The purpose of this experiment is to synthesis dibenzalacetone (trans, trans-1,5-diphenyl-1,4-pentadien-3-one) through the aldol condensation of acetone with benzaldehyde. The synthesis begins by using strong base to generate the acetone enolate ion. Water (not shown) is formed as a byproduct. The equilbrium position of this reaction strongly favors the starting acetone, and the amount of acetone enolate formed is quite small; however the enolate is extremely nucleophilic
Being a very strong nucleophile, this enolate attacks the carbonyl of benzaldehyde and forms a B-carbonyl alkoxide ion. This alkoxide ion abstracts a proton from water to form a beta hydroxy ketone. Sodium hydroxide abstracts another acidic alpha H to form a stabilized carbanion. The electron pair on carbon is used to eliminate the hyroxide ion, forming a alpha-beta unsaturated ketone in an irreversible step. This is an example of an E1CB mechanism. Note that in this reaction, the intermediate alcohol is dehydrated under basic conditions, unlike most alcohol dehydrations, which are generally E1 mechanisms under acid conditions. The E1CB mechanism is made possible by the presence of the carbonyl, which stabilizes the intermediate carbanion.
Since this newly formed ketone still posses alpha hydrogens, it too can undergo the same enolate condensation reaction with a second mole of benzaldehyde to form the final product:
Before you come into lab, make sure you have filled in your table of reagents and products. You will need these values (particularly the molecular formula and molecular weight) to determine the identify of your products and to calculate your final yield. Take care to determine the coefficients of the balance chemical reaction when determining the theoretical yield. .You will also need to come to lab with IRs of your starting materials and expected product(s) already in your notebooks (General Chemistry Links). Finally, it is important that you know exactly what you are going to be doing so you can work more efficiently:
(Created by C.R. Snelling
(Updated by J. Neilan, 8/7/06))