Optical Activity

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# Optical Activity - PowerPoint PPT Presentation

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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Presentation Transcript
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 lampand standard tube.
• You obtained a rotation of 1.6o
• What is the specific rotation of your compound?
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,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.