Isomerism is the phenomenon whereby certain compounds, with the same molecular formula, exist in different forms owing to their different organisations of atoms. The concept of isomerism illustrates the fundamental importance of molecular structure and shape in organic chemistry.
Structural Isomers have different structural formulae because their atoms are linked together in different ways.
It arises owing to:
· arrangement of Carbon skeleton
e.g. The formula C4H10 represents two possible structural formulae, butane and methylpropane:
· position of Functional group
e.g. propan-1-ol and propan-2-ol
· different Functional groups
e.g. the molecular formula C2H60 represents both ethanol and methoxymethane.
Cyclic alkanes are isomeric with alkenes, e.g. cyclopropane and propene
Stereoisomers have the same structure and bond order but their atoms and groups of atoms are arranged differently in space. They have different spatial arrangements and their molecules are not superimposable. There are two types:
· Geometric Isomerism
Involves a double bond, usually C=C, that does not allow free rotation about the double bond (unlike a C-C single bond). They are not superimposable.
e.g. cis-but-2-ene and trans-but-2-ene
Stereoisomers may possess quite different physical properties, such as melting point, density and solubility in water (look up those of maleic acid and fumaric acid). Ring structures and other steric factors also result in geometric isomerism.
· Optical isomerism
Involves an atom, usually carbon, bonded to four different atoms or groups of atoms. They exist in pairs, in which one isomer is the mirror image of the other.
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 polarised 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.