Stereo Isomers (Optical Isomers)

In stereochemistry, stereoisomerism, or spatial isomerism, is a form of isomerism in which molecules have the same molecular formula and sequence of bonded atoms, but differ in the three-dimensional orientations of their atoms in space.

1. STEREOISOMERS
Optical Isomers

2. INTRODUCTION
Isomerism in which two or more than two
compounds have the same molecular formula and
structural formulae, but different spatial
arrangements of groups or atoms is called
Stereoisomerism.
Due to different spatial arrangement of groups or
atoms the stereoisomerism have different
configuration, i.e. different three dimensional
structures.

3. CLASSIFICATIONOFSTEREOISOMERISM
Stereoisomerism
Configurational
Optical isomers
Geometrical
isomers
Conformational

4. OPTICALISOMERS
On an outline, optical isomerism arise due to the
ability of an organic compound to rotate the plane of
polarised light.
Now, to understand the phenomenon of optical
isomerism, it is necessary to study some basic
fundamentals.
Plane polarised light-
It has been well established by wave theory that an
ordinary beam of light vibrates in all the possible
directions.
This ordinary light can be split up into two beams,
each vibrating in one plane only with the help of
polarimeteror polarize. This light vibrating in one
plane only is called Plane Polarised Light.
The extent of polarisation of light is measured by a
special instrument called Polarimeteror Polarizer.

5. SCHEMATICREPRESENTATIONOFAPOLARIMETER

6. OPTICALACTIVITY
Optical property of any substance to rotate the plane of
polarized light.
Now, let a sample to be tested be placed in the sample tube-
If the sample does not affect the plane of polarisation, light
transmission is still at the maximum and the substance is
said to be Optically Inactive.
But if the substance rotates the plane of polarisation, the
transmitted light dims and now the lens near our eyes
must be rotated to conform with the new plane so that light
transmission may be maximum. In this condition, the
substance is said to be Optically Active.
If the substance rotates the plane of polarisation towards
right, i.e. clockwise, it is Dextro-rotatory(Latin:
dextermeans right) and if towards left, i.e.
anticlockwise, it is Laevo-rotatory(Latin: laeves
means left) in nature.
Dextroand Laevoforms of any optically active compound
are mirror images of each others.

7. ANGLEOFROTATION
The extent to which the plane of polarisation is
rotated either towards right or left is called
Angle of rotation.
Further, the angle of rotation depends upon;
Nature of compound
Concentration of the solution
Thickness of the layer crossed by polarized light
Nature of solvent
Temperature of the solution

8. CONDITIONSOFOPTICALACTIVITY
First and most important condition for a
compound to be optically active is Chirality.
At least one chiralcarbon atom is necessary in a
compound for optical rotation.
A chiralcarbon atom is that whose all the four
valenciesare satisfied with four different groups.
Further, besides the presence of chiralcentre in
the molecule, a compound to be optically active
must also not contain any element if symmetry.

9. ENANTIOMERS
The optical isomers which are mirror images of each
other and are non-superimposableare called
Enantiomers.This phenomena is called
Enantiomerism.
These two forms are mirror images of each other but
non-superimposable. Thus these two are enantiomers
of each others.
Both enantiomersare optically active and rotate the
plane of polarisation to the same extent but in
opposite directions.
One enantiomersis dextro-rotatorywhile other is
laevo-rotatory.

10. DIASTEROMERISM
This is the other class of optical isomers, which
are not the mirror images of each other. These
are called as Diasteromers. This phenomena is
called Diasteromerism.
e.g. Four forms can be written for 3-bromo-2-
butanol.
All these four forms are optical isomers as they
differ only in the arrangement of groups at chiral
centres.

11. DIASTEROMERISM
Out of these four forms 1 & 2 and 3 & 4 constitute
such pairs which are mirror images of each other.
Thus, they are considered to be the Enantiomeric
pairs.
But if we consider 1 & 3 and 1 & 4, similarly 2 & 3
and 2 & 4, these pairs are optical isomers but are not
the mirror images of each other. Thus, they are
considered to be the Diasteromericpairs.
They have their own individual properties. They
differ in their physical, chemical and optical
properties.
They can be easily separated from the mixture by any
convenient technique, as Distillation,
Chromatography etc.