SPATIAL STRUCTURE OF MOLECULES I
Isomerism. Structural isomerism - types. Tautomerism.
I. Isomerism. Types of isomerism. There are a number of chemical compounds (inorganic and organic) with the same molecular formula but different physical, chemical and biological properties.
In 1830 Berzelius, who studied this phenomenon, proposed to call it “isomerism” and to name the compounds which have the same composition but different properties “isomers” (from Greek “isos” = the same, and “meros”= parts).
The difference in the properties of isomers is due to the difference in the relative arrangements of atoms in their molecules.
Compounds which have the same molecular formula (i.e. the same qualitative and quantitative composition and equal molecular mass), but differ in the arrangement of the atoms within the molecules and in the properties (physical, chemical, and biological) are named isomers.
There are two main types of isomerism:
A. Structural isomerism and B. Space or stereoisomerism.
A. Structural isomerism. When isomerism arises due to the difference in the arrangement of atoms within a molecule, without any reference to space, it is termed structural isomerism. Isomers, which differ in the order in which their atoms are bonded, are often referred to as structural isomers (or constitutional isomers). They have the same molecular formula but differ in the order in which their atoms are connected. Structural isomerism can further be divided into five types. They are:
- Chain isomerism
- Position isomerism
- Functional isomerism
- Metamerism
- Tautomerism
1. Chain isomerism. Chain isomerism is a type of structural isomerism. It is possible for compounds with different carbon skeletons. This type of isomerism arises due to the difference in the nature of the carbon chain, i.e. straight or branched chain. This isomerism is possible when the compounds (hydrocarbons and their derivatives) contain four or more carbon atoms in its molecule. Because they differ in structure, they are structural isomers. Their chemical and physical properties are different and they are different and quite distinct compounds.
Examples:
a) Normal butane and iso-butane are chain isomers. They have the same molecular formula (C4H10) but differ in the structure of carbon chain:
b) pentane (C5H10) exists in three isomeric forms:
The number of isomers of alkanes grows with each number of carbon atoms from the molecule. For example, the decane C10H22 has 75 isomers and the hydrocarbon eicosane C20H42 can have approximately 366319 isomers.
c) Butyl alcohol (C4H9OH) has the following two chain isomeric forms:
d) Other chain isomers: