CBSE Class 10 Science 2017 · Paper 531 · Question 7

(a) A, B and C are alkenes: unsaturated hydrocarbons with one carbon-carbon double bond, members of one homologous series. The reason is their general formula, ${\text{C}}_n{\text{H}}_{2n}$, which is that of the alkene series: it has two hydrogen atoms fewer than the alkane with the same number of carbon atoms, ${\text{C}}_n{\text{H}}_{2n+2}$, which for an open chain means one C=C double bond (a ring compound such as cyclohexane, ${\text{C}}_6{\text{H}}_{12}$, also fits ${\text{C}}_n{\text{H}}_{2n}$, but the homologous series with this general formula in the textbook is the alkene series). (b) C has the maximum number of carbon atoms. In a homologous series the boiling point rises as the molecular mass, and so the number of carbon atoms, increases; C has the highest boiling point, $-0.5{}^{\circ }\text{C}$. (c) The second member is propene, ${\text{C}}_3{\text{H}}_6$, ${\text{CH}}_3-\text{CH}={\text{CH}}_2$. Its structural formula is $$\begin{array}{ccccccc}& & \text{H}& & \text{H}& & \text{H}\\ & & |& & |& & |\\ \text{H}& -& \text{C}& -& \text{C}& =& \text{C}\\ & & |& & & & |\\ & & \text{H}& & & & \text{H}\end{array}$$