What is the reason behind optical activity?

Question

Why do some molecules have tendency to rotate the direction of plane polarized light?

I know about chiral carbon atoms and polarimetry. I want to know why exactly do chiral carbon atoms rotate the plane of plane polarized light.

Answer 1

Optical rotation or optical activity is the rotation of linearly polarized light as it travels through certain materials. That is, the molecules will rotate in the direction in which the light is vibrating as it passes through them. This occurs in solutions of compounds that have at least one carbon that is asymmetrcially substituted. Such molecules are called "chiral" molecules. A "chiral" molecule is said to have a configurational arrangement that has a discrete mirror image that is a different molecule (and not a repositioning of the same).

In the 19th century, it was OBSERVED (discovered) that certain solutions of molecules could rotate plane-polarized light a discrete number of degrees clockwise or counterclockwise. This was developed a means of determining the composition of a mixture of chiral molecules - of "right-" (the R) and "left-handed" (the S) versions of the same compound. The greater the rotation, the more pure the solution is in the R or S enantiomer.

More on this topic if you crack ANY Organic chem textbook

Answer 2

Optical activity is a result of chiral carbon atoms in a molecule. They alter the vibration of the polarised light. The arrangement of the groups attached to the carbon will dictate the direction of rotation. If a molecule is symetrical there will be no rotation