How to make benzalacetone not dibenzalacetone from acetone and benzaldehyde?

Answer 1

The modern solution to this problem--how to stop the aldol reaction after one addition when another one is possible--is to use preformed enolates.

1. Add acetone to an LDA solution. By adding the ketone to the LDA, you decrease the possibility that the acetone enolate will add to another molecule of acetone--i.e. self-condensation.
2. Add benzaldehyde to the enolate solution. It may be necessary to heat the reaction mixture in order to cause the final elimination step to occur.

If you are curious why this is done, keep reading.

Aldol reactions begin with an equilibrium between ketone and enolate. This equilibrium is not typically favorable, but the addition step is quite favorable. This means that, after addition, another slow equilibrium followed by a fast addition is possible.

If you could have a solution where the equilibrium is essentially all enolate, you would only get one reaction. Once all the enolate gets used up, there is no reaction.

The way to shift the equilibrium in favor of the ketone is to use really strong bases. The most common one is lithium diisopropylamie (LDA). It is sterically hindered, so it does not react with the carbonyl, but has a pKa of about 45, which is twice that of a typical ketone.

As a practical note, all glassware used in these reactions should be flame or oven-dried, the reaction should be run in a flask with a rubber septum, and reagents should be added using a syringe.

Answer 2

no shiara there is nothing wrong with the question. it involves an aldol condensation mechanism.

dibenzalacetone is formed from 2 benzaldehyde and 1 acetone molecule in aqueous/ethanolic KOH. if you want a single aldehyde and ketone to be formed, then you would just follow the dibenzalacetone formation method but use acetone in excess.

this would result in only single condensations. of 1 benzaldehyde and 1 acetone molecule.

Answer 3

you need to go over naming rules because you're not naming the compounds right. to make a keytone from an aldehyde takes a lot of effort. it's a lot easier to take benzoyl chloride and a cuprate reagent to get the desired keytone