Stereochemistry

Question 1

What are isomers?

Answer 1

Compounds with the same molecular formula but different arrangements of atoms in the molecule giving rise to different physical properties and chemical properties and this phenomenon is called isomerism.

Question 2

Define optical isomerism.

Answer 2

Optical isomers are compounds which have the same molecular formula, same atom connectivity but different arrangements of the atoms in space. They have non-superimposable mirror images and rotate the plane polarise light. This phenomenon exhibited by the substance is called optical isomerism. It is observed in chiral objects.

Question 3

Define the following:
a)chirality
b)plane polarised light
c)enantiomers
d) diastereoisomers
e) geometrical isomerism
f) projection formula

Answer 3

a)Chirality: A molecule is said to be chiral if it cannot be super imposed on its mirror images.
Eg: a C atom linked to 4 different groups.

b) Plane polarised light: if a beam of light is made to pass through Nicol prism then it’s vibrations will only lie in one plane such a light is called plane polarised light. The substance which rotate the plane polarised light to the left are called Levorotatory substance. The substance which rotate the plane polarised light to the right are called Dextrotatory substances.

c) Enantiomers: optical isomers which are non-superimposable mirror images of each other, they always exist in discrete pairs.

d) Dextrotatory: optical isomers which are non-superimposable as well as not mirror images of each other.

e) Geometrical isomerism: It is a form of stereoisomerism where molecules have the same molecular formula and connectivity of atoms but differ in their spatial arrangements.
There are two type of Geometrical isomerism:
1) Trans-isomer: if two similar groups are present on opposite sides across the double bond then it is called trans-isomer.
2) Cis-isomer: if two similar groups are present on same sides across the double bond then it is called cis-isomer.

f) Projection formula: it is a 2D representation of a 3D organic molecule represented by Projection. It shows the spatial arrangements of bonds from their stereo chemical formula.

Question 4

Differentiation between Enantiomers and Diastereoisomers.

Answer 4

Enantiomers:
-They are mirror images of each other.
-Each Enantiomer pair has identical physical properties.
-They cannot be seperated by physical methods.
-Enantiomeric pairs rotate plane polarised light but in opposite direction.
-They are optically active.
-They have identical chemical properties with achiral reagents.

Diastereoisomers:
-They are not mirror images of each other.
-They have different physical properties.
-They can be seperated by physical methods.
-They may or may not rotate the plane polarised light in opposite direction or in same direction.
-They may or may not be optically active.
-They have different chemical properties with achiral reagents.