What is the mechanism of an aldol addition?

Question

Lucy Alessi · Accepted Answer

Here's what I find.

An aldol addition is the addition of an enolate of an aldehyde or a ketone to the carbonyl carbon of another molecule under basic or acidic conditions to obtain a β-hydroxyaldehyde or β-hydroxyketone (an aldol).

An example of aldol addition is the base-catalyzed reaction of acetaldehyde with itself to form 3-hydroxybutanal.

There are three steps in the aldol addition.

1. Removal of an acidic α-hydrogen by a strong base to form a resonance-stabilized enolate ion.

2. Nucleophilic attack of the enolate ion on the electrophilic C=O group of another molecule forms a new C-C bond and an alkoxide ion.

3. Protonation of the alkoxide ion to form the aldol.

A condensation is a reaction in which two molecules combine to form a larger molecule together with the loss of a small molecule such as water.

If the aldol is then dehydrated, the overall process is called an aldol condensation.

Heating the aldol with strong base results in the elimination of water and the formation of an α,β-unsaturated product.

The video below gives a good overview of aldol addition and condensation.

Skylar Alexander · Answer

undefined The mechanism of an aldol addition involves the following steps:

1. Enolate Formation: A base abstracts a proton from the α-carbon of a carbonyl compound, forming an enolate ion.
2. Nucleophilic Attack: The enolate ion attacks the electrophilic carbonyl carbon of another carbonyl compound, resulting in the formation of a tetrahedral intermediate.
3. Elimination: The tetrahedral intermediate collapses, expelling the leaving group, and forming a new carbon-carbon double bond.
4. Proton Transfer: A proton transfer occurs to regenerate the carbonyl functionality and yield the aldol product.

Ruby Allen · Answer

undefined The mechanism of an aldol addition involves the following steps:

1. Enolization: One of the reactants, usually a ketone or aldehyde, undergoes enolization to form an enolate ion in the presence of a base (e.g., hydroxide ion).

2. Nucleophilic attack: The enolate ion acts as a nucleophile and attacks the carbonyl carbon of another molecule of the ketone or aldehyde, forming a new carbon-carbon bond.

3. Proton transfer: A proton is transferred from the oxygen atom of the enolate ion to the oxygen atom of the resulting alkoxide ion, generating an aldol product.

4. Tautomerization: The aldol product undergoes tautomerization to form the more stable keto-enol tautomer.

Overall, the aldol addition reaction involves the addition of an enolate ion to a carbonyl compound, resulting in the formation of a β-hydroxy aldehyde or ketone product.