Stereochemistry - Overview

Louis Pasteur was a first person who observed that the salts of tartaric acids only those that were collected from wine production vessels could rotate light, but those from other sources did not. He suggested that the only reason for the difference in these two types of tartrate salts is optical isomerism.

In 1874, Jacobus Hoff and Joseph Le Bel explained the optical activity further in terms of the tetrahedral arrangement of atoms bound to carbon.

Introduction

Stereochemistry

It is a sub-discipline of chemistry that involves the relative spatial arrangement of atoms that form the structure of molecules and deal with their manipulation.

It mainly focuses on dealing with stereoisomers.

Stereoisomerism

Stereoisomerism is defined as the ability to visualize and manipulate molecules in three dimensional pattern to study and understand the structural features.

Stereoisomers

Stereoisomers are those isomeric molecules that have the same molecular formula as well as sequence of bonded atoms (constitution) but they only have one difference that is the 3-dimensional arrangement of their atoms in space.

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Types of stereoisomers:

• Optical stereoisomers.
• Geometric stereoisomers.
• Optical stereoisomers:

Optical stereoisomers are those isomers that only differ in the arrangement of atoms in 3-dimmentional space and create mirror images of each other.

Geometric stereoisomers:

Geometric stereoisomers are those isomers that have difference in their spatial arrangement about a double bond.

Types of optical stereoisomers:

Enantiomers: Those pairs of stereoisomers that are non-superimposable mirror images of each other. They can also rotate plane polarized light in opposite directions and have same physical properties too.

Diastereomers: Those pairs of stereoisomers that are not mirror images of each other but they have different physical properties, e.g. boiling point, melting point etc.

Epimers: They are diastereomers which differ from each other at only one chiral centre.

Geometric or cis-trans isomerism:

Cis Isomerism: If two substituents on at least one end of a double bond are same, then there is no stereoisomer.

Trans Isomerism: If two substituents on at least one end of double bond are different, they are stereoisomers.

 Importance of stereoisomers:

Stereoisomers are very important in pharmaceutical production and also assist the breakdown of drugs in body. For example the Thalidomide which was firstly prescribed as a sleeping pill to lessen the morning sickness in pregnant women but later it was banned because it causes birth defects in baby.

Our ability to taste and smell is regulated by chiral molecules in our mouth and nose that act as a receptor to 'sense' foreign substances. Different enantiomers interact differently with receptor molecules and induce different "sensations".

Stereoisomers are also useful in cancer chemotherapies. Those that are primarily use in chemotherapy include leucovorin , ifosfamide and verapamil.

Stereoisomers are also helpful in insects for their interaction and mating with others by inducing different smell and sensations.