Question

Identify compound 'A' used in the following reaction:
Benzoic acid $\xrightarrow[\Delta]{\text{A}}$ Benzoyl chloride + Phosphorus oxychloride + Hydrogen chloride

• A. $\text{SOCl}_2$
• B. $\text{PCl}_3$
• C. HCl
• D. $\text{PCl}_5$

Hint:

The presence of phosphorus oxychloride ($\text{POCl}_3$) as a reaction byproduct is an exclusive chemical signature proving that phosphorus pentachloride ($\text{PCl}_5$) was utilized as the main chlorinating reagent.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We must determine the identities of reagent 'A' which reacts with benzoic acid under heat to produce benzoyl chloride along with phosphorus oxychloride ($\text{POCl}_3$) and hydrogen chloride (HCl) as distinctive chemical byproducts.

Step 2: Detailed Explanation:
Carboxylic acids undergo nucleophilic substitution reactions where their hydroxyl group (-OH) is replaced by a halogen atom to form acyl chlorides. While multiple chlorinating agents exist, each leaves unique side products:
Treatment with $\text{SOCl}_2$ produces $\text{SO}_2$ and HCl gases.
Treatment with $\text{PCl}_3$ produces phosphorous acid ($\text{H}_3\text{PO}_3$).
Treatment with $\text{PCl}_5$ (phosphorus pentachloride) yields phosphorus oxychloride ($\text{POCl}_3$) and hydrogen chloride (HCl) gas alongside the target acyl halide structure.
Let's look at the balanced chemical transformation using benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$):
$$ \text{C}_6\text{H}_5\text{COOH} + \text{PCl}_5 \xrightarrow{\Delta} \text{C}_6\text{H}_5\text{COCl} + \text{POCl}_3 + \text{HCl} $$ The byproduct distribution listed in the question matches the reaction footprint of phosphorus pentachloride.

Step 3: Final Answer: The active compound 'A' is $\text{PCl}_5$, matching option (D).