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Unformatted text preview: TECHNIQUE: Polarimetry
Written by Albert T. Sneden, VCU Department of Chemistry with revisions by L.M. Moses
A large number of organic compounds, particularly those from natural sources, have a physical
property known as specific rotation which is reported as the term [α] in degrees (+ or - >0-360°).
Specific rotation is a measure of the compounds ability to rotate the plane of plane polarized
light, and compounds which have this ability are said to be optically active. Such compounds are
defined as chiral compounds.
Chiral compounds, in the simplest sense, are those compounds whose overall structure contains
no element (plane, axis, point) of symmetry. If the three dimensional structure of such a
compound was reflected in a mirror and the mirror image was removed from the mirror and
superpositioned on the original compound, no amount of turning or repositioning would allow all
parts of the mirror image to precisely line up with the same parts of the original. Something
would always be out of alignment. If the parts did line up exactly, then the compound would be
achiral, would not rotate the plane of plane polarized light, and would be optically inactive ([α]
As an example of these principles, consider two hands. As shown below in A, the right hand is
the mirror image of the left hand. If the right hand is brought out of the mirror and
superpositioned on top of the left hand (B or C), the various parts can never line up, no matter
how the hands ar...
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This note was uploaded on 03/06/2012 for the course CHEM 301 taught by Professor Sahli during the Spring '07 term at VCU.
- Spring '07