Aldol condensation
Aldol condensation | |||||||||
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Reaction type | Condensation reaction | ||||||||
Reaction | |||||||||
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Conditions | |||||||||
Temperature | +Δ, ~100°C[a]
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Catalyst
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−OH or H+
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Identifiers | |||||||||
Organic Chemistry Portal | aldol-condensation | ||||||||
RSC ontology ID | RXNO:0000017 | ||||||||
An aldol condensation is a
The overall reaction equation is as follows (where the Rs can be H)
Aldol condensations are important in organic synthesis and biochemistry as ways to form carbon–carbon bonds.[2][3][4][5]
In its usual form, it involves the
The term aldol condensation is also commonly used, especially in
Mechanism
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The first part of this reaction is an
Base-catalyzed aldol condensation
Acid-catalyzed aldol condensation
animation, base catalyzed | animation, acid catalyzed |
Crossed aldol condensation
A crossed aldol condensation is a result of two dissimilar carbonyl compounds containing α-hydrogen(s) undergoing aldol condensation. Ordinarily, this leads to four possible products as either carbonyl compound can act as the nucleophile and self-condensation is possible, which makes a synthetically useless mixture. However, this problem can be avoided if one of the compounds does not contain an α-hydrogen, rendering it non-enolizable. In an aldol condensation between an aldehyde and a ketone, the ketone acts as the nucleophile, as its carbonyl carbon does not possess high electrophilic character due to the
Examples
The Aldox process, developed by
Pentaerythritol is produced on a large scale beginning with crossed aldol condensation of acetaldehyde and three equivalents of formaldehyde to give pentaerythrose, which is further reduced in a Cannizzaro reaction.[14]
Scope
Ethyl 2-methylacetoacetate and campholenic aldehyde react in an Aldol condensation.[15] The synthetic procedure[16] is typical for this type of reaction. In the process, in addition to water, an equivalent of ethanol and carbon dioxide are lost in decarboxylation.
Occasionally, an aldol condensation is buried in a multistep reaction or in catalytic cycle as in the following example:[18]
In this reaction an alkynal 1 is converted into a
The reaction between
The product can
Other condensation reactions
There are other reactions of carbonyl compounds similar to aldol condensation:
- When the base is an amine and the active hydrogen compound is sufficiently activated the reaction is called a Knoevenagel condensation.
- In a anhydride.
- Claisen-Schmidt condensationbetween an aldehyde or ketone having an α-hydrogen with an aromatic carbonyl compound lacking an α-hydrogen.
- A Claisen condensation involves two ester compounds.
- A Dieckmann condensation involves two ester groups in the same molecule and yields a cyclic molecule
- In the Japp–Maitland condensation water is removed not by an elimination reaction but by a nucleophilic displacement
- A Michael reaction prior to the aldol condensation.[3]
- In the Guerbet reaction, an aldehyde, formed in situ from an alcohol, self-condenses to the dimerized alcohol.
See also
- Auwers synthesis
- Aldol addition
References
- OCLC 1201694230.
- ISBN 0-471-58589-0.
- ^ ISBN 0-306-43440-7.
- ISBN 0-13-236731-9.
- ISBN 3-527-30714-1.
- ISBN 0-08-040593-2.
- ISBN 0471264180.
- ^ Paterson, I. (1988). "New Asymmetric Aldol Methodology Using Boron Enolates". Chemistry and Industry. 12. London: Paterson Group: 390–394.
- ISBN 0471264180.
- PMID 27281298.
- ISBN 0-306-43440-7.
- ISBN 978-81-7709-605-7.
- ^ Graduated hydrogenation of aldox aldehydes to alcohols US US3118954A
- ; Collected Volumes, vol. 1, p. 425.
- doi:10.3390/M388.
- dioxane. Then campholenic aldehyde (1) is added and the mixture refluxed for 15 h. Then 2N hydrochloric acid is added and the mixture extracted with diethyl ether. The combined organic layers are washed with 2N hydrochloric acid, saturated sodium bicarbonate and brine. The organic phase is dried over anhydrous sodium sulfate and the solvent evaporated under reduced pressure to yield a residue that is purified by vacuum distillationto give 3 (58%).
- PMID 16122270.
- PMID 16866480.
- .
Notes
- aldol addition side product would be formed alongside the aldol condensation product.[1]
- chemical yield. The first step is formation of the Transition metal carbene complex 2. Acetic acid adds to this intermediate in a nucleophilic addition to form enolate 3 followed by aldol condensation to 5 at which stage a molecule of carbon monoxide is lost to 6. The final step is reductive eliminationto form the cycloalkene.