Question

Explain various types of constitutional isomerism (structural isomerism) in organic compounds.

Answer 1

Structural isomerism:

This type of isomers have the same molecular formula but differs in their bonding sequence.

1. Chain or nuclear or skeletal isomerism:
   These isomers differ in the way in which the carbon atoms are bonded to each other in a carbon chain or in other words isomers have similar molecular formula but differ in the nature of the carbon skeleton (ie. Straight or branched).
   \[\ce{CH3 - CH2 - CH2 - CH2 - CH3}\]
   n - Pentane
   \[\begin{array}{cc}\ce{CH3 - CH - CH2 - CH3}\\|\phantom{.........}\\\ce{CH3}\phantom{......}\end{array}\]
   iso pentane (or) 2 - methyl butane
   \[\begin{array}{cc}\phantom{...}\ce{CH3}\\|\\\ce{CH3 - C - CH3}\\|\\\phantom{...}\ce{CH3}\end{array}\]
   neopentane (or) 2, 2 - dimethyl propane
2. Position isomerism:
   If different compounds belonging to same homologous series with the same molecular formula and carbon skeleton, but differ in the position of substituent or functional group or an unsaturated linkage are said to exhibit position isomerism.
   Molecular formula C₅H₁₀
   \[\ce{CH3 - CH2 \underset{\text{pent-l-ene}}{- CH2 -} CH = CH2}\]  and
   \[\ce{CH3 - CH2 \underset{\text{pent-l-ene}}{- CH = C}H - CH3}\]
3. Functional isomerism:
   Different compounds having same molecular formula but different functional groups are said to exhibit functional isomerism.
   Molecular formula C₃H₆O
   CH₃ – CH₂ – CHO
   propanal (aldehyde group)
   \[\begin{array}{cc}\ce{O}\\||\\\ce{\underset{\text{propanone (keto group)}}{CH3 - C - CH3}}\end{array}\]