Question

Write the steps in the mechanism of the base catalyzed condensation of the following reaction:

acetone + Benzaldehyde (2.0 equivalents) ---(NaOH, H2O, heat)----> dibenzalacetone

a) The first step is the deprotonation of acetone to form the reactive enolate nucleophile. Draw a reasonable and detailed arrow pushing mechanism for the following transformation.

b) Next, the reactive enolate will react with the electrophile, benzaldehde. This will give the addition product of the Aldol reaction. Draw a reasoable and detailed arrow pushing mechanism for the following transformation.

c)Lastly, a dehydration reaction occurs to form benzalacetone. Please Draw a reasonable and detailed arrow pushing mechanism for the following transformation.

d) Referring back to part c, why doesn't the reaction stop after dehydration to get benzalacetone?

Answer 1

The given reaction is known as aldol condensation. Aldehydes/ketones having alpha hydrogens (alpha hydrogens are the one which are on the carbon adjacent to the carbonyl group of aldehyde/ketone) in presence of base react with another aldehyde or ketone to give the condensed product, which is known as aldol condensation reaction.

In the given reaction acetone having alpha hydrogens (total 6 such hydrogens are present in acetone) reacts with NaoH and benzaldehyde.

d) The dehydration reaction takes place to yield the more stable conjugated product.