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.
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 :
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).
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.
Try to do the following conversion:
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.
However, we do not face such problems if one of the carbonyl compounds does not have any α-hydrogen (e.g., formaldehyde, benzaldehyde etc.).
Aldehydes that do not have any α-hydrogen undergo disproportionation i.e., self oxidation-reduction, on treatment with concentrated alkali solution.