Organic Chemistry

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Organic Chemistry - Ch 11 549 Daley & DaleySlitPolarizerLampCross section of light beamFigure 11.8. Polarization of a light beam. Before passing through the polarizingfilter, the light beam has random orientations of the light waves. After passingthrough the polarizing filter, the light beam contains only one orientation of the lightwaves.A polarimeter is aninstrument thatmeasures opticalactivity.An optically activesubstance rotates aplane of polarizedlight.A dextrorotatory isomerrotates a plane ofpolarized lightclockwise or to theright.A levorotatorysubstance rotates thepolarized lightcounterclockwise or tothe left.If you pass this plane-polarized light through a polarimetercontaining a solution of a pure enantiomer, the plane of polarizationvisibly rotates a certain number of degrees. If you place a solution ofthe other enantiomer with the same concentration in the sameinstrument, the plane of polarized light rotates the same number ofdegrees in the opposite direction. The amount of rotation staysconstant for a given pair of enantiomers with the same concentration,using polarimeters of the same wavelength of light and the same tubelength. Substances that exhibit this type of behavior are opticallyactive, and the number of degrees that the plane of polarized lightrotates is a measure of optical activity.An enantiomer that rotates light clockwise, as you look towardthe light source is dextrorotatory. Designate a dextrorotatoryenantiomer as a d or (+) isomer. An enantiomer that rotates lightcounterclockwise is levorotatory. Designate a levorotatoryenantiomer as an l or (–) isomer. For example, the d form of 2-bromobutane has a rotation of +39.4 o when you measure it with the589 nm wavelength band of a sodium vapor lamp. This wavelength iscalled the sodium D line. Its enantiomer, l-2-bromobutane, has arotation of –39.4 o . Remember, there is no connection between the d or lisomers and the (R) and (S) designations. The d or l designation comesfrom the experimental observation of the sign of the optical rotationand the (R) or (S) designation comes from the Cahn-Ingold-Prelogrules.www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 550 Daley & DaleySpecific rotation is howmuch a plane ofpolarized light rotatesunder certainconditions.The normalized measurement of the optical activity of anoptical isomer is called its specific rotation. Normalization isnecessary to provide a constant measurement of a molecule's opticalactivity because not all polarimeters use the same tube length anddifferent concentrations of the isomers are used. The specific rotationof a molecule is a physical property of that molecule; thus, itsmeasurement is always the same. The symbol used to indicate specificrotation is [α] T λ . The [α] is the specific rotation, the T is thetemperature of the solution, and the λ is the wavelength of light beingused. Use the following expression to calculate the specific rotation ofa molecule:[α] T λ = [α]l [X]Where [α] is the observed rotation, l is the length of the tube indecimeters (1 dm = 10 cm), and [X] is the concentration of the solute ing/mL. Although [α] T λ is the term used for specific rotation, anothersymbol commonly used is [α] D. This symbol indicates the sodium D lineas the light source at a temperature of 25 o C.Exercise 11.4Researchers isolated a pure enantiomer of an unknown chemicalstructure from a rare flower. They then dissolved it in chloroform at aconcentration of 0.22 g/mL. When they placed the mixture in apolarimeter, it gave a rotation of +2.697 o in a 1 dm cell using thesodium D line. Calculate the value of [α] D.A racemic mixture, orracemate, is a mixturecontaining equalamounts of bothenantiomers.An equimolar mixture of two enantiomers has no effect on aplane of polarized light because the two isomers have an identical butopposite effect on the plane of polarized light. An equal mixture of apair of enantiomers is called a racemic mixture, or a racemate.When doing a chemical reaction that produces a molecule with astereocenter using symmetrical materials, you always obtain aracemic mixture of the product. In other words, you cannot produceasymmetric compounds directly from symmetric starting materials.For example, reacting phenyl magnesium bromide with 2-butanoneproduces racemic 2-phenyl-2-butanol.www.ochem4free.com 5 July 2005
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Organic Chemistry

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