Aldol Condensation

Aldol condensation occurs in aldehydes having α-hydrogen with a dilute base to give β-hydroxy aldehydes called aldols. This reaction is most commonly known as aldol condensation. If the condensation reaction occurs between two different carbonyl compounds it is called crossed aldol condensation.

Aldol Condensation can be defined as an organic reaction in which enolate ion reacts with a carbonyl compound to form β-hydroxy ketone or β-hydroxy aldehyde, followed by dehydration to give a conjugated enone. Aldol Condensation plays a vital role in organic synthesis, creating a path to form carbon-carbon bonds.

The general reaction of aldol condensation is

General Aldol Condensation Reaction

One of the common examples for base-catalyzed aldol condensation is stated below in which catalyst generally used is hydroxide ion.

Mechanism of Aldol Condensation

Step-1:

In reverse order, The hydroxide ion deprotonates the aldehyde.

Step-2:

Here Enolate ion 1 adds to the unreacted aldehyde.

Step-3:

Alkoxide ion 2 is protonated by water.

Step-4:

A small amount of aldol is converted into enolate ion (4) by hydroxide ion.

Step-5:

Here Enolate Ion(4) loses a hydroxide ion.

Step 1 to step 3 illustrates the aldol reaction.

Crossed Aldol Condensation

The condensation reaction between two different molecules of an aldehyde or ketone in a protic solvent such as water or alcohol constitutes the crossed aldol reaction. When condensation is between two different carbonyl compounds, it is called crossed aldol condensation. When both aldehydes have alpha hydrogens, both can form carbanions and can also act as carbanion acceptors. Hence a mixture of four products is formed which has little synthetic value.

If one of the aldehydes has no alpha hydrogen then it can act only as a carbanion acceptor. In such a case, only two products are formed. A common substrate for the crossed aldol reaction is an aromatic aldehyde, which has no alpha position. Furthermore, dehydration of the initial condensation product is rapid which leads to the formation of the α, β – unsaturated ketone and prevents the retro-aldol reaction from taking place.

Types of Condensation

It is important to differentiate aldol condensation from various reactions of carbonyl compounds.

  • In a case of Perkin reaction, enolate generated by anhydride is aromatic.
  • A Claisen condensation contains two ester compounds.
  • A Henry reaction contains an aliphatic nitro compound and an aldehyde.
  • Dieckmann condensation contains 2 ester groups present in the same molecule, which produces cyclic molecule.
  • In Japp–Maitland condensation, water is removed by nucleophilic displacement.

Sample Questions

Some frequently asked questions in aldol condensation reaction that may help you in exam preparation are as follows:

Question 1: What is aldol condensation?

Ans: When aldehydes and ketones having at least one α-hydrogen are treated with dilute alkali (which act as a catalyst) they form β-hydroxyaldehydes (aldol) or β-hydroxyketones (ketol) respectively. This reaction is known as aldol condensation.

Question 2: Explain the mechanism of aldol condensation.

Ans: Aldol condensation is an organic reaction in which an enolate ion reacts with carboxyl compound in order to form a β– hydroxyaldehyde or β– hydroxyketone.

  1. Hydroxide functions as a base and therefore moves the acidic a-hydrogen producing the reactive enolate ion. This reaction can be seen as an acid-base reaction.
  2. The aldehyde is attacked at the electrophilic carbonyl carbon by the nucleophilic enolate ion. This attack is a nucleophilic addition reaction and gives alkoxide intermediate.
  3. The alkoxide deprotonates water molecule, thereby producing hydroxide and the β–hydroxyaldehyde.