2/23/2017
Chirality & Polarized Light
Properties of Chiral Compounds
Newman & Fischer Projections
Polarized Light
Amount of rotation αobs depends on:
•Specific rotation [α]D
•Concentration of chiral substance
•Path length of sample tube
•Presence of enantiomer!
d,l and +,‐ convention
• If polarized light is rotated clockwise by a
chiral substance
• The angle of rotation is positive
• The substance is called “dextrorotatory”
• Prefix D‐ or d‐ or (+)
• Opposite: L‐ or l‐ or (‐)
• Examples: d‐glucose, L‐alanine
• When polarized light is passed through a chiral
substance, the plane of polarization is rotated.
• The angle of rotation varies with
concentration and path length
• The specific rotation of any given substance is
a measured property, not easily calculated.
Measuring Optical Rotation
• Align polarimeter so the
two halves are as close
to same shade as
possible
• Read Vernier scale
• Between 37‐38°
• Inner ring matches at
+6/‐4
• Reading: 37.6°
Example: Sucrose
• Make a solution of sucrose
(table sugar)
• Concentration = 1.00 g/100 mL
o 100 mL = 1 dL = 0.1 L
• Put in 10‐cm (1 dm) cell
• Measure specific rotation of
+66.47° (clockwise)
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Rotations at Other Concentrations
• Sucrose, 0.5 g/dL: 10 cm: +33°
• Sucrose, 1.0 g/dL, 20 cm: +133°
• General equation:
 25
 obs
D 
o C = conc. in g/dL
lc
o L – path lenth, in dm
o D means “D” line of Na emission (yellowish)
 obs    l  c
Physical Properties Not Involving
Chirality
Enantiomeric Excess
• Partial retention of chirality in a reaction
• Makes a mixture of products
• Example:
•
•
•
•
•
•
Optical rotation αobs 40% of pure S‐isomer
Product mix “40% ee”
Same as 60% racemic, 40% S  40% more S
30% R, 70% S
Can be a clue that 2 mechanisms both happen!
Physical Properties Involving Chirality
• Most phys props (MP, BP, density, solubility)
don’t involve chirality
• Measuring devices like thermometers far too
large to have molecular‐level “handedness”
• Enantiomers have identical properties
• Racemic mixture can have different props (MP,
solubility) from pure enantiomer
• Refractive index if using polarized light
• Use of chiral‐packed chromatography columns
Chemical Properties Not Involving Chirality
Chemical Properties Involving Chirality
• Acid‐base (H+ is not chiral!)
• Simple redox
• In general, enantiomers react identically to
non‐chiral reagents
• Reactions generating chiral centers from non‐
chiral materials give racemic mixtures
NaBH4 adds H to carbonyl C
Get R if H adds from back
(pushes O to front)
Get S if H adds from front
– Often cellulose‐based – naturally chiral
– Could also be protein‐based
• Different enantiomers might have different
retention times
• Solubility in chiral solvents (s‐2‐butanol) might
differ
• Chiral substances show reaction difference
ONLY with other chiral substances
• Take advantage to separate (resolve) mixture
• Biological molecules often chiral
– Separation of racemic mixtures by micro‐orgs
– Pharmaceuticals frequently one enantiomer
– Odor receptors
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Properties of Racemic Mixtures
• Co‐crystallization can make stabler crystals
– Lower solubility when recrystallizing
– Higher melting point
– If not stabler, will crystallize separately (Pasteur's
tartrates)
Fischer projections
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•
•
•
Often used for carbohydrates
Main backbone vertical
Horizontal lines poke forward
Remember bow‐tie
• Solution properties often identical
HO
OH
O
OH
HO
OH
Newman projections of chiral
molecules
•Remember Newman projections are odd perspective
•Looking along C‐C bond
•H are shown with lines
•Imagine in unseen H atoms
•Imagine ball‐and‐stick
•Rotate…
•S isomer
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