Question

X and Y in the below reaction are ______ and ______ respectively

C\(_6\)H\(_5\)–CO\(_2\)H + X \(\xrightarrow{\text{Heat}}\) C\(_6\)H\(_5\)–COCl \(\xrightarrow{\text{H}_2/\text{Pd/BaSO}_4\text{, Quinoline}}\) Y

• A. SOCl\(_2\) and C\(_6\)H\(_5\)CHO
• B. (COCl)\(_2\) and C\(_6\)H\(_5\)CH\(_3\)
• C. SOCl\(_2\) and C\(_6\)H\(_5\)CH\(_3\)
• D. (COCl)\(_2\) and C\(_6\)H\(_5\)CH\(_2\)OH
• E. SOCl\(_2\) and C\(_6\)H\(_5\)CH\(_2\)Cl

Hint:

Thionyl chloride (\(\text{SOCl}_2\)) is preferred for making acid chlorides because the by-products (\(\text{SO}_2\) and \(\text{HCl}\)) are gases, making the purification of the product very simple.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The sequence involves two distinct organic transformations: the conversion of a carboxylic acid to an acid chloride, and the subsequent partial reduction of the acid chloride to an aldehyde.
Step 2: Key Formula or Approach:
Identify the specific reagents required for each transformation:
Reaction 1: \(\text{R-COOH} \xrightarrow{\text{Chlorinating Agent (X)}} \text{R-COCl}\)
Reaction 2: \(\text{R-COCl} \xrightarrow[\text{Rosenmund Reduction}]{\text{H}_2\text{, Pd/BaSO}_4\text{, Quinoline}} \text{R-CHO (Y)}\)
Step 3: Detailed Explanation:
In the first step, benzoic acid (\(\text{C}_6\text{H}_5-\text{CO}_2\text{H}\)) is converted to benzoyl chloride (\(\text{C}_6\text{H}_5-\text{COCl}\)).
This conversion requires a chlorinating agent. Common reagents for this are Thionyl chloride (\(\text{SOCl}_2\)), Phosphorus pentachloride (\(\text{PCl}_5\)), or Phosphorus trichloride (\(\text{PCl}_3\)).
Among the options provided, \(\text{SOCl}_2\) and \((\text{COCl})_2\) are capable of this transformation. Let's evaluate the second step to confirm the correct pair.
The second step involves treating benzoyl chloride with hydrogen gas (\(\text{H}_2\)) in the presence of Palladium on Barium sulfate (\(\text{Pd/BaSO}_4\)) and a poison like Quinoline.
This is the classic Rosenmund reduction. The \(\text{BaSO}_4\) and quinoline intentionally "poison" or partially deactivate the palladium catalyst.
This deactivation prevents the resulting aldehyde from being further reduced into a primary alcohol.
Therefore, the acid chloride (\(\text{C}_6\text{H}_5-\text{COCl}\)) is selectively reduced to an aldehyde (\(\text{C}_6\text{H}_5-\text{CHO}\)), which is benzaldehyde.
Looking at the options, only (A) has \(\text{SOCl}_2\) for X and the correct aldehyde \(\text{C}_6\text{H}_5\text{CHO}\) for Y.
Step 4: Final Answer:
The reagent X is \(\text{SOCl}_2\) and product Y is benzaldehyde (\(\text{C}_6\text{H}_5\text{CHO}\)).