Aldol condensation

About Aldol condensation

Two molecules of an aldehyde or a ketone undergo condensation in the presence of a base to yield a β-hydroxyaldehyde or a β-hydroxyketone. This reaction is called the aldol condensation which may be exemplified by the reaction of acetaldehyde to give acetaldol (3-hydroxybutanal).

The mechanism of this reaction illustrates important characteristics of carbonyl compounds, i.e.  the acidity of their α hydrogens and the tendency of their carbonyl groups to undergo nucleophilic addition.Befor moving to mechanism of aldol read the basics of organic chemistry and class 12th chemistry 
Let us see the mechanism now:

Reaction Mechanism of Aldol condensation

Step 1.

Base abstracts a proton from the α carbon of aldehyde to give resonance stabilized enolate ion.

Step 2.

The enolate ion then acts as nucleophile and attacks the carbonyl carbon of a second molecule of aldehyde, producing alkoxide ion.

Step 3.

In step 1, OH^– is acting as a base and not as a nucleophile. OH^– will act as a nucleophile and attack to the carbonyl group only when there is no acidic hydrogen, i.e. its (OH^–) first preference is  to act as a base and then as a nucleophile.
This reaction can also be catalysed by acid. Let us see this mechanism and the product formed.

Crossed Aldol Condensation

An aldol condensation between two different carbonyl compounds so called crossed aldol condensation is not always useful as a mixture of four different possible products may be obtained. under certain conditions, a good yield of a single product can be obtained from a crossed aldol condensation.
One reactant contains no α-hydrogens and therefore is incapable of condensing with itself (e.g. aromatic aldehydes or formaldehyde).

Intramolecular Aldol Condensation

When a compound has two carbonyl groups, it can undergo intramolecular aldol condensation in the presence of dilute base (if a-H atoms are present in the compound).
An intramolecular reaction is readily favoured if the reaction leads to the formation of a 5 or 6-membered ring. When one of carbonyl group is an aldehyde and other is a ketone, it’s the ketone, which forms carbanion and this carbanion attack the carbonyl group of an aldehyde in such a manner that 5 or 6-membered ring is formed. For example, 2, 5-hexanedione in presence of dilute OH^- undergoes intramolecular aldol condensation to give 2 set of products as there are 2 different types of a-hydrogens. One of the product has a 5-membered ring and the other has a 3-membered ring. The major product of the reaction is a 5-membered ring compound as 5-membered ring has greater stability than 3-membered ring.

Mechanism of Intramolecular Aldol condensation

List of Name Reaction of Organic Chemistry consist of detail Reaction Mechanism of all name reactions of Organic Chemistry.