Aldol Condensation

by James

Description: The aldol condensation is a reaction between an enolate (or enol) and an aldehyde or ketone that leads to the formation of a new carbon-carbon double bond. This is the product if an aldol addition reaction is heated for prolonged periods. The reaction is called a “condensation” because one molecule of water is formed from the two reactants.
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{ 11 comments… read them below or add one }

David Carnevale

Are there any conditions in which the ketone and alcohol remain and the double bond does not form in the final elimination step?

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James

Yes – that’s “aldol addition”. Typically one avoids using heat in that process.

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Petr Menzel

Hi, is the “heat” always important? Or can I use another way for condensation? Thx, P.

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James

Hi Petr – for aldol condensation, heat tends to favour elimination of water. If one wants to perform the aldol condensation, the best way to go about it is to heat the reaction mixture. To avoid the elimination, and to just perform an aldol addition reaction, typically one uses lower temperatures.

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Rich Aversa

In the intramolecular reaction, the ketone you deprotonated has a primary and a secondary alpha carbon, so it has two different enolate forms. The conditions used would encourage formation of the “thermodynamic enolate”, i.e. the more substituent pi bond. Thus, the cyclic product would look quite different. For instance the remaining ketone would not “straddle” the ring but “hang off”, connected to its now tertiary alpha carbon. As I’ve seen it in other sources, a 1,6-diketone should yield a five-membered ring. Is this correct?

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James

While that enolate is in fact the more stable “thermodynamic” enolate, it would form a 4-membered ring, not a 5-membered ring. That’s why it doesn’t happen.

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James

Being the best site in organic chemistry i was hoping that i could find the mechanism for Cannizzaro reaction (which i wasn’t able to find on googling and referring to Solomon). So can you propose the mechanism for the cannizzaro reaction as well?
Thanks in advance.

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James

Hi – there is a page for the Canizarro, with the mechanism. http://www.masterorganicchemistry.com/reaction-guide/cannizarro-reaction/

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Shelby Backman

Hi James, I was wondering what would be the favored product in the aldol condensation of 3-ethyl-4-hydroxy-2-hexanone (just my guess how to name the molecule I’m looking at).

Would the product be favored towards 3-ethyl-3-hexenone or 3-ethyl-4-hexenone?
I’m leaning towards the first since the removed hydrogen (assuming a basic reaction) would be from the tertiary carbon.

But… now that I think about it, does base vs acid matter in that case?

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James

Hi Shelby – if I’m reading your question right, the answer is that the double bond would be adjacent to the carbonyl (i.e. on the 3-position). Conjugation increases the stability of the system.

As an additional note, in order for the elimination to occur, you need to form either the enolate (under basic conditions) or the enol (under acidic conditions) and this is going to occur at the 3 position, not the 5 position (as would have to be the case if the alkene formed on the 4-position).

Hope this helps! James

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Shelby Backman

Hi James– can you possibly email me? I have some MS paint drawings that I drew up (I’m very very visual), and I tried sending them in my email reply but I got a message back that says the email cannot be reached.

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