Optical activity, racemates and resolution template

Specific rotation [α] = α/(l·c), enantiomeric excess from rotation, racemic mixtures, and resolution of a racemic acid through diastereomeric salts of (S)-1-phenylethylamine.

What the template shows

• Enantiomers rotate plane-polarised light equally in opposite directions; a racemate does not rotate it
• Measuring and separating enantiomers
• α observed rotation (degrees), l path length (dm), c concentration (g mL⁻¹); D = sodium D line (589 nm), T in °C. Example: (S)-(+)-butan-2-ol, [α]D +13.5; (R)-(−)-butan-2-ol, −13.5; the racemate, 0. A sample with [α]D +6.75 has 50 % ee: 75 % (S) and 25 % (R).
• racemic acid (R)-RCOOH + (S)-RCOOH
• (R)-acid·(S)-amine crystallises (less soluble)
• (S)-acid·(S)-amine stays in solution
• filter, then add HCl → pure (R)-acid
• evaporate, add HCl → (S)-acid
• Which salt crystallises depends on the pair; the diagram shows one case.
• a common resolving agent
• (+) or d: rotates the plane clockwise (dextrorotatory); (−) or l: anticlockwise (laevorotatory). The sign cannot be predicted from R/S.
• A racemate (±) is a 1 : 1 mixture of enantiomers: optically inactive. Reactions of achiral reagents through planar intermediates (carbocations, enolates) give racemates.
• ee (%) = |% major − % minor| = [α]observed / [α]pure × 100 (optical purity, for an ideal sample).
• Enantiomers cannot be separated by crystallisation or ordinary chromatography; diastereomers can, which is why resolution goes through diastereomeric salts (or a chiral column).
• Wedge = toward you, hash = away from you · every part of this figure is editable

Drawn with 9 atoms, 24 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 14 Textbook
• Eliel & Wilen, Stereochemistry of Organic Compounds, Wiley 1994, ch. 7 Textbook
• Moss, Basic terminology of stereochemistry (IUPAC Recommendations 1996), Pure Appl. Chem. 1996, 68, 2193 IUPAC