In the following reaction sequence, X and Z respectively are :

![](https://res.cloudinary.com/ddr1kuyb3/image/upload/v1786112582/questions/pnnf9zipuobixo2nmksk.png)

Question

In the following reaction sequence, X and Z respectively are :

![](https://res.cloudinary.com/ddr1kuyb3/image/upload/v1786112582/questions/pnnf9zipuobixo2nmksk.png)

Accepted Answer

B. 1. **Look at the type of transformation happening in the sequence.** The starting alcohol is being converted step by step into other functional groups, and reagent X is used at the very first step to replace an –OH group.

2. **Recall how alcohols react with phosphorus halides.** Both $\mathrm{PCl}_3$ and $\mathrm{PCl}_5$ convert $\mathrm{R-OH}$ into $\mathrm{R-Cl}$, but $\mathrm{PCl}_5$ additionally produces phosphorus oxychloride as a by-product:
$$
\mathrm{R-OH} + \mathrm{PCl}_5 \rightarrow \mathrm{R-Cl} + \mathrm{HCl} + \mathrm{POCl}_3
$$

Based on the steps shown in the sequence, the reagent X used corresponds to $\mathrm{POCl}_3$ acting as the chlorinating agent.

3. **Follow the later steps of the sequence**, where the chloride intermediate is converted further until the final halogenated product Z is obtained.

Since the last step introduces a bromide in place of the earlier leaving group, the final product Z is the corresponding bromo-compound:
$$
\mathrm{Z} = \mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2-\mathrm{Br}
$$

4. **Match with the given options.** Only option B pairs $\mathrm{X}=\mathrm{POCl}_3$ with $\mathrm{Z}=\mathrm{CH}_3\mathrm{CH}_2\mathrm{CH}_2-\mathrm{Br}$, consistent with the reaction sequence.

Hence, the answer is **option B**.