JAMB Chemistry 2024 Paper

Concept:$\mathrm{ClCH=CHCl}$ can form isomers that have the same molecular formula but differ in the location of the chlorine atoms on the carbon chain.Explanation:The compound $\mathrm{ClCH=CHCl}$ is 1,2-dichloroethene, with the molecular formula $\mathrm{C_2H_2Cl_2}$.Positional isomerism occurs when identical substituents occupy different positions on the same carbon skeleton.For $\mathrm{C_2H_2Cl_2}$, the two chlorine atoms can both be attached to the same carbon atom, forming $\mathrm{Cl_2CH=CH_2}$ (1,1-dichloroethene).Alternatively, one chlorine atom can be attached to each carbon atom, giving $\mathrm{ClCH=CHCl}$ (1,2-dichloroethene).Since these two structures differ only in the position of the chlorine substituents, they are positional isomers.Chain isomerism is not possible because the molecule has only two carbon atoms.There is no change in functional group, so functional group isomerism does not apply.The molecule has no chiral centre, so it cannot show optical isomerism.Therefore, among the options provided, $\mathrm{ClCH=CHCl}$ can exhibit positional isomerism.Answer:B. Positional