Isomerism

Isomerism

Organic compounds having same molecular formula but differing from each other by there physical or chemical properties are called isomers.

The difference in properties of isomers is due to the difference in the relative arrangements of various atoms or groups present in their molecules. Isomerism can be classified as follows:

Structural isomerism:

Structural isomerism occurs when two or more organic compounds have the same molecular formulae, but different structures. These differences tend to give the molecules different chemical and physical properties. There are three types of structural isomerism that you need to be aware of: chain isomerism, positional isomerism and functional isomerism. There is a fourth type, known as tautomerism.
1) Chain isomerism – is a consequence of the way carbon atoms connect to each other. For example, for the pentane molecule there are three possible chain isomers.

2) Position isomerism - such isomers differ in the position occupied by a substitute or a double bond in the molecule. For example, if in the n-pentane molecule a hydrogen atom is replaced by a chlorine atom, it can have different positions in the molecule and the following position isomers will arise.

In case of pentene, position of double bond results in the existence of four isomers .

Position isomers can also occur on benzene rings.

3) Functional group isomerism - compounds which have the same molecular formula but different functional groups are called functional group isomers.

Configurational or Sterio isomerism

Stereochemistry deals with the way atoms of a molecule are arranged in tridimensional space, so the spatial structure of the molecule. Chemical compounds with the same composition and structure, but who differ by the way which atoms are arranged in tridimensional space are called stereoisomers.

1)   Geometric (cis-trans) isomerism :

This type of isomers usually occurs in unsaturated organic compounds where there is a restricted rotation somewhere in the molecule. The restricted rotation around the C=C axis is explained by the sp2 hybridization of the carbon atoms which determines the same plane placement of the substitutes chained by this atoms. Depending on their position of double bond’s plane we can have cis geometric isomers (identical substitutes are on the same side of the double bond) and trans geometric isomers (identical substitutes are on both sides of the double bond), respectively Z (zusammen) and E (entgegen, in german). Cis-trans isomers differ in their physical properties such as: boiling point, density and refractive index. Usually these values are bigger for the cis isomers.

2) Optical isomerism:

  Substances that have the capability to rotate the plane of polarized light with a certain angle, when crossed by it, are called optically active substances. Substances that rotate the plane of polarized light to the right are called dextrorotary (prefixed with +) and the ones witch rotate to the left are called levorotatory (prefixed with -). Optical activity is due either to crystal structure (for example quartz, hydrazine sulfate or potassium chlorate are optically active in the crystalline state, but inactive in solution or vapor) or molecule structure (lactic acid, tartaric acid, glucose, glycerin aldehyde are revolving the plane of polarized light in any state of aggregation, solid, liquid or gas).

e.g. butan-2-ol

These isomers are referred to as Enantiomers. The central carbon atom to which four different atoms or groups are attached, is called an asymmetrical carbon atom.

Enantiomers have identical physical constants, such as melting points and boiling points, but are said to be optically active since they can be distinguished from each other by their ability to rotate the plane of polarized light in opposite directions. A mixture of Enantiomers in equal proportions is optically inactive, and is called a racemic mixture. Use an organic chemistry textbook to find out more about optical isomerism.