5. Processing

Use standard 1D processing for ¹H NMR spectra (see Experiment 3.1). A Gaussian window (gb = 0.2, lb = −0.5 Hz) was used to process spectrum b.

6. Result

The figure shows the ¹H NMR spectrum of rac 1-phenylethanol obtained on an AM-400 spectrometer (a) without and (b) in the presence of the chiral solvating agent S-(+)-1-(9-anthryl)-2,2,2-trifluoroethanol. The signals at δ_H = 1.5 (CH₃) and at δ_H = 4.9 (CH) are doubled. Note, however, that the separation of the two CH₃ doublets is not sufficient for an integration. This is due to a weaker interaction of the chiral solvating agent with the 1-phenylethanol, which is less basic than 1-phenylethylamine (see Exp. 8.3). On the other hand the chiral auxiliary used here is not acidic enough to form strong solvation complexes. Better results were achieved for 1-phenylethanol with a chiral shift reagent (see Exp. 8.2).

Proton NMR spectra of racemic 1-phenylethanol without and with Pirkle’s reagent

7. Comments

Chiral solvating agents form diastereomeric solvation complexes, which are in rapid equilibrium with the uncomplexed species. Solvents with low solvating ability should therefore be used, such as CDCl₃, CCl₄ or C₆D₆. Frequently used commercially available agents are 2,2,2-trifluoro-1-phenylethanol, or R-(−)-O-acetyl-mandelic acid