Read the passage carefully and answer the following questions.
Aldehydes, carboxylic acids and ketones are widespread in the animal and plant kingdoms. They play a pivotal role in various biochemical processes of life. Their presence in nature adds fragrance and flavor. These compounds are widely used in food products, pharmaceuticals, paints, resins and other important product industries. These compounds are prepared by various laboratory methods, which mainly include oxidation, formylation, acylation and reduction. Due to the polar nature of the carbonyl group in aldehydes and ketones, they can exhibit different reactions like nucleophilic addition. They do exhibit redox and various condensation reactions which lead to the formation of various important compounds. On the other hand, the carboxylic acids are mainly prepared by oxidation and hydrolysis of different compounds. The carboxylic acid consists of a carbonyl group and the hydroxy group (attached to the carbonyl carbon atom). This makes it possible for the carboxylic acid to participate in various chemical reactions which involve cleavage of the C-OH bond and the O-H bond along with the reactions involving the complete -COOH group.
Identify the incorrect statement for Cannizzaro reaction.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Dilute sodium hydroxide is used as a catalyst
Aldehydes that have no α-hydrogen atom (methanal, benzaldehyde) cannot form an enolate, so they do not undergo aldol condensation. When heated with concentrated alkali they undergo self oxidation and reduction (disproportionation): one molecule is reduced to the alcohol and the other is oxidised to the salt of the carboxylic acid. This is the Cannizzaro reaction:
\( \displaystyle \ce{2HCHO} \xrightarrow[\Delta]{\text{conc. }\ce{KOH}} \ce{CH3OH + HCOOK} \)
\( \displaystyle \ce{2C6H5CHO} \xrightarrow[\Delta]{\text{conc. }\ce{NaOH}} \ce{C6H5CH2OH + C6H5COONa} \)
Option (1) The reaction involves self oxidation and reduction — Correct about the Cannizzaro reaction. One aldehyde molecule is oxidised to the acid salt while another is reduced to the alcohol.
Option (2) It is given by aldehydes which lack alpha hydrogens — Correct about the Cannizzaro reaction. Aldehydes with an α-hydrogen undergo aldol reactions with alkali instead.
Option (3) It is carried out in presence of concentrated alkali — Correct about the Cannizzaro reaction. Concentrated NaOH or KOH is used, with heating.
Option (4) Dilute sodium hydroxide is used as a catalyst — Incorrect about the Cannizzaro reaction. The alkali must be concentrated, and it is not a catalyst: it is used up in forming the carboxylate salt. Dilute alkali is the catalyst of the aldol reaction, which needs an α-hydrogen.
NCERT (2026-27 edition):
Cannizzaro Reaction
