Question

Identify the reagent used in the following reaction.
Benzoic acid $\xrightarrow{\text{Reagent}$ Benzoyl chloride + Phosphorus oxychloride + Hydrogen chloride}

• A. PCl$_3$
• B. HCl
• C. PCl$_5$
• D. SOCl$_2$

Hint:

$\text{PCl}_5$ always gives $\text{POCl}_3$ as a byproduct when reacting with oxygen-containing organic compounds.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Carboxylic acids can be converted to acid chlorides using various reagents. The identity of the reagent can be determined from the byproducts formed.
Step 2: Key Formula or Approach:
Match products to specific reactions:
- \(\text{PCl}_3 \to \text{H}_3\text{PO}_3\)
- \(\text{SOCl}_2 \to \text{SO}_2 + \text{HCl}\)
- \(\text{PCl}_5 \to \text{POCl}_3 + \text{HCl}\)
Step 3: Detailed Explanation:
When benzoic acid reacts with phosphorus pentachloride (\(\text{PCl}_5\)), it produces benzoyl chloride, phosphorus oxychloride (\(\text{POCl}_3\)), and hydrogen chloride (\(\text{HCl}\)).
\[ \text{C}_6\text{H}_5\text{COOH} + \text{PCl}_5 \to \text{C}_6\text{H}_5\text{COCl} + \text{POCl}_3 + \text{HCl} \] The other reagents do not produce this specific set of products.
Step 4: Final Answer:
The reagent is PCl$_5$.