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SYNT 743 Formal Lab Report

SYNT 743 Formal Lab Report - SYNT 743 Converting...

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SYNT 743: Converting Benzaldehyde to Benzilic Acid: A Multistep Synthesis Zach Rohr August 4, 2010 Organic Chemistry Lab II Professor Poulewi Abstract: The experiment is a multistep synthesis converting benzaldehyde to benzoin, benzoin to benzil, and benzil to benzilic acid. First we completed the benzoin condensation reaction which converted benzaldehyde to benzoin. Next we carried out the benzoin oxidation experiment in which we took the benzoin from the former experiment and made benzil. Last we took the benzil and finished with benzilic acid which was our final product. In order for perform these experiments, we used vacuum filtration, recrystallization, melting point measurement, IR spectroscopy, TLC, and reflux, the results of this experiment displayed the percent recovery for benzoin, benzil, and benzilic acid, as well as their experimental melting points and IR spectra. Benzoin had a recovery of 80% and an experimental M.P. of 132-134 Celsius, comparable to the literature M.P. of 135-137, with strong IR peaks at 3600-3650 for alcohol or phenol, and 1680 for C=O. Benzil had a recovery of 73% from benzoin and 48% from benzaldehyde. It had an experimental M.P. 94-95 Celsius, comparable to the literature melting point of 94-95 Celsius,
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with strong IR peaks at 3000 for aromatic and 1680 for C=O. The TLC plate confirmed that all of the benzoin oxidized to benzil. The Rf value was 0.3 for benzoin, 0.6 for benzil, and 0.6 for the product, which proves that benzoin completely oxidized to benzil. Benzilic acid had a percent recovery of 18% from benzil and 8% from benzaldehyde. The experimental melting point was 148-150 Celsius which is comparable to the literature melting point of 150-153 Celsius. Benzilic acid has string IR peaks at 3400 for aromatic or alcohol, and 1650 for C=O. Introduction: The main theory of this experiment is that from benzaldehyde, benzilic acid can be synthesized through several steps. The first step is to yield benzoin from benzaldehyde by crystallization, vacuum filtration, recrystallization, and refluxing. This step involves benzoin condensation which the coenzyme, thiamine pyrophosphate (TPP), is responsible for much of the bond breaking and bond making. Thiamine catalyzes the benzoin condensation. The mechanism for the benzoin condensation reaction on benzaldehyde is easily explained. A carbanion forms when a hydroxide ion deprotonates the thiamine thiazole ring. This carbanion undergoes a reversible addition to the benzaldehyde carbonyl, forming an alkylated thiazole derivative. The resonance effects of the thiazole ring increase the acidity for the α-hydrogen atom on the carbon atom adjacent to the benzene ring. Increased acidity facilitates proton transfer from the α-carbon to the oxygen, forming another carbanion, this carbanion then adds to another molecule of benzaldehyde, forming an oxyanion. The oxyanion is in equilibrium with another oxyanion, which in turn, eliminates the catalyst and forms benzoin. In this step, product purity can be
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SYNT 743 Formal Lab Report - SYNT 743 Converting...

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