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Limitations of Aldol Condensation

Carbonyl compounds contain a carbon with an oxygen attached to it by two double bonds. The carbon next to the carbonyl group is called an alpha carbon and the carbon located one position further away is a beta carbon. Aldol addition takes two such molecules and combines them to form a compound with a hydroxyl, or OH group, on the beta carbon; heating the product gets rid of the hydroxyl group. These two steps together are called an aldol condensation -- an important reaction for organic chemists, albeit one that has some limitations.

Ketones
- An aldehyde has a carbonyl group at the end of a carbon chain, while a ketone has a carbonyl group with a carbon attached on either side. The aldol reaction is faster and offers higher yields for aldehydes than it does for ketones, because aldehydes are less stable. That's because the polar carbon-oxygen bond leaves a partial positive charge on the carbonyl carbon, so the two electron-donating carbon chains (alkyl groups) attached to the carbonyl carbon in a ketone help to stabilize it; an aldehyde only has one alkyl group attached. Consequently, to make this kind of reaction happen with a high yield, chemists must remove the product as it is formed, which makes the process somewhat more complicated than the usual reaction with aldehydes.
Crossed Aldol
- Combining two different carbonyl compounds (e.g. propanal and acetaldehyde) leads to a so-called "crossed aldol condensation," creating a variety of different products. Typically, you want a reaction that produces as few products as possible, because this will increase your yield; moreover, separating mixtures of products can be very difficult, especially if they are similar compounds with similar properties. Consequently, crossed aldol condensations are only useful if one of the compounds involved does not have any hydrogens attached to its alpha carbons.
Multiple Products
- Even reactions that involve only one carbonyl compound can sometimes produce multiple products -- if they involve an asymmetric ketone. An aldol reaction involving 2-methylcyclohexanone, for example, would be less straightforward, because the base could remove a hydrogen ion from either of the two alpha-carbons and thereby lead to a reaction with multiple products.
Ring Formation
- Aldol reactions can involve an alpha-carbon and a carbonyl group in the same molecule; if so, they are called intramolecular aldol condensations. These reactions generally form only one of two types of rings: six-membered rings or five-membered rings. For the most part, this is an advantage -- you can predict what kind of product you will get and thereby use this reaction to design a specific kind of ring. It can also be a limitation, however, assuming that your goal is to make a four-membered or seven-membered ring.

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