Aldol condensation reaction PDF

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Aldol condensation reaction experimental...

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Experiment 1 Aldol Condensation reactions

Introduction (a) Objectives 

To synthesize Dibenzalacetone.

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To synthesize Benzalacetophenone.

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To measure the melting point of the sample.

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To understand the mechanism of an Aldol Reaction.

(b) Theory An Aldol condensation reaction is one in which an enol or an enolate ion reacts with a Carbonyl compound to form β-hydroxyaldehyde or a β-hydroxyketone, hence a dehydration gives a conjugate enone. In both parts of this practical experiment Benzaldehyde reacts with a ketone in the presence of a base to produce β-unsaturated ketones. The reaction that the experiments follow is an example of a crossed aldol condensation reaction where the intermediate in the reaction becomes dehydrated to produce the resonance- stabilised unsaturated ketone. The ketone loses a proton from an alpha-carbon to then form an enolate ion, which attacks the carbonyl carbon of the aldehyde to yield, after protonation alpha, beta-hydroxyketone. The intermediate formed for the aromatic aldehyde is unstable and is subjected to base-catalysed dehydration, until the unsaturated product is yielded. An important point to note is that a crossed-aldol condensation leads to a number of different products, unless one of the carbonyl compounds that is involved in the reaction cannot form an enolate ion (meaning compound has no alpha-hydrogens). An aromatic aldehyde, such as Benzaldehyde is sufficient for the reaction as only one enolate ion will form, which is from the other carbonyl compound. Hence formation, the nucleophilic enolate ion attacks the carbonyl carbon to form a β-hydroxycarbonyl product. The β-hydroxycarbonyl product then eliminates a molecule of water to form a conjugated system composed of a double bond and the carbonyl group. The elimination process with regards to part one of the experiment was particularly fast as the alkene is stabilised by conjugation to not only the carbonyl but also benzene. The theoretical melting point of Dibenzalacetone is 110-111 °C. In theory, the melting point of Dibenzalacetone is 55-57°C. Reaction Mechanisms:

See post practical part (i) for reaction mechanisms Experiment procedure Part One – Synthesis of Dibenzalacetone. Benzaldehyde (2.88g; 2.77mL) was added to a conical flask with acetone (0.79g; 1mL). The solution was shaken to ensure a homogeneous solution. Hence, 2M sodium hydroxide (40mL) and ethanol (30mL) was added to a beaker. Half of the solution from the conical flask was added to the beaker and mixed several times. After 15 minutes, the remainder of the solution was added to the beaker. Ethanol was used to ensure that all of the material had been transferred to the reaction vessel. The reaction was timed for 30 minutes and hence the product was collected by suction filtration. The solid that was collected was washed with water (3 x 60mL) and then sucked as dry as possible. The product was then weighed using a scales and the percentage yield before Recrystallisation was calculated. The Dibenzalacetone was then recrystallized from ethanol (2.5mL/g). The product was then dried in air and the percentage yield and melting point of the sample was determined. Part Two- Synthesis of Benzalacetophenone. Benzaldehyde (2.88g; 2.77mL) was added to a conical flask with acetophenone (3.26g; 3.17mL). The solution was mixed to ensure a homogeneous solution. Half of the mixture was added to a beaker containing 2M sodium hydroxide (40mL) and ethanol (30mL). The solution was swirled well and after 15 minutes the remainder of the benzaldehyde/acetone mixture was added. Ethanol was used to transfer all the solution to the reaction vessel. The solution was placed in ice. The reaction was timed for 30 minutes, swirled occasionally. The solid was recrystallized. The product was hence collected by suction filtration. Percentage yield and melting point was determined.

Results Part One: Mr of acetone: 3(12.011)+ 6(1.0079) + 15.9994= 58.0798g. Theoretical yield: 0.79g/ 58.0798g= 0.01360197521 moles. Mr of Dibenzalacetone: 17(12.011)+ 14(1.0079) + 15.9994= 234.297g. Amount of Dibenzalacetone Crystals obtained: 2.69g. Actual yield: 2.69g/234.297g= 0.01148115426 moles.

Percentage yield before Recrystallization: Act/Theoretical: 0.01148115426/0.01360197521 x 100/1= 84.40799284%. Recrystallization: Crystals attained after Recrystallization: 2.69g- 1.47g= 1.22g. Actual yield: 1.47g/234.297g=0.006274088016moles. Theoretical Yield: 0.006274088016/0.01148115426 x 100/1= 54.64684015 is approx equal to a 55% yield. Melting point of Dibenzalacetone: 95 °C. Part Two: Mr of Benzaldehyde: 6(12.011) + 5(1.0079) + 12.011+ 1.0079+ 15.9994=106.1238g. Mr of Acetophenone: 8(12.011) + 8(1.0079) + 15.9994= 120.1506g. 1 x 2.88g/ 106.1238g= 0.02713811605 moles. 120.1506g x 0.02713811605= 3.260660926g X= amount of Acetophenone required= 3.260660926g. Density of Acetophenone= 1.03g/cm^3. Amount required = 3.165690219 approx 3.17mL.

Discussion In part one of the experiment the yield dramatically dropped after recrystallization. It can be thought that this occurred for a number of reasons. One of which could perhaps be that not all of the crystals were washed with ethanol and then transferred into the filtration apparatus. Another could be that there was a number of impurities present before recrystallization and hence some of these impurities were removed after recrystallization. In addition, some of the product may have been lost due to evaporation. Finally, some of the product could also have been lost during the

filtration process. The melting point was lower (95 °C) than the expected (110-111°C). The lower melting point resulted as there was impurities present and impurities tend to lower the melting point of compounds. In Part two of the experiment the limiting reagent was found and its molar value and hence its molar value was multiplied by the molecular mass of Acetophenone in order to calculate the value of x. The X value was the amount of Acetophenone that was required to synthesis Benzalacetophenone. The value of X was divided by the density of the ketone in order to calculate the value in mL. The percentage yield and melting point for Part Two was not calculated, due to the fact the experiment was not successful. The experiment was not successful, due to not following procedures efficiently enough. The mixture of Benzaldehyde and Acetophenone was added entirely to the NaOH and ethanol, instead of in two aliquots (the second being after 15 minutes). Due to not following the correct procedure Crystals did not form in the conical flask. Perhaps, heating would have helped the solution to speed up the formation of Crystals too.

Conclusions Condensation reactions are reactions which join two or more molecules together, usually with the loss of a small molecule such as water or an alcohol. Precaution needs to be taken when following lab procedures, to ensure one obtains coherent results. An enolate ion acts effectively as a nucleophile, which can be used to synthesize Dibenzalacetone. Chalcone (Benzalacetophenone) is an important aromatic ketone and enone that forms the central core in many biological compounds....