Question

What are the necessary conditions for optical activity ? Give an example of optically active molecules.

Answer 1

Hint: Optical activity of an organic compound depends upon the presence of a special condition of the carbon atom that is arranged in it, and the special condition is obtained when the groups/atoms attached to the carbon are all different.

Complete step by step answer:
Chiral Carbon:
If all the four valencies of carbon are satisfied by 4 different groups the carbon is chiral carbon or asymmetric carbon. Presence of chiral carbon never ensures the molecule is optically active.
Compounds with one chiral carbon are always chiral. While compounds containing more than one chiral carbon may or may not be chiral.
Any molecule is optically active if
(a) it is non-superimposable on its mirror image.
(b) does not contain any element of symmetry
- Elements of Symmetry : Elements of symmetry offer a simple device to decide whether a molecule is chiral or achiral, i.e., whether the molecule is superimposable or non-superimposable on its mirror image. When a molecule lacks all elements of symmetry it is chiral.
(1) Plane of symmetry: Any imaginary plane which divides the full molecule into two halves which are mirror images of each other.
(ii) Centre of inversion : Any imaginary point, through which when inversion will be carried out it will repeat equivalent configuration.
(iii) Alternating axis of symmetry : A molecule posses an alternating axis of symmetry, if when rotated through an angle of ${\displaystyle \frac{{360}^0}{n}}$ about the axis and then followed by reflection in a plane perpendicular to the axis, molecule comes to its equivalent form.
- Enantiomers: Molecules which possess mirror images that are non-superimposable to each other are called enantiomers.
(i) Enantiomers have the same physical properties except their optical rotation which is equal in magnitude but in the opposite direction.
(ii) Enantiomers have the same chemical behaviour towards the molecules which are optically inactive, but they have different chemical behaviour towards the molecules which are optically active.
- Diastereomers: Stereoisomers which do not exhibit mirror images (superimposable) of each other are called diastereomers. Diastereomers show different chemical and physical properties.
Some examples of optically active molecules are ascorbic acid , all amino acids , tartaric acid etc.

Note: Racemic Mixture:
1:1 molar mixture of laevo and dextro forms of the same pair of enantiomers and is optically inactive because the optical rotation for the dextro is neutralized by the optical rotation of the laevo form. It is the case of external compensation.