Chain or nuclear isomerism
Two organic compounds having same molecular formula differ in the nature of the carbon atom chain. This kind of isomerism is known as chain or nuclear isomerism. For e.g., n-butane and isobutane, have the same molecular formula C4H10. While n-butane is a straight chain, isobutane has a branched structure.
When the same substituent occupies different positions in organic compounds having same molecular formula, the compounds exhibit position isomerism. For e.g.,H3C-CH2-CH2-I is n-propyl iodide or 1-iodopropane and H3C-CHI-CH3 is isopropyl iodide or 2-iodopropane.
Note that both the compounds have the same molecular formula C3H7I. In n-propyl iodide the substituent is attached to the first carbon atom whereas in isopropyl iodide it is attached to the second carbon atom.
Organic compounds having same molecular formula but different functional groups exhibit functional isomerism. For e.g., ethanol (C2H5OH) and methoxy methane (H3C-O-CH3) have the same molecular formula C3H6O. But the functional group in ethanol is the alcohol group whereas in methoxy methane, it is the ether group.
Some members of the same homologous series differ in the nature of the alkyl groups attached to the polyvalent atom of the functional group. Such compounds are said to exhibit metamerism.For e.g., diethyl ether (H5C2OC2H5), methyl n-propyl ether (H3COCH2CH2H3C) and methyl isopropyl ether [H3COCH(CH3)2]are metamers. All of them have the same molecular formula C4H10O but the alkyl groups are different.
Tautomerism is a dynamic isomerism where one isomer is constantly changing into the other and vice versa. For e.g., alky cyanides (RCN) and alkyl isocyanides (RNC) are tautomers.