MECHANISM OF ALDOL CONDENSATION
Aldol condensation is an important reaction in organic chemistry where molecules containing an α-hydrogen (usually aldehydes or ketones) react with each other in the presence of a mild base. This produces a new compound called an aldol, which contains both an aldehyde (aldo-) and an alcohol (-ol) group.
A simple example is the reaction of acetaldehyde with dilute base, which forms a β-hydroxy aldehyde (aldol).
What Compounds Undergo Aldol Condensation?
- Two same aldehydes
- Two different aldehydes
- Two same ketones
- Two different ketones
- An aldehyde and a ketone
When two different carbonyl compounds react, the reaction is called a crossed aldol condensation.
Mechanism of Aldol Condensation (Step-by-Step)
Step 1: Formation of Enolate Ion
The base removes an α-hydrogen from the aldehyde or ketone. This forms a resonance-stabilized enolate ion.
Step 2: Nucleophilic Attack
The enolate ion attacks the carbonyl carbon of another molecule of the aldehyde/ketone. This forms an alkoxide ion.
Step 3: Protonation
The alkoxide ion picks up a proton from the solvent and becomes a β-hydroxy aldehyde or ketone (the aldol product).
Overall, the reaction is basically an addition of an enolate ion to a carbonyl group.
Key Points About Aldol Condensation
- Aldehydes react more easily than ketones because ketones are more sterically hindered and their carbonyl carbon is less positive.
- Aldol products often dehydrate to form α,β-unsaturated compounds when heated.
- Using strong bases may lead to repeated condensation forming resin-like products.
- The reaction is promoted by electron-withdrawing groups (−I effect) and slowed down by electron-donating groups (+I effect).
Special Notes
- Ketones generally give poor yields in aldol condensation due to steric hindrance.
- However, acetone can form diacetone alcohol in good yield when heated with Ba(OH)2.
- Aldols are often not isolated because they form cyclic hemiacetals or quickly dehydrate.
Detailed Notes:
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