prepared by .Aram Ishmael

Bakhan Ibrahim3rd stage –chem.dept.

Sulaimani uni.

introduction Dibenzylideneacetone or

dibenzalacetone, , is an organic compound. It is a bright yellow solid insoluble in

water, but soluble in ethanol. Dibenzylideneacetone is used as a

sunscreen component and it is used as a ligand in organometallic chemistry for

instance in tris(dibenzylideneacetone)dipalladium(0). In

this case, it is a labile ligand which is easily displaced by stronger ligands like

triphenylphosphine, hence it serves a useful entry point into palladium(0) chemistry.

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:

preparationThe compound can be prepared in the laboratory

by an aldol condensation of benzaldehyde and acetone with sodium hydroxide in a water /

ethanol medium with the exclusive formation of the trans,trans isomer (melting point 110–111

°C).[1]

This reaction is frequently encountered in organic chemistry education as a laboratory procedure. The conversion proceeds via the

intermediacy of benzylideneacetone.

Information of exp.1 .Sufficient alcohol is used to dissolve the benzaldehyde

rapidly and to retain the benzalacetone in solution until it has had time to react with the second molecule of

aldehyde. 2 .Lower concentrations of base slow up the

formation of the dibenzalacetone Higher concentrations of base give added difficulty

in washing. These concentrations were suggested by, and are approximately the same as, those used in the

preparation of benzalacetophenone..3 .Only temperatures between 20 and 25° were tried; it

was assumed that a change of temperature would have the same effect that it has in the preparation of

benzalacetophenone4.Stirring is essential, as it makes considerable

difference in the uniformity of the product.

An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone.

 

Aldol condensation: Synthesis of dibenzalacetone

The aldol condensation of benzaldehyde and acetone is a textbook example of an exothermic, spontaneous reaction

which is often performed during practical courses at universities and high schools. Due to its exothermic

character, the reaction vessel is traditionally cooled in an ice bath, with controlled reagent addition to avoid the

formation of side products and evaporation of acetone. The product dibenzalacetone is used as a UV blocker and as a

ligand in organometallic chemistry.

DiscussionDibenzalacetone has been prepared by

condensing benzaldehyde with acetone using as condensing agents dry hydrogen chloride,

1 10 per cent sodium hydroxide solution,2 and glacial acetic acid with sulfuric acid.3 It

has also been obtained by condensing benzalacetone with benzaldehyde in the

presence of dilute sodium hydroxide.4 Straus and Ecker5 were the first to record the use

of ethyl acetate for crystallization.