1. 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

2. 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

3. 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

4. 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

5. 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)

6. 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.