Stereochemistry B.Sc. 2nd Year

Stereochemistry Notes B.Sc. 2nd Year


Elements of Symmetry

Elements of symmetry are a simple tool to identify whether a molecule is chiral or not. The essential conditions for optically active molecule to be chiral is that, the molecule should not possess any kind of symmetry elements. The elements of symmetry are given below:
a. Simple axis of symmetry (Cn)
b. Plane of symmetry (σ)
c. Centre of symmetry (Ci)
d. Alternating axis of symmetry (Sn)

Simple axis of symmetry (Cn)

When a rotation of 360°/n (where n is any integer like 1,2,3...etc.) around the axis of a molecule is applied, and the rotated form thus obtained is indistinguishable from the original molecule, then the molecule is known to have a simple axis of symmetry. It is represented by Cn.
proper axis of rotation

Plane of symmetry (σ)

It is defined as when a plane that devide a molecule into two equal halves which are related to object and mirror image is known as plane of symmetry. It is represented by σ.
plane of symmetry

Centre of symmetry (Ci)

It is also called centre of inversion. A molecule has a centre of symmetry when, for any atom in the molecule, an identical atom exists diametrically (diagonally) opposite to this centre and at equal distance from it.
center of symmetry
Alternating axis of symmetry (Sn)
An alternate axis of symmetry is defined as, when a molecule is rotated by 360°/n degrees about its axis and then a reflection plane is placed exactly at perpendicular to the axis, and the reflection of the molecule thus obtained is identical to the original. It is represented by Sn.


Optically active chiral molecules which are non-super imposable on its mirror images are called enantiomers and the phenomenon is known as enantiomerism. For a molecule to be an optical isomerism the molecule must have at least one asymmetric carbon atom.
We can easily understand the chirality by comparing our hands (left hand and right hand). Our left hand and right hand are the best example of non-superimposable mirror image of each other. Each hand is therefore considered as chiral.

The main Properties of enantiomers are given below-

The main properties of enantiomers are given below-
a. Enantiomers always exist in pair.
b. Enantiomers are non-super imposable on its mirror image.
c. Enantiomers have same physical properties (like boiling point, melting point, solubility, density, viscosity, refractive index etc.)and chemical properties in achiral environment.
d. Each enantiomers have opposite behavior with respect to plane polarized light, if one of them will rotate the plane polarized light towards right hand direction then definitely the other will rotate the plane polarized light towards left hand direction.
e. Each enantiomers shows the same chemical reactivity with achiral reagent; however they have different reactivity with chiral reagent.


Diastereomers are simply not enantiomers. They are compounds which have the same molecular formula and sequence of bonded elements but are not mirror image and nonsuperimposable. The must essential conditions for a molecule to be diastereomer there must be at least two asymmetric carbon.

The main properties of. Diastereomers are given below-

a. Diastereomers have different physical properties such as melting points, boiling points, densities, solubilities, refractive indices, dielectric constants and specific rotations.
b. Diastereomers other than geometrical isomers may or maynot be optically active.
c. Diastereomers show similar, but not identical chemical properties. The rates of reactions of the two diastereomers with a given reagent provided tha reagent is not rapidly active.
d. Diastereomers can be separated from one another through techniques like fractional crystallization, fractional distillation, chromatography etc.

Chiral and Achiral Molecules with Two Stereogenic Centres:

Chiral molecules are those in which the central carbon atom is bonded directly through four different atoms or group of atoms and do not have any type of symmetry element present in it and the molecule has non-super imposable mirror image.
Achiral molecules are those molecules in which central carbon atom is directly bonded through four different atoms of group of atoms and it satisfied any type of symmetry elements are called achiral molecule. Achiral molecules have super imposable mirror images.
Let us consider the stereoisomers of Tartaric acid which has two stereocenters with identical atoms or group of atoms attached to both the stereocenters (chiral carbon).
tartaric acid
The tartaric acid have two stereocenters and can have four stereoisomers (i.e. 22) out of which two stereoisomers are non-super imposable mirror image of each other called enantiomers and chiral, and the rest two are identical to each other and also have plane of symmetry hence it can be divided in to two equal halves. So, they are achiral.

Erythro (Syn) and Threo (Anti) Diastereomers

The threo and erythro naming is given only to those diastereomers having two adjacent stereocentres. Erythro and Threo are generally applied only to those molecules which do not have symmetric ends.
erythro and threo
If the similar atoms or group of atoms on adjacent stereocentres of diastereomer are on same (syn) side it is designated as Erythro, whereas if the similar atoms or group of atoms on adjacent stereocentres of diastereomer are on opposite (anti) side the diastereomer is designated as threo.
Each erythro and threo stereoisomer can have their non-super imposable mirror image (enantiomers).