Preparation of Dibenzalacetone by the Aldol condensation
David o Neill
Date of experiment: 14/12/2011
Steam bath, ice bath, Buchner funnel, beaker, conical flask, filter paper, TLC apparatus, Melting point apparatus Materials / chemicals
Benzaldehyde, acetone, ethanolic sodium hydroxide, ethanol
The synthesis of dibenzalacetone is formed from an Aldol condensation reaction. An Aldol condensation reaction is a very effective way of forming a carbon – carbon bond reaction, in which the enolate anion adds to the carbonyl group of the aldehyde. Aldol condensations are very versatile, as the enolate anion of the carbonyl compound can be added to the carbonyl carbon of another. The synthesis of dibenzalacetone is an example of a mixed Aldol condensation reaction. This experiment involves condensating acetone with two measures of Benz aldehyde (giving dibenzalacetone, an organic sun screen). The carbonyl group on the aldehyde is more reactive than that of the keytone. Therfore the enolate ion of Benz aldehyde adds to the carbonyl group of the acetone giving b-hydroxykeytone. This then undergoes base catalysed reaction with ease. Depending on the quantity of reactants used the reaction may produce mono or dibenzalacetone. Because we want dibenzalacetone, 2 moles of Benz aldehyde is reacted with one mole of acetone in the presence of sodium hydroxide.
The reaction scheme is given by:
Initially in the synthesis of Dibenzalacetone I mixed 5 mL (0.049 moles) of Benz aldehyde with 1.8 mL (0.0245 moles) of acetone in a small beaker. ·
I then added half of this mixture to 90 mL of ethanoic sodium hydroxide in a 100 mL conical flask. ·
I allowed this mixture to stand for 15 mins.
I then placed the remainder of the previous aldehyde- keytone mixture into this conical flask and swirled the solution for 30 mins. ·
The product was collected under suction using the Buchner funnel and the suction pump. ·
The collected product was then washed with 3 x 100 mL of water to remove all traces of sodium hydroxide. ·
Using filter paper I dried the product as much as possible by pressing it into the paper using my spatula. ·
The product was recrystallized form ethanol ( 10 mLs of ethanol of every 4 g of dibenzalacetone ) ·
I carried out the TLC analysis of dibenzalacetone and recorded the melting point by using the melting point apparatus. ·
The percentage yield was calculated and the melting point noted.
Functional group test
Schiff’s test for aldehydes
Many aldehydes in solution on in a suspension give a red colour with Schiff’s reagent. ·
I added 1 mL of Benz aldehyde solution to 1 mL of Schiff’s reagent and mixed the two of them well. ·
In the cold a pink- red colour developed
The pink – red colour indicated that the aldehyde functional group is present in Benz aldehyde. Note:
Schiff’s reagent consists of the dye rosaniline bleached with sulphurous. Acid
A negative result will appear in the presence of keytones and also choral hydrate Tollens silver mirror test
Tollens reagent is reduce by aliphatic and aromatic aldehydes R-CHO + Ag (NH3)2OH
+ RCOO-NH4+ + H2O + NH3
In this test I added 2 drops of dilute NaOH followed by dilute ammonia (drop wise) to 1 mL of AgNO3 until a precipitated silver oxide is almost dissolved. ·
I then added 2 – 3 drops of Benz aldehyde solution to the mixture. ·
The mixture was warmed gently
A silver mirror of metallic precipitate was formed giving a positive result for reducing aldehydes, concluding that Benz aldehyde has an aldehyde functional group. Keytones give a negative result.
Reaction with 2, 4-Dinitrohenylhydrazine
2, 4-Dinitrohenylhydrazine reacts with the aldehyde carbonyl group to give 2, 4-dinitrophenylhydrazones which are orange/yellow crystalline precipitates that are insoluble in water. ·
I added 2, 4-Dinitrohenylhydrazine reagent solution and 500 mg of the Benz aldehyde in an aqueous solution. ·...
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