(A)Introduction of aldol condensation
In an aldol condensation reaction, the α-carbon of an aldehyde or ketone molecule reacts with another carbonyl carbon to form a β-hydroxyl aldehyde or β-hydroxyl ketone, followed by dehydration to give a conjugated enone if an α-H is present. It is a very useful reaction for the synthesis of α,β-unsaturated carbonyl compounds.
The aldehyde or ketone molecule is first converted to enol or enolate by acid or base catalyst respectively. In this experiment, the base-catalyzed method is used and the α-proton of the aldehyde or ketone molecule is removed by a strong base like hydroxides and alkoxides to form an enolate ion:
Although hydroxide ion is not strong enough to convert all of the aldehyde or keton molecules to the corresponding enolates for both aldehydes and ketones (i.e. the equilibrium lies well to the left), there are usually enough enolate ions for the reaction to proceed at a reasonable rate.
The enolate ion will then react with the carbonyl compound to form an intermediate anion, which is quickly protonated by water. When the β-hydroxyl aldehyde or β-hydroxyl ketone is formed, it will be sometimes spontaneously dehydrated to give an α,β-unsaturated carbonyl compound since a C=C bond can be formed by this process and it is very stable due to the conjugation with the carbonyl group. Therefore if the dehydration is not spontaneous, it can also be easily induced by gentle heating.
However, self condensation will occur, especially if both of the reactants are ketones, and a mixture of products will be formed. This is mainly due to the similar electrophilicity of the reactants. For example, when 3-pentanone is reacted with cyclopentanone:
A mixture of products will be formed as both the reactants can become enolate ion and react with itself or another reactant. Also, more kinds of products will be formed after dehydration as different kinds of α-proton is available.