Question

When benzaldehyde is subjected to oxidation using acidified potassium permanganate ($\text{KMnO}_4$), what organic product is formed?

• A. Benzyl alcohol
• B. Benzoic acid
• C. Benzene
• D. Acetophenone

Hint:

Whether using mild (Tollens') or strong ($\text{KMnO}_4$) oxidizing agents, aldehydes consistently convert into their corresponding carboxylic acids.

Answer 1

The Correct Option is B

Concept: Aromatic aldehydes are highly susceptible to oxidation due to the presence of the single carbon-hydrogen bond on the formyl carbonyl group ($-\text{CHO}$). When treated with a powerful transition metal oxidant, this carbonyl C-H bond is cleaved and converted into a carbon-hydroxyl ($\text{C-OH}$) bond. This process transforms the aldehyde functional group into a stable carboxylic acid derivative.

Step 1: Evaluating the chemical behavior of the strong oxidant with benzaldehyde.
Potassium permanganate ($\text{KMnO}_4$) under acidified conditions ($\text{H}_2\text{SO}_4$) serves as an exceptionally strong, non-selective oxidizing agent. The manganese center in the permanganate anion ($\text{MnO}_4^-$) is in its highest oxidation state ($+7$) and acts as an efficient electron acceptor. When introduced to benzaldehyde ($\text{C}_6\text{H}_5\text{CHO}$), the reagent readily attacks the formyl carbon atom, oxidizing the functional group to form a carboxylic acid.

Step 2: Determining the final structure of the aromatic product.
The oxidation converts the formyl group ($-\text{CHO}$) directly into a carboxylic acid group ($-\text{COOH}$) attached to the benzene ring. The aromatic core remains completely intact throughout this transformation, as the benzene ring is stabilized by resonance and resists oxidative cleavage. The resulting organic product is benzoic acid ($\text{C}_6\text{H}_5\text{COOH}$): \[ \text{C}_6\text{H}_5\text{CHO} \xrightarrow{\text{Acidified KMnO}_4} \text{C}_6\text{H}_5\text{COOH} \] This confirms option (B) as the correct choice.