Organic Chemistry

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