CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 9(a)

Structural isomers are compounds that have the same molecular formula but different structures, that is, different arrangements of their atoms. Two isomers of butane, ${\text{C}}_4{\text{H}}_{10}$ (structures drawn in the figure): (1) butane (n-butane), the straight chain ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$; (2) 2-methylpropane (isobutane), the branched chain ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$. Propane, ${\text{C}}_3{\text{H}}_8$ (${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_3$), has no isomers because its three carbon atoms can be joined in only one way, as the chain C–C–C. Attaching the third carbon to the middle carbon gives the same chain drawn with a bend, and a branch needs a carbon joined to three other carbons, which requires at least four carbon atoms.