What is Aldol Condensation? – Reaction, Mechanism, Example

Aldol condensations are a particularly significant class of reactions in chemical synthesis. In 1872, Charles – Adolph Wurtz and Alexander Porfyrevich Borodin independently discovered the reaction. The word aldol was selected because the result of an aldol condensation frequently contains both an aldehyde and an alcohol group. 

In general, an aldol condensation is a nucleophile assault on a carbonyl that results in a ketone or aldehyde of β -hydroxy. In most cases, the nucleophile is an enolate of an aldehyde or ketone engaging another aldehyde or ketone molecule. The condensation of aldol can be catalysed by an acidic or basic solution. 

Aldol Condensation Reaction

Aldol condensation is a chemical process in which an enolate ion combines with a carbonyl molecule to make a β-hydroxy ketone or β-hydroxy aldehyde, which is then dehydrated to produce a conjugated enone. Aldol condensation is necessary in organic synthesis because it creates a channel for the creation of carbon-carbon bonds. 

General Aldol Condensation Reaction

A common example of base-catalyzed aldol condensation is shown below, where the catalyst is typically a hydroxide ion. 

Mechanism of Aldol Condensation

Step-1

In the reversed order, the hydroxide ion deprotonates the aldehyde. 

Step-2

Enolate ion 1 reacts with the unreacted aldehyde. 

Step-3

Water protonates alkoxide ion 2. 

Step-4

Hydroxide ion converts a little quantity of aldol into an enolate ion (4).  

Step-5

Enolate Ion (4) loses a hydroxide ion. 

What is Cross Aldol Condensation?

The crossing aldol condensation reaction occurs with two different motes of an aldehyde or ketone condense in a protic liquid similar to water or alcohol. Crossed aldol condensation occurs when condensation occurs between two distinct carbonyl molecules. Both aldehydes with alpha hydrogens can create carbanions and behave as carbanion acceptors. As a result, a combination of four products of limited synthetic value is generated. 

If one of the aldehydes does not contain alpha hydrogen, it can only act as a carbanion acceptor. In this situation, just two goods are produced. An aromatic aldehyde with no alpha position is a typical substrate for the crossing aldol reaction. Furthermore, the first condensation product dehydrates quickly, resulting in the creation of the, α, β – unsaturated ketone and preventing the retro-aldol reaction. 

Types of Condensation

• Enolate produced by anhydride in a Perkin reaction is aromatic. 
• Two ester compounds are present in a Claisen condensation. 
• An aliphatic nitro molecule and an aldehyde are used in the Henry reaction. 
• Dieckmann condensation involves the presence of two ester groups in the same molecule, resulting in a cyclic molecule. 
• Water is removed by nucleophilic displacement in Japp-Maitland condensation. 

Final Notes

Use the book Organic Chemistry by Morrison and Boyd to have a deeper knowledge of this concept. Solving previous year’s JEE papers would also help you or get expert help from Tutoroot.