JEE Main · 2026 · 02 Apr 2026 · Shift 1 · Hydrocarbons

The isomers of ${\mathrm{C}}_5{\mathrm{H}}_{10}$ that do not decolourise $\mathrm{KMn}{\mathrm{O}}_4$ are the saturated cycloalkanes: cyclopentane, methylcyclobutane, 1,1-dimethylcyclopropane, 1,2-dimethylcyclopropane and ethylcyclopropane. Counting distinct hydrogen environments (structural isomers only): cyclopentane gives $1$; methylcyclobutane gives $4$ (C-1, C-2, C-3 and the methyl); 1,1-dimethylcyclopropane gives $2$ (the ring $\mathrm{C}{\mathrm{H}}_2$ and the methyl); 1,2-dimethylcyclopropane gives $3$ (the equivalent ring methines, the ring $\mathrm{C}{\mathrm{H}}_2$ and the methyl); ethylcyclopropane gives $4$ (the ring methine, the ring $\mathrm{C}{\mathrm{H}}_2$, and the two ethyl carbons). The total number of monochloro structural isomers is $1+4+2+3+4=14$.