Question

Racemic mixtures are optically inactive. Give reason.

Answer 1

Hint : Racemic mixtures are those which have equal amounts of left and right handed enantiomers of a chiral molecule and substances which cannot rotate planes of polarized light are optically inactive.

Complete step by step answer:
Enantiomers are pairs of molecules that exist in two forms. These forms are chemically identical in every order. These forms are non-superimposable mirror images of each other. A chiral molecule is a molecule which cannot be superposed on its mirror image by any combination of rotation or translation. We know racemic mixtures are those which have equal amounts of left hand and right hand enantiomers of a chiral molecule. As enantiomers have equal amount of left and right handed enantiomers i.e. they contain d and l forms ( d means dextrorotatory and l means levorotatory dextrorotatory means right handed or clockwise rotation and levorotation means left or anti clockwise rotation ) in equal proportions so , rotation due to one isomer will be cancelled by rotation due to other .Therefore net rotation will be zero. We know molecules with zero optical rotation are optically inactive. Since rotation of one isomer is cancelled with another, racemic mixtures are optically inactive.
Examples - Racemic acid (mixture of two enantiomeric isomers of tartaric acid) is an example of racemic mixture. Racemic mixture can also be formed with lactic acid and its mirror image. As seen differently they both are optically active but their racemic mixture is optically inactive.

Additional information: The separation of racemic mixture into its components i.e. pure enantiomers is called chiral resolution. Various methods for this process are crystallization, chromatography, and use of enzymes.

Note: Racemic mixture is also called racemate. Easy way of finding chiral carbon atoms is to find carbon atoms with four different groups. Carbon atom having all different groups is chiral.