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Optical Activity

Article ID: 137
Last updated: 25 Jan, 2013
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Check out this interactive stereochemistry tutorial on the ChemEdDL

Optical activity is the ability of a chiral molecule to rotate plane-polarized light. It is measured using a polarimeter, which consists of a light source, polarizing lens, sample tube and analyzing lens.


When light passes through a sample that can rotate plane polarized light, the light appears to dim to the eye because it no longer passes straight through the polarizing filters.  The amount of rotation is quantified as the number of degrees that the analyzing lens must be rotated by so that it appears as if no dimming, of the light has occurred.
 

Measuring Optical Activity:

When rotation is quantified using a polarimeter it is known as an observed rotation, because rotation is affected by path length (l, the time the light travels through a sample) and concentration (c, how much of the sample is present that will rotate the light).  When these effects are eliminated, a standard for comparison of all molecules is obtained, the specific rotation, [a].

[a] = 100a / cl      when concentration is expressed as g sample /100ml solution

Specific rotation is a physical property like the boiling point of a sample and can be looked up in reference texts.   

Take a look at a solved problem.

Given that (S)-bromobutane has a specific rotation of +23.1o and (R)-bromobutane has a specific rotation of -23.1o, what is the optical purity and % composition of a mixture whose specific rotation was found to be +18.4o?

The positive sign indicates that the (S)-isomer is in excess.

Optical purity, %  = 100 [a]mixture / [a]pure sample
                            = 100 (-18.4)  /  +23.1o
                            = 80%  this indicates a 80% excess of S over R!

The 20% leftover, which is optically inactive, must be equal amounts of both (R)- and (S)-bromobutane.  The excess 90% is all S so there is a total of 10% (R) and 90% (S).

Enantiomers will rotate the plane of polarization in exactly equal amounts (same magnitude) but in opposite directions.

Dextrorotary designated as (+), clockwise rotation (to the right)
Levorotary designated as (-), anti-clockwise rotation (to the left)

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Department of Chemistry & Chemical Biology