Question

1. In the dibenzylideneacetone synthesis by an aldol condensation procedure

a) Explain why it is not necessary to acidify and warm in the final step of the reaction

b) Explain why a mixture of products is not obtained in this cross-aldol reaction and reflect on the stereochemistry of the product around the double bond.

c) State the solubility of dibenzylideneacetone in different solvents as this influences the experimental procedure.

Answer 1

1. a) Dibenzylideneacetone is synthesised from benzaldehyde and acetone . The mechanism of this cross aldol reaction can be given as follows-

The elimination process is particularly fast in this case and it don't need any acidification or warming because the alkene formed is stabilized by conjugation to not only the carbonyl but also the benzene.

b) Generally , in cross aldol condensation a mixture of product is obtained. But here we get only a specific product . This is because acetone has an alpha hydrogen to the C=O which can be picked by the base and there is no alpha hydrogen in benzaldehyde . Hence only acetone forms the enolate ion and drives the reaction in forward direction.Also, the aldehyde carbonyl is much more electrophilic than that of a ketone, and therefore reacts rapidly with the enolate. So acetone does not give any self aldol condensed product.

Note: In the experimentYou are adding acetone last, after the benzaldehyde electrophile is already available in the reaction vessel. If the acetone goes in first, it could do aldol condensation on itself, in which enolate anions just attack neutral acetone carbonyls.
Ketone carbonyls aren’t competitive with aldehyde carbonyls as electrophiles, but if there are no aldehydes available, ketones are better than nothing!

c) Here is the solubility data of dibenzylidenediacetone in different solvents-
acetone: soluble
alcohol: slightly soluble
chloroform: soluble
diethyl ether: slightly soluble
water: insoluble(as it is less polar compound)