Aldol condensation
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An Aldol condensation is a organic reaction where a enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone followed by dehydration to a conjugated enone. The first part of this reaction is a aldol reaction, the second part a elimination reaction. Dehydration may be accompanied by decarboxylation when a activated carboxyl group is present. The base used in this reaction is a strong base like t-butoxide,potassium hydroxide or sodium hydride.
Condensation types
It is important to distinguish the Aldol condensation from other addition reactions to carbonyl compounds.
- When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
- In a Perkin reaction the aldehyde is aromatic and the enolate generated from an anhydride.
- A Claisen condensation involves two ester compounds.
- A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
- A Henry reaction involves an aldehyde and an aliphatic nitro compound.
Examples
M0388.gif
Ethyl 2-methylacetoacetate (2) (727 mg, 4.53 mmol) is added to a stirred solution of sodium hydride (184 mg, 4.60 mmol) in dioxane (25 mL). Then campholenic aldehyde (1) (707 mg, 3.72 mmol) is added and the mixture refluxed for 15 h. Then 2N hydrochloric acid (15 mL) is added and the mixture extracted with diethyl ether (3x15 mL). The combined organic layers are washed with 2N hydrochloric acid (2x15 mL), saturated sodium bicarbonate (2x15 mL) and brine (3x15 mL). The organic phase is dried over anhydrous sodium sulfate and the solvent evaporated under reduced pressure to yield a residue (900 mg) which was purified by vacuum distillation to give 3 (505 mg, 2.63 mmol, 58%). [1]
References
- [1] (E)-6-(2,2,3-Trimethyl-cyclopent-3-enyl)-hex-4-en-3-one Concepcion Bada, Juan M. Castro, Pablo J. Linares-Palomino, Sofia Salido, Joaquan Altarejos Manuel Nogueras, Adolfo Sanchez, Molbank 2004, M388 [1] (http://www.mdpi.net/molbank/molbank2004/m0388.htm) open access publication