Q.Assertion: Phosphorus chlorides (tri and penta) are preferred over thionyl chloride for the preparation of alkyl chlorides from alcohols.
Reason: Phosphorus chlorides give pure alkyl halides.

Key idea: The assertion is about preference in laboratory preparation, not about purity alone — thionyl chloride actually gives a purer product because the byproducts (SO₂, HCl) are gases that escape, leaving no residue.

Reasoning:

1. Thionyl chloride ($\ce{SOCl2}$) reacts with alcohols to give alkyl chlorides, with $\ce{SO2}$ and $\ce{HCl}$ as gaseous byproducts — these escape, yielding a very pure product directly.
2. Phosphorus chlorides ($\ce{PCl3}$ or $\ce{PCl5}$) produce byproducts like $\ce{H3PO3}$ or $\ce{POCl3}$ that are non-volatile and contaminate the alkyl halide, requiring additional purification. …

Why thionyl chloride is preferred, not phosphorus chlorides

Thionyl chloride's byproducts, sulfur dioxide and hydrogen chloride, are both gases that escape the reaction mixture on their own. What's left behind is the alkyl chloride with essentially nothing else to remove — no extra purification step is needed. This is exactly why thionyl chloride, not phosphorus tri- or pentachloride, is the standard preferred reagent.

Why the reason is also wrong

Phosphorus trichloride's byproduct, phosphorous acid ($H_3PO_3$), and phosphorus pentachloride's byproduct, phosphoryl chloride ($POCl_3$), are both liquids with boiling points close to many alkyl chlorides. They do not simply escape the way $SO_2$ and $HCl$ gases do — separating them from the alkyl halide product requires extra work (fractional distillation, washing), and some contamination is common in practice. So "phosphorus chlorides give pure alkyl halides" is not actually true; if anything, this is precisely the drawback that makes chemists prefer thionyl chloride instead. …

Method: Assertion-Reason Analysis

Concept: Preparation of alkyl chlorides from alcohols — comparing reagents (PCl₃, PCl₅ vs. SOCl₂) based on purity and byproducts.

Steps:

1. Analyze the Assertion

   • PCl₃ and PCl₅ do convert alcohols to alkyl chlorides.
   • However, SOCl₂ is generally preferred because it produces gaseous byproducts (SO₂ and HCl) that escape, leaving a pure product.
   • PCl₃ and PCl₅ produce H₃PO₃ or POCl₃ as byproducts, which are difficult to separate from the alkyl halide.
   • Result: Assertion is wrong.

2. Analyze the Reason

   • The reason states: "Phosphorus chlorides give pure alkyl halides." …

Here's a breakdown of the common mistakes students make on this concept and how to avoid each.

Common Mistake #1: Thinking "preferred" and "purity" point different ways

• The mistake: Students think the assertion must be true because phosphorus chlorides are cheap and commonly used, so they must be "preferred."
• Why it's wrong: This chapter's own text is explicit about WHY thionyl chloride is preferred: its byproducts (SO₂, HCl) are gases that escape, giving pure alkyl halide directly with no extra purification step. Phosphorus chlorides give non-volatile byproducts (H₃PO₃ from PCl₃, POCl₃ from PCl₅) that stay mixed with the product.
• How to avoid: Don't invent a rationale ("industrial vs lab scale") the chapter never states. The stated reason for the preference IS purity — and thionyl chloride wins on that count, not phosphorus chlorides. The assertion (phosphorus chlorides preferred) is false.

Common Mistake #2: Assuming the Reason is correct because it "sounds right"

• The mistake: Students assume "phosphorus chlorides give pure alkyl halides" must be true since PCl₃/PCl₅ are standard textbook reagents.
• Why it's wrong: Write the reactions out: $3R\text{-}OH + PCl_3 \rightarrow 3R\text{-}Cl + H_3PO_3$ and $R\text{-}OH + PCl_5 \rightarrow R\text{-}Cl + POCl_3 + HCl$. $H_3PO_3$ and $POCl_3$ are not gases — they remain mixed with the alkyl halide, requiring further purification. The reason is false.
• How to avoid: Contrast directly with $SOCl_2$: $R\text{-}OH + SOCl_2 \rightarrow R\text{-}Cl + SO_2\uparrow + HCl\uparrow$ — both byproducts escape as gas, which is exactly why thionyl chloride (not phosphorus chlorides) is the preferred reagent for a pure product.

Common Mistake #3: Picking (iii) or (i) because the Reason "sounds plausible"

• The mistake: Since both the assertion and reason individually mention "phosphorus chlorides" and "pure," students assume at least one of (i)/(iii) must be right. …