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Kachi Anyanwu April 17th, 2019 Section 807 2:00PM

Aldol Condensation Reaction: Preparation of trans-p-anisalacetophenone

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Purpose: Acetophenone and 4-methoxybenozldehyde will be crossed through an aldol reaction to produce chalcone. This will then be purified by recrystallization of 95% ethanol. The product will be then analyzed by its melting point & UV spectroscopy

Balanced Chemical Equation:

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Procedure: 0.6 mL of acetophenone was added to a pre-weighed Erlenmeyer flask and the flask was then reweighed to determine the exact weight of the acetophenone. 0.65 mL of 4methoxybenzaldehyde, 4.0 mL of 95% ethanol, and 0.5 mL of 15.0 M NaOH were added to the flask. This flask was stirred with a glass rod until a solid began to precipitate. After the solid formed, 10 mL of ice water was added to the flask and the solid was isolated using vacuum filtration using cold water to rinse the solid. The solid collected was recrystallized using 95% ethanol and the solid was collected by using vacuum filtration again. The mass of the solid was taken and percent yield was calculated. To analyze the product, the melting range, IR spectra, and UV-visible spectra were taken. The UV-visible spectra of acetophenone was taken to compare.

In-lab Data and Observations: 

Acetophenone o 0.6 mL o Clear liquid

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4-methoxybenzaldehyde o 0.65 mL o Clear liquid

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95% ethanol

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o 4.0 mL o Clear liquid 

NaOH o 0.5 mL o Clear liquid

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Stirring turned mixture into a yellow/orange liquid

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Stirring caused yellow solid to form

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Addition of water resulted in yellow chunky water

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Vacuum filtration resulted in light yellow crystals

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Recrystallization resulted in dark yellow liquid

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Second vacuum filtration resulted in light yellow, long, thin crystals o 0.6680 g o 66.8 – 67.7°C

Calculations: 

Theoretical Yield = (0.6 mL acetophenone) x (1.033 g acetophenone / 1 mL) x (1 mol / 120.15 g acetophenone) x (1 mol trans-p-anisalacetophenone / 1 mol acetophenone) x (238.28 g / 1 mol trans-p-anisalacetophenone) = 1.23 g trans-p-anisalacetophenone

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Percent Yield = (0.6680 g / 1.23 g trans-p-anisalacetophenone) x 100 = 54.27% yield

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Discussion: The mechanism for the preparation of trans-p-anisalacetophenone is shown below. To begin the mechanism, the hydroxide attacks the α-hydrogen of the acetophenone, creating a double bond which results in an alkoxide making an enolate. The enolate then attacks the 4methoxybenzaldehyde, causing the C-O double bond to become a single bond and the alkoxide attacks a hydrogen atom of a water molecule. A hydroxide attacks the α-hydrogen of the resulting molecule while results in a lone pair which makes the carbon negatively charged. The lone pair turns into a double bond, breaking the C-O double bond making the oxygen negatively charged. This negatively charged oxygen recreates the double bond, moving the C-C double bond which forces the alcohol group to leave the molecule. Once the alcohol group leaves, the trans-p-anisalacetophenone is left over, with water as a byproduct. The trans product is formed because the molecule rotates to the most stable form which is the trans product due to steric hindrance. The percent yield for this reaction was 63.5%. Possible sources of loss could be the loss of products during transfers, not allowing the precipitate to fully form before the vacuum filtration, or not allowing the product to fully form after the recrystallization. The λmax increased from 280 nm to 340 nm when comparing the starting material and the final product. This makes sense because the more conjugated a molecule is, the higher the λmax should be. The acetophenone is much less conjugated than the aldol product which has 8 pi bonds, whereas the acetophenone molecule has only 4 pi bonds. The IR spectrum of the acetophenone showed the C-H bonds at 3062.11 cm-1 and the ketone bond at 1681.93 cm-1. The 4-methoxybenzaldehyde IR spectrum showed the aldehyde functional group at 1681.36 cm-1.

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The IR spectrum of the product, trans-p-anisalacetophenone, correctly showed the C-C double bond at 1656.24 cm-1 and the bonds of the benzene rings around 3000 cm-1. The melting range of the product was found to be 66.8 – 67.7°C which is lower than the literature value of the melting range of trans-p-anisalacetophenone, which is known to be about 76°C. The low melting range could be due to the fact that the sample used was not completely dry, or that the sample was not fully pure. Overall the data from the UV-visible spectra, IR spectra, and melting range support that the product formed was relatively pure, but not completely.

Mechanism

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Acetophenone IR Spectrum

4-methoxybenzaldehyde IR Spectrum

trans-p-anisalacetophenone IR Spectrum

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UV-visible Spectrum Starting material: 280 nm Product: 340 nm

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Post-lab Questions: 1) The UV-visible spectrum will show a higher λmax value when the pH is at 13 rather than a pH of 2 because the basicity will deprotonate the alcohol group, resulting in a lone pair that makes the molecule more conjugated. The more conjugated a molecule is, the higher the λmax value.

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