One of the most important concepts in organic chemistry is the class of reactions that deal with Aldol condensation reactions. This reaction was developed by Charles Adolf Wurtz and Alexander Porfyrevich Borodin independently in the year 1872. The reaction was given the term Aldol because the products formed at the completion of this reaction are generally an aldehyde and alcohol. A noteworthy feature of the Aldol reaction is that the carbon-carbon bond formation takes place between the alpha-carbon atom of one aldehyde and the carbonyl group of the other. This is because the generation of the carbonate can involve the proton abstraction from the alpha-carbon atom only. Here, the carbonate refers to the enolate formed.
The process of aldol condensation involves a nucleophile attack on a carbonyl to produce a ketone or aldehyde of β-hydroxy. The catalyzation of the condensation process can be carried out with the help of an acidic or basic solution.
Bases such as hydroxide ions and alkoxide ions are used for the partial conversion of an aldehyde to its enolate anion.
The enolate undergoes a nucleophilic addition to the carbonyl group. This is achieved in a solution that contains both an aldehyde and its enolate. Aldol condensation is prominent in those aldehydes that have an α-hydrogen along with a dilute base to give β-hydroxy aldehydes, also called aldols. When the above reaction takes place between two different carbonyl compounds, it is referred to as Crossed Aldol Condensation Reaction. When both the aldehydes possess α-hydrogens, then they can both form carbanions (enolate) that can also act as acceptors of enolates. Thus, the reaction results in a mixture of four products that have a minute synthetic value. In case any of the aldehydes does not have α-hydrogen, then it can only act as an acceptor of Carbanion. Thus, only two products will be formed on the completion of the reaction. Also, the dehydration of the initial condensation products takes place rapidly and thus leads to the formation of alpha, beta-unsaturated ketone and prevent the occurrence of the retro-aldol reaction.
Its general form is -
STEP 1: Aldehyde is subjected to deprotonation done by the hydroxide ion that functions as a base and thus moves the a-hydrogen which results in the formation of the enolate.
STEP 2: Enolate ion attaches itself to the aldehyde at the site of the electrophilic carbonyl carbon, and results in the formation of an alkoxide intermediate.
STEP 3: Alkoxide ion now deprotonates the water molecule resulting in the formation of hydroxide and the β–hydroxy aldehyde.
STEP 4: Aldol 3 is now an aldehyde that is ready for enolation.