Question

Which of the following sequence of reagents converts benzoic acid to benzaldehyde?

• A. \( \text{C}_2\text{H}_5\text{OH}, \text{H}^+ \); (i) DIBAL-H (ii) \( \text{H}_2\text{O} \)
• B. \( \text{SOCl}_2 \); \( \text{H}_2, \text{Ni} \)
• C. \( \text{C}_2\text{H}_5\text{OH}, \text{H}^+ \); \( \text{LiAlH}_4, \text{H}_2\text{O} \)
• D. \( \text{LiAlH}_4, \text{H}_2\text{O} \); \( \text{KMnO}_4, \text{H}^+ \)

Hint:

- Use DIBAL-H to selectively reduce esters to aldehydes at low temperatures.
- \( \text{LiAlH}_4 \) and \( \text{NaBH}_4 \) are powerful reducing agents that typically reduce carboxylic acids all the way to alcohols, not stopping at the aldehyde stage.
- \( \text{KMnO}_4 \) and \( \text{CrO}_3 \) are used for oxidation reactions and would not facilitate the reduction of carboxylic acids to aldehydes.

Answer 1

The Correct Option is A

Step 1: Understanding the Reduction of Benzoic Acid to Benzaldehyde 
- The conversion of benzoic acid (\( \text{C}_6\text{H}_5\text{COOH} \)) to benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)) is typically achieved by first transforming benzoic acid into its ester, then reducing the ester to an aldehyde using DIBAL-H. 

Step 2: Reaction Mechanism 
1. Esterification Step: 
- Benzoic acid (\( \text{C}_6\text{H}_5\text{COOH} \)) is esterified using ethanol (\( \text{C}_2\text{H}_5\text{OH} \)) and an acid catalyst (\( \text{H}^+ \)) to form ethyl benzoate (\( \text{C}_6\text{H}_5\text{COOCH}_2\text{CH}_3 \))

2. Selective Reduction to Aldehyde: 
- DIBAL-H selectively reduces the ester to benzaldehyde (\( \text{C}_6\text{H}_5\text{CHO} \)) under controlled conditions, typically at low temperatures, followed by careful hydrolysis with water (\( \text{H}_2\text{O} \)). 

Step 3: Incorrect Options Explained 
- Option 2 (\( \text{SOCl}_2; \text{H}_2, \text{Ni} \)): This sequence is used for reducing acids to alcohols, not aldehydes. 
- Option 3 (\( \text{C}_2\text{H}_5\text{OH}, \text{H}^+; \text{LiAlH}_4, \text{H}_2\text{O} \)): \( \text{LiAlH}_4 \) is too strong a reducing agent and would further reduce the aldehyde to an alcohol. 
- Option 4 (\( \text{LiAlH}_4, \text{H}_2\text{O}; \text{KMnO}_4, \text{H}^+ \)): \( \text{KMnO}_4 \) is an oxidizing agent that would not typically be used to produce an aldehyde from a carboxylic acid.