Chem. 262 Notes 5

Chem. 262 Notes 5 - the hydrogen attached to the carboxyl...

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Experiment V: The Wittig reaction is one of the most useful reactions for olefination (preparation of alkenes from carbonyls). The Wittig reaction involves the reaction of an alkyl phosphonium ylide with an aldehyde or ketone. If the alkyl group being transferred contains an electron-withdrawing group, however, the resulting ylide is too stable to react with carbonyls. In these cases, the Horner-Wadsworth-Emmons modification of the Wittig reaction is followed: an ylide reagent is derived from a phosphonate. Hydrolysis of the ester produced by the phosphonate ylide produces the carboxylic acid. 1. Show the mechanism of the Wittig reaction. 2. A proton NMR spectrum of a sample obtained from a student who just finished the Wittig experiment is given to you. How will you know if the student’s sample was: (a) the product? A key peak that would be on the solid carboxylic acid’s NMR spectrum would be the peak for
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Unformatted text preview: the hydrogen attached to the carboxyl group. The peak would be located downfield with a chemical shift around 11-12 ppm. (b) the ester precursor? A characteristic NMR peak for the ester precursor of this reaction would be the three hydrogens located on the methoxy group. This would produce a tall, singlet peak around 3-4 ppm. (c) a sample containing a mixture of the phosphonate and carbonyl starting material? The phosponate would have a very tall hydrogen peak for the two collections of ester hydrogens which would peak around 4 ppm on the NMR spectrum. The carbonyl starting material would have a peak downfield around 10 ppm due to the hydrogen next to the carbonyl group. The combination of the ester and aldehyde hydrogen peaks would illustrate the combination of these two materials....
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