Acid Catalysed Aldol Condensation

What is a Condensation Reaction?

Condensation is an addition reaction in which two or more molecules combine to form a single molecule, eliminating small molecules like water, hydrogen sulphide, ethanol, ammonia or acetic acid. If a water molecule is eliminated, it is a dehydration reaction.

Aldol reaction, Knoevenagel reaction, Claisen reaction and the Dieckman reaction are examples of condensation reactions.

Table of Content

• Introduction of Aldol Condensation
• Acid Catalysed Aldol Condensation
• Mechanism of Acid Catalysed Aldol Condensation
• Base Catalysed Aldol Condensation
• Frequently Asked Questions – FAQs

Introduction of Aldol Condensation

‘Aldol’ is the abbreviation of alcohol and aldehyde. It is a condensation reaction in which dimerisation of aldehyde or ketone occurs. In it, an enolate ion reacts with a carbonyl compound to form a beta-hydroxy aldehyde or beta-hydroxy ketone, accompanied by elimination of water molecule to yield a conjugated enone.

• The carbonyl compound must have alpha hydrogen for aldol condensation to take place.
• If the condensation occurs between the same carbonyl compounds, it is known as aldol condensation.
• If the condensation occurs between two different carbonyl compounds, it is known as crossed aldol condensation.

CH₃-CHO + CH₃-CHO → CH₃-CH(OH)-CH₂-CHO

Both acids and bases catalyse aldol condensation reaction, but the best result is obtained under basic conditions below room temperature.

Acid Catalysed Aldol Condensation

Acid atalysed Aldol Condensation is an aldol condensation reaction taking place in the presence of an acid. In it, dimerisation of aldehyde or ketone occurs. An enol reacts with a carbonyl compound to form a beta-hydroxy aldehyde or beta-hydroxy ketone, accompanied by the elimination of water molecules to yield a conjugated enone.

Mechanism of Acid Catalysed Aldol Condensation

Step 1: Activation of Carbonyl Centre

In the first step, the lone pairs of oxygen attract the proton released by the acid, thus activating the carbonyl oxygen into a protonated form.

Step 2: Enolisation

In the second step, intermediate 1 reacts with the acid’s conjugate base to form an enol, which acts as a nucleophile.

Step 3: Nucleophilic Addition

In the third step, the enol formed in the second step acts as a nucleophile that attacks the activated carbonyl compound to give an aldol addition product.

Step 4: Dehydration

In the fourth step, spontaneous elimination of water takes place to yield a corresponding beta-hydroxy aldehyde or beta-hydroxy ketone.

Base Catalysed Aldol Condensation

Base Catalysed Aldol Condensation is an aldol condensation reaction taking place in the presence of a base. In it, dimerisation of aldehyde or ketone occurs. An enolate ion reacts with a carbonyl compound to form a beta-hydroxy aldehyde or beta-hydroxy ketone, accompanied by the elimination of water molecules to yield a conjugated enone.