Question

Which of the following is true for the isomer $(-)$-Lactic acid ?

• A. It rotate plane polarised light in clockwise direction
• B. It rotate plane polarised light in anti-clockwise direction
• C. It rotate plane polarised light in both clockwise and anti-clockwise direction
• D. Insufficient data to predict the rotation of plane polarised light.

Answer 1

The correct option is B It rotate plane polarised light in anti-clockwise direction

Compounds that rotate plane polarised light is called optically active compounds. Based on the direction of rotation of plane polarised light, they are classified into two types.
Compounds that rotate the plane of polarised light in clockwise direction or to the right of direction of propagation. It is indicated by (+) or d- before the compound name. These compounds are called dextrorotatory.

Compounds that rotate the plane of polarised light in anti-clockwise direction or to the left of direction of propagation. It is indicated by (-) or l- before the compound name. These compounds are called laevorotatory.

Here, the given isomer is $(-)$-Lactic acid so it rotate the plane polarised light in anti-clockwise direction.