Optical Activity | Importance and Measurement of Optical Activity |

Optical Activity.

Measurement of 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,

g = no. of grams of solute in 100 ml solution
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.

Measurement of Optical Activity.

  1. A glass tube pT containing optically active liquid or solution is placed in the polarimeter.

  2. A beam of monochromatic light passes through it after being polarized by polarizer P.

  3. The sample rotates the plane of polarized light through a certain angle.

  4. The analyzer A is rotated through the same angle to get a dark field of view again.

  5. This angle and its direction are noted with the help of a graduated disc.

  6. 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.

  1. 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.
  2. 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:
    Importance In Chemistry.

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