The figure shows two reactions of an arenediazonium chloride (Ar = aryl group).
What is the role of hypophosphorous acid (H3PO2) and of ethanol in these reactions?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → They act as mild reducing agents and are themselves oxidised to H3PO3 and CH3CHO.
In both equations \( \ce{ArN2+Cl-} \) becomes ArH: the diazonium group is replaced by H, so the diazonium salt is reduced to the arene.
At the same time \( \ce{H3PO2} \) becomes \( \ce{H3PO3} \) (it gains an oxygen atom) and \( \ce{CH3CH2OH} \) becomes \( \ce{CH3CHO} \) (it loses two hydrogen atoms). Both changes are oxidations, so the two reagents are mild reducing agents.
Why the other options are incorrect
Option (1) They act as oxidising agents and are themselves reduced to H3PO3 and CH3CHO. — Incorrect. Going from H3PO2 to H3PO3 and from ethanol to ethanal is oxidation, not reduction; a reagent that is oxidised acts as a reducing agent.
Option (2) They act as catalysts and are recovered unchanged at the end of the reaction. — Incorrect. The figure shows them changed into new products, H3PO3 and CH3CHO, so they are used up.
Option (3) They act as nucleophiles and become attached to the aromatic ring. — Incorrect. The ring gets only an H atom (ArH); no group from H3PO2 or ethanol is attached to it.
NCERT (2026-27 edition):
9.9 Chemical Reactions – “4. Replacement by H: Certain mild reducing agents like hypophosphorous acid (phosphinic acid) or ethanol reduce diazonium salts to arenes and themselves get oxidised to phosphorous acid and ethanal, respectively.”
Replacement by H: reduction of arenediazonium chloride by hypophosphorous acid and by ethanol