JAMB Chemistry 2019 Paper

Concept:Isomers are compounds with the same molecular formula but different structural arrangements of atoms.Explanation:2-methylprop-1-ene has the structure $\mathrm{CH_2=C(CH_3)-CH_3}$.Counting the atoms gives its molecular formula as $\mathrm{C_4H_8}$.Therefore, any isomer of it must also have the formula $\mathrm{C_4H_8}$.But-2-ene has the structure $\mathrm{CH_3-CH=CH-CH_3}$ and the same molecular formula $\mathrm{C_4H_8}$.It is a structural isomer of 2-methylprop-1-ene because the carbon skeleton and the position of the double bond are different.The other options — 3-methylbut-1-ene, 2-methylbut-1-ene, and pent-2-ene — each contain 5 carbon atoms.Their molecular formula is $\mathrm{C_5H_{10}}$, so they cannot be isomers of a four-carbon alkene.Answer:C. but-2-ene