WAEC Chemistry 2020 Paper

Concept:The parent chain is chosen so that the double bonds get the lowest possible locants, and the substituent is then named as a prefix.Explanation:The given structure has a straight chain of four carbon atoms containing two double bonds, so the parent hydrocarbon is $\text{buta-1,3-diene}$.The chain is numbered from the end nearer the double bond, giving the double bonds the locants $1$ and $3$.A chlorine atom is attached to carbon $2$, so the substituent is named as $\text{chloro}$ at position $2$.Thus, the correct IUPAC name becomes $\text{2-chlorobuta-1,3-diene}$.Answer:A. $\text{2-chlorobuta-1,3-diene}$