Question

Explain the optical activity of lactic acid

Answer 1

1) Lactic acid contain one asymmetric carbon atom (chiral carbon ) which is attached to 4 different groups - COOH ,CH3,OH & H
2) Two different arrangements of these groups around the carbon atom are posible as shown in fig.
3) Hence,lactic acid can exist in two different form, l(L)- form and d- form ,which are non-superimpossable mirror images of each-other
l - form rotates the plane of polarisation to left while d - form rotates plane of polarisation to right
4)Racemic mixture:- A mixture containing equimolar amount of d - form amd l - form of optically active compound
Note :- Racemic mix. is always inactive (dl - form) due to extarnal / internal compensation.
In case of lactic acid racemic mix. is inactive due to external compensation.