Question

Which of the following reagents converts an aldehyde selectively into a primary alcohol?

• A. PCC
• B. \( KMnO_4 \)
• C. \( LiAlH_4 \)
• D. \( CrO_3 \)

Hint:

While \( LiAlH_4 \) is strong, \( NaBH_4 \) (Sodium Borohydride) is a milder alternative that also selectively reduces aldehydes and ketones to alcohols without affecting functional groups like esters or carboxylic acids.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The conversion of an aldehyde to an alcohol is a reduction process.
An aldehyde (\( R-CHO \)) contains a carbonyl group that must gain hydrogen to become a primary alcohol (\( R-CH_2OH \)).
Step 2: Key Formula or Approach:
The reaction can be represented as:
\[ R-CHO \xrightarrow{[\text{Reduction}]} R-CH_2OH \]
Step 3: Detailed Explanation:
- PCC (Pyridinium Chlorochromate): This is an oxidizing agent. It is used to convert primary alcohols into aldehydes, not the other way around.
- \( KMnO_4 \) (Potassium Permanganate): This is a very strong oxidizing agent. It would oxidize an aldehyde further into a carboxylic acid.
- \( LiAlH_4 \) (Lithium Aluminium Hydride): This is a powerful and selective reducing agent. It provides hydride ions (\( H^- \)) that attack the electrophilic carbon of the aldehyde group, reducing it to a primary alcohol.
- \( CrO_3 \) (Chromic anhydride): This is also an oxidizing agent (often used in Jones reagent) which oxidizes alcohols/aldehydes to carboxylic acids.
Step 4: Final Answer:
Among the given options, \( LiAlH_4 \) is the reagent that reduces an aldehyde to a primary alcohol.