Question

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

• A. $PCl_{3}$
• B. HCl
• C. $PCl_{5}$
• D. $SOCl_{2}$

Hint:

Chemistry Tip: Identifying chlorinating agents relies purely on memorizing their distinct byproducts: $SOCl_2 \rightarrow (SO_2 + HCl)$, $PCl_3 \rightarrow (H_3PO_3)$, and $PCl_5 \rightarrow (POCl_3 + HCl)$.

Answer 1

The Correct Option is C

Concept: Chemistry (Organic Chemistry) - Preparation of Acyl Chlorides from Carboxylic Acids.

Step 1: Identify the reactants and the main product. The reactant is Benzoic acid ($C_6H_5COOH$). The primary organic product is Benzoyl chloride ($C_6H_5COCl$). This transformation involves replacing the hydroxyl group ($-OH$) of the carboxylic acid with a chlorine atom ($-Cl$).

Step 2: List common reagents for this functional group transformation. Carboxylic acids can be converted into acyl chlorides using specific chlorinating agents. The most common reagents for this specific nucleophilic acyl substitution are Thionyl chloride ($SOCl_2$), Phosphorus trichloride ($PCl_3$), and Phosphorus pentachloride ($PCl_5$).

Step 3: Analyze the reaction with Thionyl chloride ($SOCl_2$). If $SOCl_2$ is used, the reaction is: $R-COOH + SOCl_2 \rightarrow R-COCl + SO_2\uparrow + HCl\uparrow$. The byproducts are sulfur dioxide gas and hydrogen chloride gas. This does not match the given byproducts.

Step 4: Analyze the reaction with Phosphorus trichloride ($PCl_3$). If $PCl_3$ is used, the reaction requires three moles of the acid: $3R-COOH + PCl_3 \rightarrow 3R-COCl + H_3PO_3$. The byproduct is phosphorous acid ($H_3PO_3$). This also does not match the given byproducts.

Step 5: Analyze the reaction with Phosphorus pentachloride ($PCl_5$) and finalize. If $PCl_5$ is used, the chemical equation is: $R-COOH + PCl_5 \rightarrow R-COCl + POCl_3 + HCl$. For benzoic acid specifically: $C_6H_5COOH + PCl_5 \rightarrow C_6H_5COCl + POCl_3 + HCl$.
The generated byproducts are exactly $POCl_3$ (Phosphorous oxychloride) and $HCl$ (Hydrogen chloride). Since these byproducts perfectly match the ones explicitly listed in the reaction prompt, the reagent must definitively be $PCl_5$. $$ \therefore \text{The reagent used is } PCl_{5}. $$