Two or more compounds having the same molecular formula but different chemical and physical properties are called isomers and the phenomenon is known as isomerism. There are two types of isomerism
Compounds having the same molecular formula but different structures are known as structural isomers. Structural isomerism is further divided into six subcategories. They are one by one discussed below :
Compounds having same molecular formula but different carbon skeletons are known as chain isomers and the phenomenon is known as Chain isomerism.
Compounds having same molecular formula but differ in the position of the multiple bond (double or triple bond) or functional group are known as position isomers and the phenomenon is known as position isomerism.
Compounds having same molecular formula but different functional groups are called functional isomers and the phenomenon is called functional isomerism.
General formula for ether is R-O-R' where R and R' are different (or similar) carbon chains.
Compounds having same formula but different alkyl chains on either side of the functional group are known as metamers and the phenomenon is known as metamerism.
Notice that the above compounds are also position isomers.
Tautomerism arises due to 1,3-migration of a hydrogen atom within the same molecule. The best example of tautomerism is keto-enol tautomerism, where one form contains the keto group (>C=O) and the other form contains the enol (en + ol) group.
Acetaldehyde and ethenol
You may note that in keto-enol tautomerism, enol form is usually negligible.
Compounds having same molecular formula but one has an open chain while the others have cyclic structures are called ring-chain isomers and the phenomenon is known as ring-chain isomerism.
Compounds having same formula but different spatial arrangement of atoms or group of atoms in space are known as stereoisomers and the phenomenon is called stereoisomerism. Stereoisomerism has the following two subcategories :
Conformational isomers are those isomers in which the relative positions of some of the atoms differ in the molecule in the three-dimensional space due to rotation about sigma bonds. We will discuss conformations later in Alkanes Isomerism.
Configurational isomerism arises due to breaking and remaking of covalent bonds. They are further divided into :
Geometrical isomerism is discussed in Alkene.
Optical isomerism is a relatively vast topic. We will upload its tutorial very soon.