Aldol Condensation Reaction

Study Notes

Acidity of α-Hydrogen Atoms

The acidity of α-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.

Aldol Condensation

Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively.
The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds, which are aldol condensation products.
The reaction is known as Aldol reaction, and the name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products.

Cross Aldol Condensation

When aldol condensation is carried out between two different aldehydes and/or ketones, it is called cross aldol condensation.
If both of them contain α-hydrogen atoms, it gives a mixture of four products.

Cannizzaro Reaction

Aldehydes which do not have an α-hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali.
The position of the equilibrium lies largely to the right-hand side for most aldehydes and to the left for most ketones due to steric reasons.

Addition of Grignard Reagents and Alcohols

Aldehydes react with one equivalent of monohydric alcohol in the presence of dry hydrogen chloride to yield alkoxyalcohol intermediate, known as hemiacetals.
Hemiacetals further react with one more molecule of alcohol to give a gem-dialkoxy compound known as acetal.
Ketones react with ethylene glycol under similar conditions to form cyclic products known as ethylene glycol ketals.

Mechanism of Nucleophilic Addition Reactions

A nucleophile attacks the electrophilic carbon atom of the polar carbonyl group from a direction approximately perpendicular to the plane of sp hybridised orbitals of carbonyl carbon.
The hybridisation of carbon changes from sp2 to sp3 in this process, and a tetrahedral alkoxide intermediate is produced.
The intermediate captures a proton from the reaction medium to give the electrically neutral product.

Reactivity

Aldehydes are generally more reactive than ketones in nucleophilic addition reactions due to steric and electronic reasons.
Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent.

Description

This quiz covers the Aldol condensation reaction, a process where aldehydes and ketones undergo a reaction in the presence of dilute alkali to form β-hydroxy aldehydes or β-hydroxy ketones. It explains the acidity of α-hydrogen atoms of carbonyl compounds and the electron withdrawing effect of the carbonyl group.