Dibenzalacetone - student lab report PDF

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1. CHE 204, 04/14/2020 2. Dibenzalacetone by the aldol condensation. 3. Purpose: Synthesize dibenzalacetone which is aldol compound by Claisen-Schmidt reaction to prove that reaction of an acetone with aldehyde, catalyzed by a strong base, yields an aldehyde + alcohol mixed compound known as (aldol). 4. Introduction: Aldol condensation is a synthetic reaction used in organic chemistry, and a type of coupling reaction in which an enol or an enolate is joined with a carbonyl compound to produce conjugated enone and intermediated by the dehydration/elimination of β-hydroxyaldehyde or β-hydroxyketone. However, they can be catalyzed by either strong acid or strong base. All aldol condensation reaction involves the formation of new carbon-carbon bond through the nucleophilic attack of ketone enolate to an aldehyde to form a β-hydroxy ketone, known as " aldol" (aldehyde + alcohol). Claisen-Schmidt reaction is one type of many aldol condensation reactions that in particularly basecatalyzed reaction of α-hydrogen aldehyde or ketone with non-α-hydrogen aromatic compound. We will conduct an experiment of Claisen-Schmidt reaction by preparation of dibenzalacetone to observe how aldol condensation reaction happens and how every step of the reaction illustrates the mechanism of formation. Dibenzalacetone is used in sunscreens and sunblock preparations because of its spectral properties. Benzaldehyde, a non-α-hydrogen aromatic compound, and acetone (ketone compound) are used as reactants. Sodium hydroxide is used as strong base to catalyze the reaction. Ethanol is used as organic solvent to dissolve the starting materials. Heat is required to speed up the processes of the reaction. Because acetone has three hydrogens of its α-hydrogen carbon and benzaldehyde does not, it’s easier to be attacked by the base. Once acetone is deprotonated by the base, it forms water molecule and an enolate which readily nucleophilic attack one molecule of very reactive aldehyde carbonyl to form an alkoxide, then is protonated to form a β-hydroxyketone followed by the dehydration/elimination (E2) by the base catalyst to give an ester called enone and known as benzalacetone. E2 reaction is multiple steps exothermic reaction; heat will be produced. Nonetheless, the Claisen-Schmidt reaction can occur between two esters depending on the quantities of the reactants in the presence of the strong base, thus two moles of benzaldehyde is

required to form β-diketone known as dibenzalacetone, because one molecule of benzalacetone readily react with the other molecule of benzaldehyde. The final product may exist in many conformation (trans, trans), (trans,cis), or (cis,cis) but the major product is trans, trans because it’s the most stable and forms readily.

two moles of water and sodium hydroxide are regenerated at the end of the reactions as a result of protonation and deprotonation. We will use crystallization followed by vacuum filtration as a purification technique to remove impurities or any basic filtrate crude from the product. Heating and cooling our impure solid compound with solvent will initially create saturated solution that allow crystals to form by decreasing its solubility.

5. Balanced chemical equation:

( CH 3 )2 CO +2C 6 H 5−CHO + OH C 6 H 5−HC =CH −CO −CH = CH −C6 H 5 +2 H 2 O →

6. Reaction mechanism: deprotonation

Nucleophilic attack

protonation

E2

This figure is retrieved from Macroscale and Microscale Organic Experiments 7 th edition, by Kenneth L. Williamson & Katherine M. Masters

7. limiting reagent:

1 mol rxn=1mol acetone=2 mol benzaldehyde=1 mol dibenzalacetone we need 0.05 mol benzaldehyde ×

2 mol acetone =0.10 mol of acetone to react with 1 mol benzaldehyde

benzaldehyde for the reaction to produce 1 mol of dibenzalacetone

0.05 mol benzaldehyde × 0.10 mol acetone ×

1 mol rxn =0.025 rxn 2 mol benzaldehyde

1 mol rxn =0.10 rxn 1 mol acetone

The more moles of reaction you have, the more times the reaction can occur. Therefore, the reactant with fewer moles of reaction is the limiting reagent.

benzaldehyde is limiting reagent based on textbook calculations .

8. Theoretical yield:

0.025 mol dibenzalacetone ×

234.29 g =5.9 g of dibenzalacetone 1 mol

A typical student yield is 4 grams

percent yield=

4.0 experimental yield × 100= ×100=68 % theoretical yield 5.9

9. Table of chemical and physical properties: Benzaldehyde

MW= 100.13 g/mol

Clear liquid

Bp= 178°C Density= 1.04 g/ml Acetone

MW= 58.08 g/mol

Clear liquid

Bp =56°C Density= 0.790 g/ml

Dibenzalacetone

MW= 234.4 g/mol

Yellow solid crystals

mp= 110°C Ethanol

MW= 46.07 g/mol

Clear liquid

Sodium hydroxide

39.997 g/mol

White powder

NaOH

Density: 2.13 g/cm³

10. Safety: Wear gloves and goggles. If sodium hydroxide gets on your skin, wash until the skin no longer has a soapy feeling. Ethanol is flammable. Benzaldehyde

11.

NaOH

acetone

First recrystallization retrieved from ellesmerealevelchemistry

Ethanol

Coffee filtration

drying

Coffee filtration

Coffee filtration

washing

First Melting

Second Melting point

12. Experimental procedure: Procedure

observations

Start with adding 250 ml pf 95% ethanol in a 1000 ml beaker. Add a magnetic bar and turn on the stirring. Measure and add 32 ml of benzaldehyde to the beaker.

The benzaldehyde quickly dissolves. Few minutes after, the solution turn cloudy we left with clear solution. No changes to the mixture after adding extra ethanol

Try adding some extra ethanol to remove the cloudiness of the mixture. Dissolve 18 g of NaOH in a 150 ml of water. Mix well the solution Add the NaOH solution to the reaction mixture. As quickly as possible initiate the reaction by adding 10 ml of acetone. Keep stirring for 30 minutes Turn off the stirring

The solution turns yellow very slowly as a yellow precipitate forms slowly at the bottom at the beaker Orange red solution forms above

Add 400 ml of water to dissolve orange dibenzalacetone Setup Buchner funnel. Break the suction into small patches Wash the powders with some distilled water Reapply the vacuum. Repeat these steps 3 times. Determine the melting point of the crude Transfer the final product to a 1000 ml beaker. add 70:30 ethanol: water under stirring and heat partially cover the beaker add some water to reduce the solubility. Add some ethanol followed by water until solution reach saturation point. Setup hot filtration Pour the beaker content over coffee filter. Keep heating and stirring the solution. Turn off the heat and stirring. allow the solution to cool. Do another Buchner filtration. Transfer the product to another paper towel. Press the powder with the paper. Let the crystals dry. Test the melting point of the crystals.

dibenzalacetone The reaction mixture turns all yellow

90-95°C All the dibenzalacetone dissolved

This technique failed to remove the cloudiness of the solution. Dibenzalacetone precipitated on the bottom of the flask The cloudiness of the solution has been reduced. The crystals formed quickly. The product fell off around the solution Melting point after Recrystallization 109-111°C

13. Waste handling: Dilute the aqueous waste with water and neutralized it with HCL before flushing down the drain. Place ethanol filtrate in the organic solvent waste container.

14. Experimental data and observations: Product yield: 28 g dibenzalacetone Melting point: crude- 90-95°C Recrystallized 109-111°C The reaction readily occurs. Benzaldehyde has low solubility in ethanol. Dibenzalacetone dissolves quickly in water and ethanol. Adding too much water and ethanol ruin the crystallization of the product. Impurities are hard to extract from dibenzalacetone. Adding

ethanol neither increase the yield nor increase the purity. The formation of dibenzalacetone crystals takes place when the solution gets darker.

15. Limiting reagent: Sodium hydroxide and ethanol can’t be limiting reagent because they are catalyst in this reaction. They are formed at the end of the reaction.



32 ml ×

1.04 g benzaldehyde 1 mol × =0.33 mol benzaldehyde ml 100.13



10 ml ×

0.79 g acetone 1 mol =0.14 mol acetone × 58.08 ml



1 mol rxn=1mol acetone=2 mol benzaldehyde=1 mol dibenzalacetone



0.33 benzaldehyde ×



0.14 acetone ×

1mol rxn =0.16 mol rxn 2 mol benzaldehyde

1 mol rxn =0.14 mol rxn 1 mol acetone

 The more moles of reaction you have, the more times the reaction can occur. Therefore, the reactant with fewer moles of reaction is the limiting reagent.

acetone is our limiting reagent .

16. Theoretical yield: Because acetone is the limiting reagent, the amount of mole dibenzalacetone is produced theoretically is 0.05: 0.025: 0.025 mole ratio

0.14 mol dibenzalacetone×

234.29 g =33 g of dibenzalacetone 1mol

17. Percent yield:

¿

experimental yield 28 ×100= ×100=85 % 33 theoretical yield

18. Discussion: 95 % of ethanol has 5 % water which might explain why surprisingly benzaldehyde has low solubility in ethanol despite the fact the like dissolves like. Unlike the dibenzalacetone quickly

dissolves in water and ethanol only under heating. However, we added too much hot solvents of water and ethanol combination which did not help dibenzalacetone reach saturation point. As the solubility decreases supposingly with cooling the solution, the product failed to crystallize, and we ended-up having too much solvents flowing above dibenzalacetone. To get rid of the solvents and impurities we did coffee filtration and another Buchner filtration followed by melting point (through determining the melting point before and after) which helped improving purity and form better crystals slowly; the melting point after recrystallization jumped from broad range of 90-95°C to small range of 109-111°C in between the real melting point of dibenzalacetone 110 °C. this means that our desired product is pure. In future labs, to avoid such errors we recommend adding just 95% ethanol in small measured amounts to achieve better recrystallization then final washing crystals with cooled ethanol to decrease dibenzalacetone solubility at the same time. The reaction had been initiated once benzaldehyde/ethanol was added to the NaOH solution followed by the acetone. The slow coloration of the mixture followed by the slow precipitation of dibenzalacetone indicated that reaction takes time to happen which does not support the theory that dibenzalacetone is readily prepared. However, other references, such as our textbook, suggest initiate the reaction by mixing benzaldehyde with acetone followed by dissolving in sodium hydroxide and ethanol. There is no big difference between our technique and the textbook technique since the reaction takes place in the presence of all the reagents. We chose this method to ensure that enolate quickly attacks the carbonyl on the benzaldehyde as soon as enolate forms by the fast addition of acetone. In future educational labs we recommend using the textbook method for many reasons. First it carries lower risk of Cannizzaro effect. Second, it can manage the temperature associated with exothermic reaction. On the other hand, we had great yield of 85% with applying our method greater compared to the percent yield according to textbook 68 % . Therefore, we recommend using our method for synthetic purposes because it ensures and maximize product yield by increasing the formation of enolate ion. During precipitation the reaction mixture turned into orange color as it reaches saturation point due to the increase in dibenzalacetone concentration followed by the crystallization of yellow dibenzalacetone which supports our theory that solubility decrease after solution becomes saturated. Overall the calculated percent yield and the determination of melting point indicted that reaction was completed. 19. Conclusion: The lab was successful because reaction was completed as the theory says and the final product of was synthesized in its physical and chemical properties with great yield and maximum purity. We conclude that the aldol condensation of benzaldehyde with acetone is dibenzalacetone.

References: Macroscale and Microscale Organic Experiments 7 th edition, by Kenneth L. Williamson & Katherine M. Masters

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