JAMB Chemistry 2024 Paper

Concept:Geometrical isomerism arises when each carbon atom of a double bond is attached to two different groups. So only the alkene whose double-bonded carbons satisfy this condition can show cis-trans isomerism.Explanation:The acyclic alkenes with molecular formula $\mathrm{C_4H_8}$ are $\text{but-1-ene}$, $\text{but-2-ene}$, and $\text{2-methylpropene}$. In $\text{but-1-ene}$, $\mathrm{CH_2=CHCH_2CH_3}$, one double-bonded carbon carries two identical hydrogen atoms, so it cannot show geometrical isomerism. In $\text{2-methylpropene}$, $\mathrm{(CH_3)_2C=CH_2}$, one double-bonded carbon carries two identical methyl groups, so it also cannot show geometrical isomerism. In $\text{but-2-ene}$, $\mathrm{CH_3CH=CHCH_3}$, each double-bonded carbon is bonded to a methyl group and a hydrogen atom. Therefore, but-2-ene can exist as two geometrical isomers: $\text{cis-but-2-ene}$ and $\text{trans-but-2-ene}$. Hence, the total number of geometrical isomers of butene is two. Answer:$2$ (Option A).