Question

Write short notes on the following:
(i) Aldol condensation
(ii) Cannizzaro's reaction

Hint:

Aldol needs an alpha-hydrogen with dilute base; Cannizzaro is disproportionation of aldehydes without alpha-hydrogen using concentrated alkali.

Answer 1

(i) Aldol condensation - key points.
\(Step\ 1\): The tell-tale feature is the alpha-hydrogen. Only carbonyl compounds having a hydrogen on the alpha carbon can react.
\(Step\ 2\): A base pulls off the alpha-hydrogen to form a resonance-stabilised carbanion. This carbanion attacks the carbonyl carbon of a second molecule, producing a beta-hydroxy carbonyl compound (the aldol). Example: two molecules of ethanal give \(3\)-hydroxybutanal.
\(Step\ 3\): The word condensation refers to loss of water. On heating, the aldol dehydrates to an \(\alpha,\beta\)-unsaturated carbonyl compound, here but-2-enal, \(CH_3CH=CHCHO\).

(ii) Cannizzaro's reaction - key points.
\(Step\ 1\): This is the reaction of aldehydes that lack an alpha-hydrogen (for example HCHO, benzaldehyde, trimethylacetaldehyde).
\(Step\ 2\): With strong concentrated alkali, one molecule hands over a hydride to the other. This is a disproportionation: the same species is both oxidised and reduced.
\(Step\ 3\): The oxidised molecule becomes the salt of the carboxylic acid and the reduced molecule becomes the primary alcohol. For benzaldehyde the products are benzyl alcohol and sodium benzoate.
\[\boxed{2\,C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + C_6H_5COONa}\]