Class 12 NEET Chemistry Alcohols, Phenols And Ethers

Concept:The reaction sequence involves converting an alcohol into an alkyl halide via chlorination with $\mathrm{POCl}_3$, followed by nucleophilic substitution to give propyl bromide.Chemical Equation / Formula:Alcohol chlorination: $\mathrm{R-OH + POCl_3 \rightarrow R-Cl + H_3PO_3}$.Bromination: $\mathrm{R-Cl + Br^- \xrightarrow{SN2} R-Br + Cl^-}$.Explanation:Step 1: The starting alcohol (likely propanol) reacts with $\mathrm{POCl}_3$ to form the corresponding chloroalkane and phosphorous acid ($\mathrm{H_3PO_3}$). Thus, $X$ is $\mathrm{POCl}_3$.Step 2: The chloroalkane undergoes SN2 displacement with a bromide ion (e.g., from $\mathrm{NaBr}$ or $\mathrm{KBr}$) to yield the bromoalkane. Therefore, $Z$ is $\mathrm{CH_3CH_2CH_2Br}$ (propyl bromide).No other reagent among the options produces the correct bromo product under these conditions.Shortcut:Recall that $\mathrm{POCl}_3$ is a common reagent for converting alcohols to alkyl chlorides. The subsequent halogen exchange with $\mathrm{Br^-}$ gives the corresponding bromide.Answer:Option B: $X = \mathrm{POCl}_3$, $Z = \mathrm{CH_3CH_2CH_2-Br}$.