Chem 3BL - Lab Report E#21

Chem 3BL - Lab Report E#21 - Kylie 20 October 2010 Chem 3B...

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Kylie 20 October 2010 Chem 3B Lab Experiment #21: Wittig Reaction Discussion: NMR Spectra To determine the major alkene isomer formed in this reaction, I first drew out the possible isomers of the alkene product that would result in this Wittig reaction. I then looked at the reagents and searched for information on the stereoselectivity of Wittig reactions, and found that stabilized ylides (such as this experiment's ylide) lead to mainly cis alkenes. Although this produces the less stable product, the reaction is under kinetic control since the stabilized ylide leads to a lower activation energy and preferable transition state. The alkyl group was made trans- to the nitro group because this would more likely be favored since it provides less steric hindrance. It's advantageous to combine the reactants with silica gel because the silica gel adds mass to the mixture, making it easier to mix the starting aldehyde and phosphorus ylide for even distribution. The silica gel may also help maintain anhydrous conditions by absorbing any water molecules (atmospheric moisture) present, preventing the reagents from hydrolyzing (phosphorane reacts violently with moisture). The three lanes on my first TLC plate showed different results: the triphenylphosphine oxide lane had the small stain with the lower Rf value (~0.15), lane #2 had the larger stain with the higher Rf value (~0.62), and lane #3 had both components present. Since the second lane (which represented portion #1) lacked the triphenylphosphine oxide stain below that of the other product, it was safe to assume that portion #1 contained the purified and desired alkene
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product. All the three lanes on my second TLC plate showed both stains (almost identical in
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Chem 3BL - Lab Report E#21 - Kylie 20 October 2010 Chem 3B...

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