please help..?

Question

can you discuss briefly how a polarimeter work?

and also some examples of dextrorotatory and levorotatory substances or mixtures..

thanks a lot!!

Answer 1

Optically active samples, such as solutions of chiral molecules, often exhibit circular birefringence. Circular birefringence causes rotation of the polarisation of plane polarised light as it passes through the sample.

A simple polarimeter to measure this rotation consists of a long tube with flat glass ends, into which the sample is placed. At each end of the tube is a Nicol prism or other polarizer. Light is shone through the tube, and the prism at the other end, attached to an eye-piece, is rotated until all light is shut off. The angle of rotation is then read off of a scale. The specific rotation of the sample may then be calculated. Temperature can affect the rotation of light which should be accounted for in the calculations.

Many people like using this technique for integrated lab. However it usually seems to be a waste of time.

Dextrorotation is the property of rotating plane polarized light clockwise. Laevorotation is the opposite of dextrorotation.

Compounds with these properties are said to have optical activity and consist of chiral molecules. Chiral centers that have opposite configurations rotate polarized light the same number of degrees, but in opposite directions. If one molecule that has a chiral center with an "R" configuration rotates plane polarized light 25 degrees clockwise, its enantiomer with the corresponding chiral center having the "S" configuration will rotate plane polarized light 25 degrees counter-clockwise. However it is not possible to determine whether any chiral center will be laevorotatory or dextrorotatory directly from its configuration. Enantiomers have equal and opposite optical rotation, but both "R" and "S" stereocenters have the ability to be dextrorotatory or laevorotatory.

The prefixes D- or (+)- and L- or (-)- are applied in the naming of a compound. D-glucose is dextrorotatory.

Answer 2

in brief, a beam of monochromatic light is passed through a solution of the substance. If the light beam is rotated to the right, the substance is said to be dextrorotary. If the substance rotates the light left, it is said to be levorotary. The polarimeter is a monochromator to produce the monochromatic beam, and a sensor and recorder.