Which reagent converts aldehydes to primary alcohols?

Solution

Chemistry ma’am · 2 steps

Aldehyde to primary alcohol is a reduction, so pick the reducing agent: LiAlH₄.

1. What must happen. RCHO has to become RCH₂OH. Two hydrogen atoms are added across the C=O bond, so this is a reduction.

   RCHO →((i) LiAlH₄, (ii) H₃O⁺) RCH₂OH

   Reagent	Type	Effect on an aldehyde
   LiAlH₄	reducing agent (hydride donor)	RCHO → RCH₂OH, a primary alcohol
   PCC	mild oxidising agent	No reduction. PCC oxidises primary alcohols to aldehydes, so it stops at the aldehyde
   K₂Cr₂O₇/H⁺	strong oxidising agent	RCHO → RCOOH, a carboxylic acid
   O₃	oxidising agent (ozonolysis)	Cleaves C=C bonds. It does not reduce C=O

2. So. Only LiAlH₄ supplies hydride (H⁻) to the carbonyl carbon. After acidic work-up the aldehyde becomes a primary alcohol, so the answer is (a).

The idea to keep. Aldehydes sit between primary alcohols and carboxylic acids. A reducing agent (LiAlH₄, NaBH₄, H₂/Ni) takes them down to the alcohol. An oxidising agent takes them up to the acid.

Answer from the NTA final answer key. Solution by PrepWiser.