Optical Activity
• Chiral compounds
bend plane polarized
light
• Angle depends on
concentration, path
length, solvent,
temperature,
wavelength of light,
which enantiomer
The specific rotation of sucrose is reported:
[a]20D = +66.4o (H2O)
D = sodium D-line = 589 nm
Specific Rotation
• Since the actual rotation is so dependent on
experimental conditions, there must be a way
to standardize so that we can compare
• Observed rotation: a
– The rotation measured at experimental conditions
• Specific rotation: [a]
– The rotation standardized to a sample of
concentration 1 g/1 mL, with pathlength of 1
decimeter
Calculation
• Specific rotation is calculated from observed
rotation by [a] = a / (c . l)
– C (concentration) is in grams/mL
– L (pathlength) is in decimeters
• Most samples are actually measured at more
dilute concentrations (g/ 100mL), then the
rotation is converted to specific rotation
• Dilution does affect rotation, so to compare to
literature, you must obtain data at same
concentration they did
Example
• Reported data: Compound A has a reported
specific activity [a]23D = 93o (c = 1.9, CH2Cl2)
• To repeat the experiment, you mixed 190 mg of
compound in 10mL of methylene chloride at 23o
C and used a sodium lamp and standard tube.
• You obtained a rotation of 1.6o
• What is the specific rotation of your compound?
– Answer: 1.6o /(0.019g/ml)= 84o
Example, continued
• Using the last example, what is the enantiomeric
excess of your sample? And what is the % of
each enantiomer in your mixture?
• Solution: Divide your determined specific activity
by the accepted specific activity.
– 84o / 93o = 90%
– Therefore, there is a 90% excess of the (d) enantiomer,
meaning that the other 10% is a racemic mix of (d)
and (l). So 5% is (d) and 5% is (l)
– Overall: 95% (d) and 5% (l)
From JOC Guideline for Authors
• Specific Rotation. Specific optical rotations should be
reported for isolated natural products,e nantiopure
compounds, and enantioenriched isomer mixtures
when sufficient sample is available.
• Specific rotations based on the equation [α] =
(100⋅α)/(l⋅c) should be reported as unitless numbers as
in the following example: [α]D 20 −25 (c 1.9, CHCl3),
where the concentration c is in g/l00 mL and the path
length l is in decimeters.
• The units of the specific rotation, (deg⋅mL)/(g⋅dm), are
implicit and are notincluded with the reported value.