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Optical Activity. Enantiomers have virtually identical physical properties. Exactly how do they differ from each other? Solutions of them rotate plane-polarized light in opposite directions. They react with enzymes differently (actually, one usually reacts and the other does not).
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Optical Activity • Enantiomers have virtually identical physical properties. Exactly how do they differ from each other? • Solutions of them rotate plane-polarized light in opposite directions. • They react with enzymes differently (actually, one usually reacts and the other does not).
Plane-Polarized Light • Light with an E (or H) field that oscillates in a single plane. Figure 5-10. Schematic diagram of a polarizing filter.
Polarimeter • A solution of a pure enantiomer rotates plane-polarized light. Figure 5-13. Schematic diagram of a polarimeter.
Optical Activity-Designation • A solution of a pure enantiomer that rotates plane-polarized light to the • left (CCW) is designated (-) or l (levorotatory) • right (CW) is designated (+) or d (dextrorotatory) • These do NOT correlate with (R) and (S)!
Specific Rotation • The specific rotation [α] of a solution of a pure enantiomer is [α] = α(observed) c l • where c is the concentration of enantiomer in g/mL and l is the length of the sample cell in dm.
Specific Rotation • A solution of 0.5 g of (-)-epinephrine dissolved in 10 mL of dilute HCl was placed in a 20-cm polarimeter tube. Using the sodium D line, the rotation was found to be -5.0° at 25°C. Determine the specific rotation of epinephrine. [α] = α(observed)/(c l) = -5.0°/[(0.5 g/10 mL)(20 cm x 0.1 dm/1 cm) ] = -5.0°/[(0.05 g/mL)(2.0 dm)] =-5.0°/0.10 = -50.°
Racemic Mixture • A 50/50 mixture of enantiomers. • Since solutions of pure enantiomers rotate plane-polarized light in opposite directions, a 50/50 solution will NOT rotate plane-polarized light. • The solution is optically inactive. • A racemic mixture is also called • a racemate • a (d,l) pair • a (±) pair
Other Mixtures of Enantiomers • If the mixture contains only the pure enantiomers, these two equations give the same value. If the mixture contains other optically active compounds, op will differ from ee.
Enantiomeric Excess (e.e.) • Problem 5-12: When optically pure (R)-(-)-2-bromobutane is heated with water, 2-butanol is the product. Twice as much (S)-2-butanol forms as (R)-2-butanol. Find the e.e. and the observed rotation of the product. [α]=13.50° for pure (S)-2-butanol. Let x = amount of (R) enantiomer formed e.e. = 100 |d-l|/|d+l| = 100 |2x-x|/|2x+x| = 100 (1/3) = 33%
Using Optical Purity Since 2-butanol is the only product, e.e. = o.p. Since o.p. = 100 (observed rotation)/(rotation of pure enantiomer) 33 = 100 (observed rotation)/ 13.50° observed rotation = 33(13.50)/100 = +4.5° The predominance of the (S)-(+)-2-butanol causes the specific rotation of the solution to be positive.
Specific Rotation of a Mixture • Sucrose is a disaccharide of D-glucose and D-fructose. Sucrose has a specific rotation of +66.5°. • When sucrose is hydrolyzed by an invertase, a mixture of glucose (C6H12O6, +52.7°) and fructose (C6H12O6, -92.4°) results and the observed rotation of the mixture is (-). • Because of this, the mixture is called an invert sugar. • Can you calculate the specific rotation of invert sugar? • (-39.7º, since glucose and fructose have the same molar mass.)
Chirality of Conformationally Mobile Systems • A molecule cannot be optically active if its chiral conformations are in equilibrium with their mirror images.
Chirality of Conformationally Mobile Systems • Such enantiomers are in equilibrium with each other through ring flipping. • One enantiomer cannot be separated from the other.
Chirality of Conformationally Mobile Systems • To determine whether a conformationally mobile molecule can be optically active, consider tis most symmetric conformation. cis-1,2-dibromocyclohexane is achiral…is the trans isomer achiral?
Chirality of Conformationally Mobile Systems • Chiral or achiral? • Optically active or optically inactive?
Enantiomers with No Chiral Carbon Atoms • Conformers that cannot interconvert (due to steric hindrance) can be enantiomers.
Enantiomers with No Chiral Carbon Atoms • H2C=C=CH2 is allene. • A disubstitutedallene can be chiral, because the substituents are in perpendicular planes. Why aren’t all the H’s and Cl’s in the same plane?