CBSE Class 10 Science 2017 · Paper 31(B)/C · Question 19

(a) Isomers are compounds that have the same molecular formula but different structures. The two isomers of ${\text{C}}_4{\text{H}}_{10}$ are butane (n-butane), ${\text{CH}}_3-{\text{CH}}_2-{\text{CH}}_2-{\text{CH}}_3$, a straight chain, and isobutane (2-methylpropane), ${\text{CH}}_3-\text{CH}({\text{CH}}_3)-{\text{CH}}_3$, a branched chain (full structures in the figure). (b) The reaction is oxidation: alkaline ${\text{KMnO}}_4$ oxidises ethanol to ethanoic acid, and its purple colour disappears while ethanol is left. ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{alkaline KMnO}}_4+\text{heat}}{\to }{\text{CH}}_3\text{COOH}$ (with the oxygen shown: ${\text{CH}}_3{\text{CH}}_2\text{OH}+2[\text{O}]\to {\text{CH}}_3\text{COOH}+{\text{H}}_2\text{O}$). (c) The product is ethene, ${\text{CH}}_2={\text{CH}}_2$. Concentrated ${\text{H}}_2{\text{SO}}_4$ acts as a dehydrating agent, removing water from ethanol: ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{hot conc. H}}_2{\text{SO}}_4,443\text{K}}{\to }{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$.