Physical Properties of Enantiomers
• Enantiomers share identical physical
properties
– m.p., b.p., nD, density, heats of formation etc.
• Example: Lactic Acid
– m.p. = 53°C
– b.p. = 122°C
Lactic Acid
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• Enantiomers rotate a plane of polarized light in
equal, but opposite directions
– Called “Optical Activity”
– Use Polarimeter
2
Optical Activity
• If the sample rotates the plane of polarized light CW
→ dextrorotatory (+)
– Latin: Dexter = Right
• If the sample rotates the plane of polarized light CCW
→ levorotatory (-)
– Latin = Laevus = Left
• If one enantiomer is +, the other will be –
(+)-lactic acid
(–)-lactic acid
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• The sign of optical rotation is unrelated to R and S
configuration of a compound
(S)-(+)-lactic acid
(R)-(–)-lactic acid
• Optical rotation (a) is a quantitative measure
of optical activity






[a] = a/cl
[a] = specific rotation
a = degrees
c = concentration (g/mL)
l = path length (dm)
Often, temp and wavelength indicated
(R)-(+)-glyceraldehyde
[α]20 = +13.5° mL g-1 dm-1
(S)-(-)-glyceraldehyde
[α]20 = -13.5° mL g-1 dm-1
Problems
1) A 1.50 g sample of (S)-(+)-coniine, the toxic
extract of poison hemlock, was dissolved in
10.0 mL of ethanol and placed in a sample
cell with a 5.00 cm pathlength. The observed
rotation at the sodium D line was +1.21°.
Calculate the [α]D for coiine.
2) What will the specific rotation be for the R
enantiomer?
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Racemic Mixtures
• Racemic mixture/Racemate: a mixture
containing equal amounts of two enantiomers
– typically have different physical properties from
that of the pure enantiomers
– Example: Lactic Acid
• m.p. (R or S) = 53°C
• m.p. (R and S)= 17°C
– Indicating racemic mixture:
• Racemic Lactic Acid
• (±)-Lactic Acid
– Optical rotation = 0
6.4 Racemates
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Making Racemic Mixtures
• Racemization: the process of forming a racemic mixture
– Easy: mix = amounts of each enantiomer together
– More difficult: chemical rxns
• Heat, enzymes, acids, bases
Celexa vs. Lexapro
R-(−)-citalopram
• Celexa = racemic mixture
• Lexapro = enantiopure S
– escitalopram
S-(+)-citalopram
Resolution of Enantiomers
• Enantiomeric resolution: the separation of a pair of
enantiomers from a racemic mixutre
– Also called chiral resolution, optical resolution,
and mechanical resolution
– Often difficult
• Enantiomers have same chemical properties
• Separation often through
–Selective crystallization
–Diastereomeric salt formation
Selective Crystallization
• Pasteur’s experiment (1848)
• 1882 – “Seeding” of a supersaturated solution
– Now often used in industry
• Methadone
• L-glutamic acid
Diastereomers
• Diastereomers/Diastereoisomers:
stereoisomers that are not mirror images
(enantiomers) of each other
– possible when a molecule has two or more
asymmetric carbons
– Differ in all their physical properties
6.6 Diastereomers
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Isomer Identification Flowchart
6.6 Diastereomers
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• Molecules with more
than one chirality
center have mirror
image stereoisomers
that are enantiomers
• In addition they can
have stereoisomeric
forms that are not
mirror images, called
diastereomers
Diastereomers and Enantiomers
6.6 Diastereomers
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Problem: Can you identify the diastereomers?
2R,3R
2S,3S
2R,3S
2S,3R
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Physical Properties of Stereoisomers
6.6 Diastereomers
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