JEE Main · 2026 · 06 Apr 2026 · Shift 2 · Hydrocarbons

For complete combustion of ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2n+2}$ the oxygen needed is $\frac{3n+1}{2}$ moles; setting this to 8 gives $n=5$, so the alkane is ${\mathrm{C}}_5{\mathrm{H}}_{12}$. A single monochlorinated product requires every hydrogen to be equivalent, which holds only for 2,2-dimethylpropane, $(\mathrm{C}{\mathrm{H}}_3{)}_4\mathrm{C}$. Its four methyl carbons are all primary, so the number of primary carbon atoms is 4.