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Preparation of Dibenzalacetone by the Aldol Condensation

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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)2OH2Ag + 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. ·I covered the top of the test tube with a cork and shook the test tube vigorously ·A few drops of water were added to aid precipitation.

Hydroxylamine hydrochloride test
To 1mL of aldehyde solution I added 2-3 mL of hydroxylamine hydrochloride reagent solution with 2 drops of methyl orange solution. The mixture was warmed gently.
After a few minutes a red colour had developed.

Results and data
Melting point
The melting point of dibenzalacetone was determined using the melting point apparatus. The melting point was found to be 108°C.the sharp melting point range indicates that the dibenzalacetone produced was very pure. Percentage yield

Percentage yield of pure dibenzalacetone
Final mass = 4.268 g
Theoretical yield = 0.0245 M
Dibenzalacetone 1M = 234.3 g
Therefore 4.268 g = 0.0182 M
Theoretical yield = 0.0245 due to the 1:1 ratio in the reaction scheme. Percentage yield = actual yield obtained (M)/ theoretical yield (M) x 100 Percentage yield = 0.0182/0.0245 x 100 = 74.3 %
Purification analysis
Melting point determination
The efficacy of a recrystallisation can be concluded by a melting point. A pure compound usually has a sharp and characteristic melting point. Even small amounts of impurity may depress the melting point greatly. Generally, one crystallises to constant melting point. Therefore the melting point of a substance is an indicator of purity and also a specification for labelling many drug substances Thin layer chromatography

Thin layer chromatography is a very popular and widely used for of chromatography as it gives a fast, qualitative analysis of mixtures and determination of purity of elements. The separation is carried out on a flat plate coated with a thin layer of absorbent material (stationary phase). The crystals that we were examining were dissolved in an appropriate solvent and spotted on the plate at the base line. This plate is then placed in a developing jar with a little of the attraction developing solvent (mobile phase). The various compounds in the mixtures ascend the plate (due to capillary action) at different depending on their differing affinity for the absorbent (polarity).The higher up the plate the mixtures ascend the more non-polar they are. The RF values can also indicate the polarity of the compounds. Examining the chromatogram

In this experiment the material in which we used in the TLC was colourless so to visualise the chromatogram an ultra violet light had to be used. When illuminated with an ultraviolet light the absorbent then glows a pale green and the organic compounds turn up as dark spots because they quench the fluorescence. Using the positions of the mixtures of the TLC I calculated the RF values. RF value = distance travelled by compound/ distant travelled by solvent front RF of Benz aldehyde = 6/15 = 0.4

RF of dibenzalacetone = 6.4/15 = 0.4266

Mobile phase used: ethyl acetate hexane in a 1:1 ratio.

In conclusion I can say that in the presence of sodium hydroxide, 2 moles of Benz aldehyde and 1 mole of acetone can be reacted to synthesize 1mole of dibenzalacetone. The synthesis gave a reasonable yield of 74.3%. The loss of the other 25.7% could have been due to personal experimental error throughout the experiment. My purified dibenzalacetone had a melting point of 108°C, which indicated that the substance must be pure as the precise melting point of dibenzalacetone is 130°C. This low margin of difference between my melting point results and the actual melting point can prove the purity of my crystals and the success of the experiment. References used

School of Pharmacy and Pharmaceutical sciences laboratory manual (PH1001) British pharmacopeia
Organic chemistry – Heart et al

1.Mechanism for the formation of benzalacetone.

Mechanism for the formation of dibenzalacetone.

2.Write the formula to show the possible geometric isomers of dibenzalacetone. Which isomer would you find to be the most stable and why?

The E-E isomer of dibenzalacetone is most stable because it has the highest melting point. This tells me that it is the most stable and less volatile than the other isomers which have a lower melting point. 3.I can tell that my product consists of a single isomer because I found that my product had a distinct melting point, indicating that a pure substance was formed. The product did not begin to melt over a range of different temperatures which concludes that several isomers were not present in the compound. 4.In order to synthesis benzalactone, I would :

·React 1 mole of Benz aldehyde with 1 mole of acetone. ·I would not have a 2:1 ratio between them.
In order to synthesis benzalacephonone, I would:
·React 1 mole of Benz aldehyde with 1 mole of acetophenone. 5.

According to the BP, 98% to 101% of the E isomer is allowed.

According to the BP, 13% to 18.5% of the Z isomer is allowed. By carrying out the thin layer chromatography it is possible to differentiate between the E and Z isomer. The TLC process will result in different RF values for each of the isomers and will allow you to differentiate between them.

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