Specific Rotation

Specific Rotation Definition:

The observed rotation of plane-polarized light exhibited by an optically active compound under standard conditions is called its specific rotation.

Specific Rotation Explained:

The measured/observed rotation of plane-polarized when passing through an optically active compound depends on various factors.

  • Concentration –  Observed rotation is directly proportional to the concentration of the solution .i.e the number of molecules that the light has to encounter.  doubling the concentration doubles the amount of rotation.
  • Path length – Observed rotation is also directly proportional to the amount of solution (horizontal distance) that the light has to pass through. Doubling the path length doubles the amount of rotation.

Because of such dependence, the specific rotation is used which allows scientists to compare the rotation of various compounds. The standard conditions use a concentration of 1 gm/mL and a path length of 1 dm.

Temperature and the wavelength of the incident light also affect the observed rotation. However, it is not incorporated into the equation below because the relationship is non-linear and unpredictable. Hence, the specific rotation is always reported along with the temp. and wavelength of the incident light.

(The equation below is a means to calculate the specific rotation from the observed rotation)

See Plane-polarized light, Optically Active compounds, observed rotation