op determined from the experimental SORs need not match the enantiomeric excess (ee) (determined by mixing known amounts of enantiomers or from chiral chromatography). The plot of %op against %ee generated from the data reported (Horeau 1969) for (+)-(α-methyl-α-ethyl) succinic acid is shown in Figure 9.2. From this plot, it can be seen that for a sample with 50%ee, the observed op is ~36%, which amounts to a deviation of −28%. These deviations are referred to as the Horeau effect. Chiral aggregation is considered to be responsible for such observed deviations (Eliel and Wilen 1994).
