The reagent, from the following, which converts benzoic acid to benzaldehyde in one step is

Solution

Chemistry ma’am · 3 steps

Acid to aldehyde is a reduction that must stop halfway, and only MnO stops it there.

1. What is needed. The -COOH carbon must become a -CHO carbon. That is a reduction, and it has to stop at the aldehyde, which is itself easy to reduce or oxidise.

   Reagent	What it does to C₆H₅COOH	Product
   LiAlH₄	Strong reducer, does not stop at the aldehyde	C₆H₅CH₂OH
   KMnO₄	Oxidising agent, cannot reduce anything	no change
   MnO, 573 K	Catalyses a controlled reduction with formic acid	C₆H₅CHO
   NaBH₄	Too mild to reduce a carboxylic acid	no change

2. The MnO route. Pass vapours of the acid and formic acid over MnO at about 573 K. Formic acid supplies the hydrogen, and the aldehyde forms in one step.

   C₆H₅COOH + HCOOH →(MnO, 573 K) C₆H₅CHO + CO₂ + H₂O

3. So. The one-step reagent is MnO, option (c).

The idea to keep. Before picking a reagent, ask two things: is this an oxidation or a reduction, and does the reagent stop at the right stage? Aldehydes sit in the middle, so strong reagents run past them.

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