Question

Write short notes on the following:
(i) Aldol and Crossed Aldol Condensation in Aldehyde and Ketone
(ii) Cannizzaro Reaction

Hint:

Remember: Aldol condensation requires $\alpha$-hydrogen. Compounds lacking $\alpha$-hydrogen cannot undergo this reaction.

Answer 1

(i) Aldol and Crossed Aldol Condensation in Aldehyde and Ketone:
Step 1: Aldol condensation.
When aldehydes or ketones containing at least one $\alpha$-hydrogen are treated with dilute alkali (NaOH, Ba(OH)\(_2\)), they undergo
aldol condensation. The $\alpha$-hydrogen is removed to form an enolate ion which attacks another carbonyl group, producing $\beta$-hydroxy aldehydes or ketones (aldols). On heating, they lose water to give $\alpha, \beta$-unsaturated compounds. \[ 2CH_3CHO \;\xrightarrow{NaOH}\; CH_3CH(OH)CH_2CHO \;\xrightarrow{\Delta}\; CH_3CH=CHCHO \] Step 2: Crossed Aldol condensation.
When two different carbonyl compounds are used, the reaction is called
crossed aldol condensation. For example: \[ CH_3CHO + CH_3COCH_3 \;\xrightarrow{NaOH}\; CH_3CH(OH)CH_2COCH_3 \;\xrightarrow{\Delta}\; CH_3CH=CHCOCH_3 \] Conclusion:
Aldol condensation gives $\alpha, \beta$-unsaturated aldehydes/ketones, while crossed aldol involves two different aldehydes/ketones.