Question

Explain optical activity of lactic acid

Answer 1

The optical activity of lactic acid can be discussed as:
1. Presence of asymmetrical carbon atom:
   i. Lactic acid contains one asymmetrical carbon atom.
   ii. According to van’t Hoff’s rule: a = 2ⁿ, where, a is the number of isomers and n is the  number of asymmetric carbon atom.
   iii. Thus, two isomers of lactic acid are possible.

2. Non-superimposable mirror image structures:

Hence, lactic acid can exist as d-form and l-form which are non-superimposable mirror images of each other.

3. (dl) Racemic mixture and its optical inactivity:
A mixture containing equal moles of the d and l forms of lactic acid is a racemic mixture which is optically inactive i.e., dl or (±) form. This inactivity arises due to external compensation.