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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

‘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

Aldol Condensation

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.

Acid Catalysed Aldol Condensation

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.

Acid Catalysed Aldol Condensation

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.

Acid Catalysed Aldol Condensation

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.

Acid Catalysed Aldol Condensation

Step 4: Dehydration

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

Acid Catalysed Aldol Condensation

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.

Base Catalysed Aldol Condensation

Frequently Asked Questions on 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 sulfide, ethanol, ammonia or acetic acid. If a water molecule is eliminated, it is a dehydration reaction.

What is aldol reaction?

‘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.

What is acid catalysed aldol condensation?

Acid catalysed 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.

What is 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.

What is crossed aldol condensation reaction?

Crossed aldol condensation is a variant of the aldol condensation reaction. In it, condensation occurs between two different carbonyl compounds.

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