CBSE Class 10 Science 2017 · Paper 531 · Question 10

Structural isomers are compounds that have the same molecular formula but different structures, that is, different arrangements of their atoms (for example, of their carbon chain). The first three alkanes are methane (${\text{CH}}_4$), ethane (${\text{C}}_2{\text{H}}_6$) and propane (${\text{C}}_3{\text{H}}_8$). In an alkane, with one, two or three carbon atoms there is only one way of joining the carbon atoms, as a single open chain; a branch needs a carbon atom bonded to three other carbon atoms, which requires at least four carbon atoms. So each of them has only one possible structure, and isomers are not possible. The fourth member, butane (${\text{C}}_4{\text{H}}_{10}$), has two structural isomers: n-butane, ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$ (straight chain), and isobutane or 2-methylpropane, ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$ (branched chain). Their full structural formulas are drawn in the figure.