Presentation on theme: "Optical Activity Enantiomers are different compounds:"— Presentation transcript:
1 Optical Activity Enantiomers are different compounds: Same boiling point, melting point, densitySame refractive indexRotate plane polarized light in opposite directions (polarimetry)Different interaction with other chiral moleculesEnzymesTaste buds, scent
2 Optical ActivityPolarimetry is a laboratory technique that measures the interaction between a compound and plane polarized light.Since enantiomers interact with plane polarized light differently, polarimetry can be used to distinquish between enantiomers.
3 Optical Activity“Regular” (unpolarized) light vibrates in all directions.Plane-polarized light:light composed of waves that vibrate in only a single planeobtained by passing unpolarized light through a polarizing filter
4 Optical ActivityWhen plane polarized light passes through a solution containing a single chiral compound, the chiral compound causes the plane of vibration to rotate.Polarimeter
5 Optical Activity Chiral compounds are optically active: capable of rotating the plane of polarized lightEnantiomers rotate the plane of polarized light by exactly the same amount but in opposite directions.(R)-2-butanol(S)-2-butanol+13.5o rotation-13.5o rotation
6 Optical ActivityCompounds that rotate the plane of polarized light to the right (clockwise) are called dextrorotatory.d(+) IUPAC conventionCompounds that rotate the plane of polarized light to the left (counterclockwise) are called levorotatory.l(-) IUPAC convention
7 Optical Activity+13.5o rotation-13.5o rotation(+)-2-butanol(-)-2-butanol(S)-(+)-2-butanol(R)-(-)-2-butanolThe direction and magnitude of rotation must be determined experimentally.There is NO CORRELATION between (R) and (S) configuration and the direction of rotation.
8 Optical Activity(S)-(-)-thyroxinebiologically active(R)-(+)-thyroxineinactiveUnlike (R)-(-)-2-butanol, (R)-thyroxine rotates light to the right.
9 Optical ActivityThe angular rotation observed in a polarimeter depends on:the optical activity of the compoundthe concentration of the samplethe path length of the sample cellA compound’s specific rotation [a] can be used as a characteristic physical property of a compound:the rotation observed using a 10-cm sample cell and a concentration of 1 g/mL.
10 Optical Activity where a = specific rotation c = concentration in g/mL l = path length in dma (observed) = rotation observed for a specific sample
11 Optical ActivityExample: A solution of 2.0 g of (+)-glyceraldehyde in 10.0 mL of water was placed in a 100. mm polarimeter tube. Using the sodium D line, a rotation of 1.74o was observed at 25oC. Calculate the specific rotation of (+)-glyceraldehyde.
12 Optical ActivityGiven: a (obs) = 1.74oFind: [a]
13 Optical Activity+13.5o rotation-13.5o rotation(S)-(+)-2-butanol(R)-(-)-2-butanolA mixture containing equal amounts of (+)-2-butanol and (-)-2-butanol gives an observed rotation of zero degreesJust like an achiral molecule
14 Optical ActivityA solution containing equal amounts of two enantiomers is called a racemic mixture.Racemate(+) pair(dl) pairRacemic mixtures are optically inactive.Racemic mixtures are designated using the prefix (+):(+)-2-butanol
15 Optical ActivityRacemic mixtures are often formed during chemical reactions when the reactants and catalysts used are achiral.
16 Optical ActivitySome mixtures are neither optically pure (all one enantiomer) nor racemic (equal mixture of both enantiomers).Optical purity:Ratio of the rotation of a mixture to the rotation of a pure enantiomero.p. = observed rotation x 100%rotation of pure enantiomer
17 Optical ActivityExample: (-)-2-butanol has a specific rotation of o while the specific rotation of (+)-2-butanol is +13.5o. A mixture containing (+) and (-)-2-butanol has an observed rotation of – 8.55o. Does the mixture contain more (+) or more (-)-2-butanol? Calculate the optical purity of the mixture.
18 Optical ActivityAnother method to express (or determine) the relative amounts of enantiomers present in a mixture is enantiomeric excess.Numerically identical to optical puritye.e. = o.p. = excess of one over the other x 100%entire mixture
19 Optical ActivityExample: Calculate the e.e of a mixture containing 25% (+)-2-butanol and 75% (-)-2-butanol.
20 Optical ActivityExample: Calculate the relative proportions of (+)-2-butanol and (-)-2-butanol required to give an observed rotation of +0.45o if the specific rotation of (+)-2-butanol is 13.5o.
21 Optical ActivityAny (or all) of a set of diastereomers may be optically active (if it has a non-superimposable mirror image)Pairs of optically active diastereomers rotate light by different amounts.(+)-glucose+ 52.5o(+)-galactose+ 83.9o
22 Separation of Stereoisomers & Structural Isomers Structural isomers and diastereomers have different physical properties:BP, MP, density, refractive index, solubilityCan be separated through conventional means (distillation, recrystallization, chromatography)MP = 158oCMP = 256oC
23 Resolution of Enantiomers Since enantiomers have identical physical properties, they cannot be separated by conventional methods.Distillation and recrystallization fail.The process of separating enantiomers is called resolution.Two methods:chemical resolutionchromatographic resolution
24 Resolution of Enantiomers Chemical resolution of enantiomers:temporarily convert both enantiomers into diastereomersreact with an enantiomerically pure (natural) productseparate the diastereomers based on differences in physical propertiesconvert each diastereomer back into the original enantiomer
26 Resolution of Enantiomers Chromatographic resolution of enantiomers:Prepare column containing stationary phase coated with a chiral compoundEnantiomers form diastereomeric complexes with the chiral stationary phaseSeparate the diastereomeric complexes based on differences in affinity for stationary phasestrongly complexed: elutes slowlyweakly complexed: elutes more quickly
27 Chiral Compounds w/o Asymmetric Atoms Although most chiral compounds have at least one asymmetric atom, there are some chiral compounds that have zero asymmetric atoms:conformation enantiomersallenes
28 Chiral Compounds w/o Asymmetric Atoms Conformational enantiomers:compounds that are so bulky or so highly strained that they cannot easily confert from one chiral conformation to the mirror-image conformation“locked” into one conformation
29 Chiral Compounds w/o Asymmetric Atoms Allenes:compounds containing a C=C=C unitcentral carbon is sp hybridizedlinear