Formaldehyde is the first member of the aldehyde series. It is present in green leaves of plants where its presence is supposed to be due to the reaction of \(CO_2\) with water in the presence of sunlight and chlorophyll. Commercially, formaldehyde is obtained by the controlled oxidation of methane in the presence of molybdenum oxide \((MoO)\) acting as a catalyst. Formaldehyde is a constituent of a number of polymers also called resins. These are non volatile solids or semi solids which are obtained directly from certain plants and can be synthesized in the laboratory.
Formaldehyde on reaction with benzaldehyde in basic medium gets converted to:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Formic acid
Neither formaldehyde nor benzaldehyde has an α-hydrogen, so with concentrated alkali they undergo the Cannizzaro reaction instead of aldol condensation. When two such aldehydes are mixed (cross Cannizzaro reaction), formaldehyde, the more reactive of the two, is the one that is oxidised; the other aldehyde is reduced to its alcohol:
\( \displaystyle \ce{HCHO + C6H5CHO + NaOH} \xrightarrow{\text{conc.}} \ce{HCOONa + C6H5CH2OH} \)
\( \displaystyle \ce{HCOONa + H+ -> HCOOH + Na+} \)
So formaldehyde ends up as sodium formate, that is, formic acid after acidification, while benzaldehyde gives benzyl alcohol.
Option (1) Methanol — Incorrect. Methanol forms from HCHO only when HCHO reacts with itself (2HCHO → HCOO− + CH3OH). In the crossed reaction HCHO is oxidised, and the reduction product is benzyl alcohol.
Option (3) o/p-hydroxymethylbenzaldehyde — Incorrect. This is a ring-hydroxymethylated compound, with a –CH2OH group on the ring. Such a product forms when HCHO attacks a strongly activated ring, as in phenol; the –CHO group deactivates the ring of benzaldehyde, so HCHO/OH− does not attack it.
Option (4) o/p-hydroxymethyl benzoic acid — Incorrect. This too is a ring-substituted compound. The Cannizzaro reaction takes place only at the –CHO groups and puts no –CH2OH group on the ring.
NCERT (2026-27 edition):
Cannizzaro Reaction – “Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali. In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt.”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | – | 12-06 S3 08-06 S3 | – |