Question

Optical rotation of a racemic mixture is always:
(i) Negative
(ii) Zero
(iii) Positive
(iv) Cannot be predicted

Answer 1

First try to understand what the different components present in a racemic mixture are. Then find out in which direction the different components rotate. If two components have clockwise or anticlockwise rotation then they will add up but if one has clockwise rotation and the other has anticlockwise rotation then they will cancel out each other.

Complete answer: A mixture that contains two enantiomers of a molecule in equal proportion $\left( {1:1} \right)$ then that mixture is known as a racemic mixture.Enantiomers are pairs of stereoisomers having non-superimposable mirror image relationship with each other. They have identical physical properties; but they rotate a plane polarized light (direction of oscillation of electric field vector is contained in a single plane) in the opposite direction but to an equal magnitude. Since the racemic mixture contains equal proportion of both the enantiomers of a compound the clockwise rotation of an enantiomer is cancelled by the anticlockwise rotation of the other enantiomer (since the rotation is equal in magnitude).Therefore the net optical rotation of a racemic mixture zero.

Additional Information: Enantiomers are a type of configurational stereoisomer. Since the enantiomers have identical physical properties with an exception that they rotate a plane polarized light in equal but opposite direction they are also regarded as optical isomers.

Hence the correct answer is (ii) Zero.

Note: The most important part to solve this question is to understand that racemic mixture contain enantiomers in equal proportions. Also, you should not confuse between enantiomers and diastereomers as enantiomers are mirror images of each other while diastereomers are not.