Aldol Condensation || An Universal Video for Jee Advanced | NEET | Mains | AIIMS || IITian Explained

Hello, Guys. In this Lecture we are going to explain all the important points about Aldol Reaction and Aldol Condensation. As You all know that Aldol condensation is one of the very important topic for Jee Mains/Advance/NEET and AIIMS examination. Here, We will discuss this topic from very basic level to Jee Advanced level.

The Aldol addition reaction involves the addition of α-carbon of an enolizable aldehyde or ketone to the carbonyl group of another aldehyde or ketone and thus by giving a β-hydroxy carbonyl compound also known as an aldol (indicating both aldehyde and alcohol groups). The reaction is catalyzed more commonly by a base or some times by an acid.

If the β-hydroxy carbonyl compound containing an α-hydrogen undergoes subsequent dehydration to yield an α,β-unsaturated carbonyl compound, the entire process is also called as Aldol condensation. The dehydration step is possible under the aldol reaction conditions or mostly carried out by heating in presence of an acid or sometimes during acidic workup.

Mechanism In basic medium:

The first step in base catalyzed aldol reaction is the abstraction of an α-hydrogen from the enolizable carbonyl compound to give a resonance stabilized enolate anion. Usually this step is slow and rate determining.

Note: Usually hydroxide ion is used as a base, since it is less basic than the enolate ion. This ensures the formation of only small amount of enolate ion and is a necessary condition for further reaction of this ion with unionized carbonyl compound. Otherwise, if strong bases like amide are used, all the reactant molecules are converted to enolate ions and no further addition is possible.

*The next step is the nucleophilic addition of enolate anion to the carbonyl group of second aldehyde or ketone molecule. The final product formed after protic workup is a β-hydroxy carbonyl compound (an aldol).

Note: In aldol reaction, the enolizable aldehyde or ketone acts as nucleophile, whereas the carbonyl group of other molecule acts as electrophilic centre.

Mechanism In Basic medium:

Initially an enol is generated from the enolizable carbonyl compound during the acid catalyzed aldol reaction.
Thus formed enol reacts with the protonated carbonyl group of another molecule.

Dehydration:

The spontaneous dehydration of β-hydroxy carbonyl compound containing an α-hydrogen may be possible under the aldol reaction conditions or by heating in acid medium. The formation of stable α,β-unsaturated carbonylcompound is the driving force for this step. Hence the initial aldol addition product is often not isolated.

Dehydration in basic medium:

It proceeds via E1cB (Elimination Unimolecular conjugate Base) mechanism. Initially, the hydroxide ion removes acidic α-hydrogen to yield an enolate ion, which is a conjugate base of aldol. In the second step, the hydroxide ion is lost from the enolate ion to furnish the α,β-unsaturated carbonyl compound.

And Many other aspects of this reaction has been discussed in details.

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