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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 ] 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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  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 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

  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,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.

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