When the isomers differ only in the arrangement of atoms or groups within the molecule, without any reference to space, these are arrangement of atomisomers and the phenomenon as structural isomerism. Thus the structural isomers structuralismolecular formula, but possess different structural formulae.
Structural isomerism may again be of several types.
1. Chain, nuclear or skeleton isomerism : This type of isomerism is due to the difference in the nature of the carbon chain (i.e. straight or branched) which forms the nucleus of the molecule, e.g.,
(a) \(\underset {n-Butane}{CH_3 - CH_2 - CH_2 - CH_3}\) \(\underset {Isobutane}{CH_3 - \overset{\overset {CH_3}{|}}{C}H} -CH_3\)
(b) \(\underset {n-Pentane}{CH_3 .CH_2. CH_2 .CH_2 .CH_3.}\) \(\underset {Isopentane}{CH_3- \overset {\overset{CH_3}{|}}{C}H-CH_2 - CH_3}\) \(\underset {Neopentsane}{CH_3 - \underset{CH_3}{\underset{|}{\overset{CH_3}{\overset{|}C}}}-CH_2}\)
2. Position isomerism : It is due to the difference in the position of the substituent atom or group or an unsaturated linkage in the same carbon chain. Examples are
\(C_3 H_7 OH :\) \(\underset {n-Propanol\,(Propanol - 1)}{CH_3 - CH_2 - CH_2 - OH}\) \(\underset {iso-Propanol\,(Propanol - 2)}{CH_3 - \overset {\overset {OH}{|}}{C}H - CH_3}\)
\(C_4 H_8 :\) \(\underset {Butene -1}{CH_2 = CH - CH_2 - CH_3}\) \(\underset {Butene - 2}{CH_3 - CH =CH- CH_3}\)
\(CH_3 . C_6 H_4 .CH_3 :\) or \(C_8 H_{10}\)
3. Ring-chain isomerism : This type of isomerism is due to difference in mode of linkage of carbon atoms and the isomers may have either open chain or closed structure, e.g.,
\(C_3 H_6 :\) \(\underset {Propene}{CH_3 - CH = CH_2}\)
4. Functional isomerism : This type of isomerism is due to difference in the nature of functional group present in the isomers.
The following pairs of compounds always form functional isomers with each other.
(i) Dienes and alkynes
(ii) Alcohols and ethers
(iii) Aldehydes and ketones
(iv) Carboxylic acids and esters
(v) 1º, 2º and 3º amines
(vi) Nitroalkanes and alkylnitrites
(vii) Cyanides and isocyanides
(viii) Alcohols and phenols
5. Metamerism : It is due to difference in nature of alkyl groups attached to the same functional group. This type of isomerism is shown by compounds of the same homologous series. For example,
\(\underset {Methylpropyl\,ether}{CH_3 - O - C_3 H_7}\) \(\underset {Diethyl\, ether}{C_2 H_5 - O - C_2 H_5}\)
6. Tautomerism : Tautomerism1 may be defined as a phenomenon in which a single compound exists in two readily interconvertible structures that differ markedly in the relative position of at least one atomic nucleus, generally hydrogen.
