wittig - Na Hye Eunice Kim Chem 3BL (Jessicas) October 28,...

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Na Hye Eunice Kim Chem 3BL (Jessica’s) October 28, 2009 The Wittig Reaction In this week’s experiment, we reacted a stable phosphorane, methyl(triphenylphosphoranylidene) acetate, with 2-nitrobenaldehyde, and observed the wittig reaction. Then we used column chromatography to separate the possible products and based on TLC results and NMR spectra, we identified the major product. I determined the major alkene isomer formed in this reaction by drawing out the mechanism and looking at the C and H NMRs of the product. The mechanism allowed me to figure out the basic structure of the product, while the NMR gave me the stereochemistry of the product. After looking at 1H NMR and 13C[1H]NMR of alkene product, I figured out that the product has to be a trans isomer. In order to do this I had to look at the doublets that were present. The hydrogen I looked at was the one next to the carbonyl group, Ha and Hb. I looked up each peak’s Hz according to its ppm value and found the difference in Hz of each doublet. For the double with greater intensity, the difference in Hz was 15.81Hz and for the doublet with smaller intensity, the difference in Hz was 11.936Hz. It was given to use that for trans isomers the couplet constant is between 12 and 18, and for cis isomers it is between 6 and 12. Therefore, all this showed that our major product is trans isomer. A rational arrow-pushing mechanism for the formation of this product is shown below: Simply heating the two reactants together also results in product. The advantage of performing the reaction on silica gel compared with just heating is that it makes it easier to load column. Silica gel helps to purify the product, allowing different fractions of product,
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wittig - Na Hye Eunice Kim Chem 3BL (Jessicas) October 28,...

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