electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. (i) Aldol condensation: Aldehydes and ketones having at least one a -hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form b -hydroxy aldehydes (aldol) or b -hydroxy ketones (ketol), respectively. This is known as Aldol reaction . The name aldol is derived from the names of the two functional groups, aldehyde and alcohol, present in the products.
The aldol and ketol readily lose water to give a , b -unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation . Though ketones give ketols (compounds containing a keto and alcohol groups), the general name aldol condensation still applies to the reactions of ketones due to their similarity with aldehydes. ( ii ) 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 a -hydrogen atoms, it gives a mixture of four products.
This is illustrated below by aldol reaction of a mixture of ethanal and propanal. Ketones can also be used as one component in the cross aldol reactions. . Other reactions ( i ) Cannizzaro reaction: Aldehydes which do not have an a -hydrogen atom, undergo self oxidation and reduction (disproportionation) reaction on heating with concentrated alkali.
In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidised to carboxylic acid salt. D D ( ii ) Electrophilic substitution reaction : Aromatic aldehydes and ketones undergo electrophilic substitution at the ring in which the carbonyl group acts as a deactivating and meta -directing group.