Question

Organic compounds containing carbonyl group as the functional group are aldehydes, ketones and carboxylic acids. The carbon-oxygen double bond is polarised due to higher electronegativity of oxygen as compared to carbon. Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons. They react with HCN, Sodium bisulphite, Grignard reagent and a number of ammonia derivatives like Hydroxylamine, Hydrazine, Semicarbazide, etc. Aldehydes and ketones undergo various reactions due to the acidic nature of \( \alpha \)-hydrogen. Aldehydes and ketones having at least one \( \alpha \)-hydrogen atom undergo a reaction in the presence of dilute alkali as a catalyst to form \( \beta \)-hydroxy aldehydes or \( \beta \)-hydroxy ketones. On the other hand, aldehydes which do not possess an \( \alpha \)-hydrogen undergo self-oxidation and reduction in the presence of concentrated alkali.

From the compounds given below, identify which would undergo aldol condensation and which would undergo Cannizzaro reaction ?

Cyclohexanone, Benzaldehyde, Methanal, 2-Methylpentanal

Hint:

Compounds with an alpha-hydrogen give aldol condensation. Aldehydes with no alpha-hydrogen give the Cannizzaro reaction.

Answer 1

Step 1: Recall the deciding rule:
Aldol condensation needs at least one \( \alpha \)-hydrogen, with dilute alkali as catalyst.
The Cannizzaro reaction is given by aldehydes with no \( \alpha \)-hydrogen, with concentrated alkali.

Step 2: Check each compound:
Cyclohexanone: the ring carbons next to C=O carry H atoms, so it has \( \alpha \)-hydrogens. It gives aldol-type condensation.
2-Methylpentanal: the carbon next to CHO is C-2, and it carries one H. So it has an \( \alpha \)-hydrogen and gives aldol condensation.
Benzaldehyde: the CHO group is attached directly to the benzene ring, which has no \( \alpha \)-carbon bearing H. It gives the Cannizzaro reaction.
Methanal: HCHO has no \( \alpha \)-carbon at all. It gives the Cannizzaro reaction.

Final Answer:
Aldol condensation: cyclohexanone and 2-methylpentanal.
Cannizzaro reaction: benzaldehyde and methanal.