Chem 344 lab 11 crossed aldol PDF

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Summary

This is a completed lab report for the second organic chemistry lab. The labs do not change; they remain the same year to year. ...

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What was in That Crossed Aldol? Chem 344, section 10 April 5, 2019

Objective:

The objective of this experiment was to perform a crossed aldol reaction with 2 moles of an unknown aldehyde and 1 mole of an unknown ketone. Sodium hydroxide was utilized as the base and assisted with the abstraction of the α-proton in the ketone. Characterization techniques, such as 1H NMR and melting point, were obtained to identify the structure of the product and to identify the unknown ketone and aldehyde. Overall Reaction:

Mechanism:

Stoichiometry Table:

Compound

Molar Mass (g/mol) 136.15

panisaldehyde Acetone 58.08 NaOH 39.99 Limiting Reagent: Acetone

Density (g/mL) 1.12

Amount Used 365µL

Mmol 3.00

Molar Equivalent 1.0

0.79 1.01

110µL 2.0mL

1.50 ---

1.0 Solvent

Theoretical Yield: 0.441g Procedure & Observations: Procedure 1. An unknown aldehyde and ketone were obtained. Approximately 365µL of the unknown aldehyde was obtained in a small beaker using a calibrated pipette. And another 2mL of ethanol NaOH was added to the solution with a stir bar. This was stirred at a steady pace for a couple of minutes. The aldehyde was then added to the reaction flask and the solution mixture continued to stir. Then, 110µL of the unknown ketone was obtained in a different beaker using another calibrated pipette. 2. The ketone was added dropwise to the reaction flask using a pipette. The solution was then left to stir allot time for all the reagents to react. 3. The reaction flask was gently heated under reflux conditions at a temperature of 40ºC for approximately 15-20 minutes. After this, heat was removed and the reaction flask was allowed to cool to room temperature before it was placed in an ice bath for an additional 10 minutes. 4. Vacuum filtration was used to obtain the solid product that formed at the bottom of the reaction flask. The product was dried and weighed, and the melting point was taken. From there, the crude product was recrystallized using an ethanol/water system, where the ethanol served as the warm recrystallization solvent, and cold water was used to develop and collect the purified products. The weight and the melting point was then taken for the final product. The H-

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Observations Had a pleasant smell Smelled like baby lotion Before ketone, the solution had a pink hue

The addition of ketone turned it an orange color Stilled pleasant still, like baby lotion After all ketone was added, the reaction mixture had a yellow color, like pineapple juice Precipitate formed after 10 minutes Precipitate was thick and pasty

Crude mp.: 96.9-97.3ºC Crude weight: 0.5263g Final weight: 0.001355g Final mp: 127.7-128.8ºC Crude product was a bold, golden rod color Pure product was a light soft yellow Looked like the coating of peeps

NMR Spectrum was taken for the recrystallized product. Data/Results: Product

Mass of Crude Product (g)

Crude Product Yield (%)

Dianisylideneaceton e

0.5263

119.34

1

Pure Product Yield (%)

Percent Recovery from Recrystallization (%)

30.73

25.75

Crude Melting Point Range (ºC) 96.9-97.3

Mass of Pure Product (g) 0.1355

Pure Experimental Melting Point Range (ºC) 127.7-128.8

Theoretical Yield (g)

0.441

Actual Melting Point Range (ºC) 127-129

H NMR Characterization: Proton

Ha Hb Hc Hd He

Integration 6.00 4.16 4.05 2.01 2.01

Shift (ppm) 3.80 7.52 6.88 6.92 7.66

Splitting Singlet Triplet Doublet Doublet Doublet

J-values (Hz) ------16 Hz 16 Hz

Calculations: *Mmol of p-anisaldehyde: 365µL x

1 mL 1000 µL

x

1.12 g 1mL

x

1mol 136.15 g

x

1000 mmol 1 mol

*Theoretical Yield: 1.5mmol of acetone x

1 mol 1000 mmol

x

294.35 g 1mol

= 0.441g

= 3.00mmol

*Molar Equivalent of p-anisaldehyde: 3.00mmol/1.50mmol = 2.0/2 = 1.0 *Crude Product Yield: 0.5264g/0.441g x 100 = 119.34% *Pure Product Yield: 0.1355g/0.441g x 100 = 30.73% *Percent from Recrystallization: 0.1355g/0.5263g x 100 = 25.75%

Discussion: A classic crossed aldol reaction was performed with 2 moles of an unknown aldehyde and 1 mole of an unknown ketone. Characterization techniques, such as 1H NMR and melting point, were obtained to identify the structure of the product and to identify the unknown ketone and aldehyde. Upon completion of the experiment, the ketone was identified as acetone and the aldehyde was identified as p-anisaldehyde. Sodium hydroxide was used as the base and helped with the abstraction of the α-proton in acetone to form the enolate structures. The first step of the mechanism was the deprotonation of one of the α-protons in acetone by the OH- base. This generated a resonance stabilized enolate and water as a by-product. The enolate then acted as a nucleophile and attacked the carbonyl carbon of p-anisaldehyde, which was the electrophile. This shifted the double bond up onto the oxygen, creating a lone pair and a negative charge. The negatively charged oxygen deprotonated the water and formed a β-hydroxy carbonyl compound and a by-product of OH- base. The base then attacked the second α-proton creating a new C=C double bond. Simultaneously, the OH group on the β-hydroxy carbonyl compound was able to leave due to the basic conditions of the reaction. This produced an alkene and by-products of OH and water. The second equivalent of p-anisaldehyde was reacted and the reaction then occurred in the same manner as before with the other side of the ketone, forming an α,β-unsaturated ketone. The product that formed, Dianisylideneacetone, was highly conjugated and stable. The stability allowed for spontaneous dehydration to occur because it wanted to remain as stable as possible. The reaction was based catalyzed and only the ketone possessed αprotons. This was important because it decreased the chances of the formation of any undesired aldol products. When the experiment was complete, characterization techniques such as melting point and 1H NMR were performed to identify the structure of the product and to identify the unknown ketone and aldehyde. First, the melting point of the crude product obtained was 96.9-97.3ºC. This was significantly lower than the pure melting point range obtained: 127.7-128.8ºC.

However, this was expected because the crude product contained multiple impurities that interrupted the crystal lattice of the product, resulting in a lower temperature range. Recrystallization was performed to purify the product, and as stated, the pure product melting point range was 127.7-128.8ºC. When comparing this melting point range to the chart, it fell into the melting point range of acetone and p-anisaldehyde, which was 127-129ºC. Thus, it was concluded that the crossed aldol condensation products were acetone, which was the ketone, and p-anisaldehyde, which was the aldehyde. From there, the product was able to be identified as Dianisylideneacetone. A 1H NMR was performed on the pure product to confirm its structure. The NMR obtained revealed only five protons due to the symmetricity of the product. Proton Ha appeared the most upfield and this was because they were apart of a methyl group. Methyl groups are electron donating, and as such, protons Ha were able to keep majority of their electron density. Consequently, they were shielded by this, causing them to appear more upfield. Protons Hb and Hc were the aromatic protons and appeared more downfield. However, proton Hb appeared more downfield at a chemical shift of 7.53ppm. The was due to its close proximity to the oxygen atom that is electronegative and electron withdrawing. As such, it pulled majority of the electron density towards itself causing proton Hb to be more deshielded, causing it to appear more downfield. Protons Hd and He appeared downfield as well with chemical shifts of 6.92 and 7.66. This was due to their proximity to the carbonyl oxygen atom, which is electronegative. The Jvalues for protons Hd and He were calculated at 16Hz. This indicated that the dihedral angle between both of them is approximately 180ºC, confirming that they are in the trans position of each other. Lastly, the crude and pure products were weighed and theoretical yields were calculated and obtained for both of them. The crude product weight was 0.5263g which gave a theoretical yield of 119.34%. This high yield could have been due to the presence of multiple impurities in the sample. After recrystallization, the pure product weighed 0.1355g, which gave a low yield of 30.73% and a 25.75% recovery from recrystallization. This could have been due to reactants not reacting all the way. This reaction occurred in equilibrium and as such the final state could have contained both reactants and products in a state of chemical equilibrium, resulting in a low yield. Furthermore, during the purification steps, product could have been lost when transferring it between different reaction vessels. Also, when performing recrystallization, enough solvent may have not been used to completely rid the product of all impurities, resulting in a low yield....