CBSE Class 10 Science 2019 · Paper 31/2/2 · Question 14

(i) Alkanes are saturated hydrocarbons in which the carbon atoms are joined by single bonds only; general formula ${\text{C}}_n{\text{H}}_{2n+2}$ (for example ethane, ${\text{C}}_2{\text{H}}_6$). (ii) Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond; general formula ${\text{C}}_n{\text{H}}_{2n}$ (for example ethene, ${\text{C}}_2{\text{H}}_4$). (iii) Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond; general formula ${\text{C}}_n{\text{H}}_{2n-2}$ (for example ethyne, ${\text{C}}_2{\text{H}}_2$). ${\text{C}}_4{\text{H}}_{10}$ fits ${\text{C}}_n{\text{H}}_{2n+2}$ with $n=4$, so it is an alkane (butane). Its two structural isomers are 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 shown in the figure.