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: Identify the type of reaction. 
Changing an aldehyde into an alcohol involves a reduction reaction.
In this transformation, the carbonyl group (–CHO) of an aldehyde gains hydrogen atoms to form a primary alcohol (–CH₂OH).

Step 2: Represent the chemical change.
The general conversion can be written as:

R–CHO ⟶ R–CH₂OH

This reaction requires a reagent that can donate hydride ions (H⁻) to the carbonyl carbon.

Step 3: Examine each reagent.
PCC: An oxidizing agent; commonly used to convert alcohols into aldehydes. It cannot reduce aldehydes.

KMnO₄: A strong oxidant; it oxidizes aldehydes further to carboxylic acids, not alcohols.

LiAlH₄: A strong reducing agent; it supplies hydride ions that convert aldehydes into primary alcohols efficiently.

CrO₃: Another oxidizing agent; typically used for oxidation reactions, not reduction.

Step 4: Final conclusion.
The reagent capable of reducing an aldehyde to a primary alcohol is LiAlH₄.