chem262lexp5 - presence of water. It was determined that...

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AtLee Watson Chem 262L Section 415 TA: Jeff Mighion Experiment 5: Wittig Reaction: Preparation of a para-methoxycinnamic acid In this experiment, a phosphonate ylide solution was prepared and treated with p- anisaldehyde to make α,β-unsaturated ester. The α,β- unsaturated ester was then treated with hydrolyzed to form the carboxylic acid that was crystallized from solution. The crystallized sample appeared yellow, and the experimental yield of the para-methoxycinnamic acid was 0.100g and the melting point was determined to be 116.4 0 C – 120.6 0 C. The actual melting point of pure para-methoxycinnamic acid is 170 0 C – 173 0 C. Such a large discrepancy between the two melting points was caused by impurities in the crystallized product. Possible sources for the impurities were the contamination of the sample during the evaporation of the ethanol using the Thermowell, and not allowing the sample to filtered and dried properly to crystallize without the
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Unformatted text preview: presence of water. It was determined that there was water in the crystallized product because the vapor pressure of the product caused the sample to rise in the tube in the Mel-Temp. 1. 2. The sodium methoxide is stored under nitrogen in the buret because it reacts with H 2 0 molecules in the air. 3. The NMR provides evidence that our product is indeed the carboxylic acid because the chemical shift of the hydrogen on the –COOH group would have occurred in between the range of 10-12 ppm. If it had been the aldehyde starting material the hydrogen chemical shift would have occurred in the region between 9.5-10 ppm. Likewise, the hydrogen in the ester intermediate would have shifted around 3.5-4.5 ppm. Therefore, as long as we were to see a shift past 10 ppm then it can be concluded that the product was the carboxylic acid....
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This note was uploaded on 09/16/2008 for the course CHEM 262L taught by Professor Mbadugua during the Spring '08 term at UNC.

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