When isomers have the same structural formula but differ in relative arrangement of atoms or groups in space within the molecule, these are known as stereoisomers and the phenomenon as stereoisomerism. The spatial arrangement of atoms or groups is also refferred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration.
Stereoisomerism is of three types
1. Geometrical isomerism : The isomers which possess the same structural formula but differ in the spatial arrangement of the groups around the double bond are known as geometrical isomers and the phenomenon is known as geometrical isomerism. This isomerism is shown by alkenes or their derivatives. When the similar groups lie on the same side, it is the cis-isomer; while when the similar groups lie on opposite sides, the isomer is trans.
For example,
2. Optical isomerism : This type of isomerism arises from different arrangement of atoms or groups in three dimensional space resulting in two isomers which are mirror image of each other. Optical isomers contain an asymmetric (chiral) carbon atom (a carbon atom attached to four different atoms or groups) in their molecules. For example, lactic acid having four different groups on the central carbon atoms is optically active'; while propanoic acid having two similar atoms on the central carbon atom is optically inactive.
The only essential condition for a chiral molecule is that it does not superimpose on its mirror image.
Optical isomers have similar chemical and physical properties and differ only in their behaviour towards plane polarised light. The isomer which rotates the plane polarised light towards left is known as laevo (l) while that which rotates the plane polarised light to right is known as dextro (d).
For example,
3. Conformational Isomers : The isomers which differ in the conformation are known as conformational isomers. This type of isomerism is found in alkanes and cycloalkanes and their substituted derivatives.
Conformation of alkanes : According to Kemp and Pfitzer there is restricted (but not free) rotation about a single bond. The restricted rotation lead to the existence of a single compound of one configuration into more than one spatial arrangements (conformations). So now conformation, also called rotational isomer or rotamer, may be defined as the term used to denote any one of the infinite number of spatial arrangements of the atoms of a molecule that can arise from rotation about a single bond.
Difference between conformation and configuration : The term conformation should not be confused with the configuration which relates to those spatial arrangements of the atoms of a molecule that can be changed only by the breaking and making of bonds whereas the spatial arrangements in conformation are changed simply by rotation about a single bond. In other words we can say that the various stereoisomers that differ in configuration can be interconverted only by the breaking and making of bonds whereas the conformational arrangement may be interconverted by rotation of one part of the molecule with respect to the rest of the molecule about a single bond joining these two parts. The various conformations of some of the important compounds are dealt in the book to clarify the point.
Representation of conformation : Conformation of alkanes can best be represented with the help of Newmann projection formulae since eclipsing (overlapping or crowding) of hydrogen atoms can best be represented by this formula. In the Newmann projection formula, the carbon atoms nearer to the eye (ie, the front carbon) and the groups attached to it are represented by equally spaced radii and the distant carbon atom (ie. the carbon atom farther from the eye or the rear carbon) and the groups attached to it are represented by a circle with three equally spaced radial extensions.
Front carbon
Rear carbon
Newmann projection formula for H3C-CH3
Conformations of Ethane. The conformation of ethane, H3C-CH3 affords the simplest possible introduction of the subject. Imagine that one of the methyl groups is rotated along the C-C axis keeping the rest of molecule undisturbed. An infinite number of possible arrangements of the rotated methyl group with respect to the undisturbed methyl group are possible, each of these possible arrangement represents a conformation.
