JEE Main · 2026 · 05 Apr 2026 · Shift 2 · General Organic Chemistry

A molar mass of $72\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ corresponds to the saturated hydrocarbon ${\mathrm{C}}_5{\mathrm{H}}_{12}$. Counting primary (terminal $-\mathrm{C}{\mathrm{H}}_3$) carbons: 2,2-dimethylpropane has four, n-pentane has two, and 1,1-dimethylcyclopropane (which is in any case ${\mathrm{C}}_5{\mathrm{H}}_{10}$, molar mass $70$) has two, whereas 2-methylbutane, $\mathrm{C}{\mathrm{H}}_3\mathrm{CH}(\mathrm{C}{\mathrm{H}}_3)\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{H}}_3$, has exactly three primary carbons. The hydrocarbon of molar mass $72\mathrm{g}\mathrm{mo}{\mathrm{l}}^{-1}$ with three primary carbons is 2-methylbutane.