Optical Activity.
Some liquids and solutions, when placed in the path pf plane-polarized light, rotate it through a certain angle.”The property of rotating plane-polarized light to the left or right is called optical activity and the substance is called the optical activity“.Those rotating in a clockwise direction are called dextrorotatory and those in anti-clock direction are called laevorotatory.
Measurement of Optical Activity.
Optical activity is measured in terms of specific rotation. | α | tλ, at a particular temperature t with the help of the light of wavelength λ,
Where,
g = no. of grams of solute in 100 ml solution
d = density of liquid
d = density of liquid
l = length of the column of liquid through which light passes.
The angle of rotation, α is measured with the help of polarimeter as shown in the figure.
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A glass tube pT containing optically active liquid or solution is placed in the polarimeter.
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A beam of monochromatic light passes through it after being polarized by polarizer P.
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The sample rotates the plane of polarized light through a certain angle.
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The analyzer A is rotated through the same angle to get a dark field of view again.
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This angle and its direction are noted with the help of a graduated disc.
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The substance is laevorotatory if the analyzer is rotated in the anticlockwise direction. It is Dextrorotatory if the analyzer is rotated in the clockwise direction.
Importance In Chemistry.
- Optical activity depends upon the arrangement of atoms in the molecule. Thus it is a constitutive property. It is employed in ascertaining the structure of compounds.
- Only those organic compounds containing asymmetric carbon atom are optically active. A carbon atom with four different groups of atoms is called asymmetric carbon, e.g., lactic acid and maleic acid are optically active and have been assigned the formulas:
