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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 ] 20 D = +66.4 o (H 2 O). D = sodium D-line = 589 nm. Specific Rotation.

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Optical Activity


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optical activity
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
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
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
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 lampand 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
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
From JOC Guideline for Authors
  • Specific Rotation. Specific optical rotations should be reported for isolated natural products,enantiopurecompounds, 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]D20−25 (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 notincludedwith the reported value.