Question

Assertion (A) : Phosphorus halides are preferred over thionyl chloride for the preparation of alkyl halides from alcohols.

Reason (R) : Reaction of alcohols with thionyl chloride forms pure alkyl halides.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Thionyl chloride is preferred because its by-products SO2 and HCl are gases, so A is false and R is true.

Answer 1

The Correct Option is D

Step 1: Write the thionyl chloride reaction:
\[ \mathrm{R-OH + SOCl_2 \rightarrow R-Cl + SO_2 \uparrow + HCl \uparrow} \]
Both by-products, \( \mathrm{SO_2} \) and HCl, are gases.
They leave the mixture on their own, so the alkyl chloride is obtained pure.

Step 2: Check the Reason:
The Reason says that thionyl chloride gives pure alkyl halides. This is true.

Step 3: Write the phosphorus halide reactions:
\[ \mathrm{3R-OH + PCl_3 \rightarrow 3R-Cl + H_3PO_3} \]
\[ \mathrm{R-OH + PCl_5 \rightarrow R-Cl + POCl_3 + HCl} \]
The by-products \( \mathrm{H_3PO_3} \) and \( \mathrm{POCl_3} \) are not gases. They must be separated from the product.

Step 4: Check the Assertion:
Because of the easy work-up, thionyl chloride is the preferred reagent.
So the Assertion, which says phosphorus halides are preferred, is false.

Final Answer:
A is false and R is true, option (D). \[ \boxed{\text{(D)}} \]