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Reactions of aldehydes and ketones with alkali

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Let's get acquainted with α carbon and α hydrogen in aldehydes and ketones before starting this section.

α carbon : The carbon atom next to the carbonyl group is called an α carbon.

α hydrogen : Hydrogen atoms attached to the α carbon are known as α hydrogens.

alpha beta and gamma carbons w.r.t carbonyl group

Acidic nature of α−hydrogen

The carbonyl group exhibits −I-Effect, and thus withdraws electrons from adjacent carbon-carbon bond. This makes α-carbon electron deficient. The α-carbon, in turn, withdraws electron from Cα−Hα bond. As a consequence, the Cα−Hα bond becomes weaker and can be separated easily by a strong base to give enolate anion (a conjugate base). The enolate anion is stabilised by resonance as shown below :

Acidity of alpha hydrogen w.r.t carbonyl group Formation of enolate anion

There is a negative charge on α−carbon in structure (I) but no charge on α−cabon in structure (II). This suggests that electron density on α−carbon decreases due to resonance, and consequently, increases the acidity of α−hydrogen.

Conclusion : The acidity of α−hydrogen is due to (i)−I-effect and (ii) resonance stabilisation of the conjugate base (enolate anion).

Aldol condensation

Aldehydes and ketones having α−hydrogen atom when treated with dilute alkali, form β-hydroxy aldehydes and β−hydroxy ketones respectively (known as aldols), which on heating give unsaturated compounds.

aldol condensation between two acetaldehydes
aldol condensation between two acetones aldol condensation between two acetones

Try to do the following conversion:

Convert ethanol to 3-hydroxybutanal.

Cross aldol condensation

When aldol condensation is carried out with two different aldehydes or ketones having α-hydrogen, the reaction is called cross-aldol condensation. Cross-aldol condensation can take place between :

The major problem in cross-aldol condensation is that a mixture of four different products is formed which are difficult to separate.

Cross aldol condensation between ethanal and propanal
Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product Cross aldol condensation between ethanal and propanal product

However, we do not face such problems if one of the carbonyl compounds does not have any α-hydrogen (e.g., formaldehyde, benzaldehyde etc.).

Cross aldol condensation between ethanal and methanal

Cannizzaro Reaction

Aldehydes that do not have any α-hydrogen undergo disproportionation i.e., self oxidation-reduction, on treatment with concentrated alkali solution.

Cannizzaro reaction between two methanal groups
Cannizzaro reaction in benzaldehyde

Conversion

Convert benzaldehyde to 3-phenylprop-2-enal.

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