11_03_07_Wittig Reaction Lab Report

11_03_07_Wittig Reaction Lab Report - Wittig Reaction Lab...

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Wittig Reaction Lab Report (November 4, 2007) Norbert Wang (Section 113) 1. When an alkene is formed the first thing one thinks of is which product, E or Z, will be major.  In the case of the Wittig reaction in this lab experiment, the E isomer is the major product.  Figure A shows the mechanism of the Wittig reaction using the reagents for this lab. Ultimately  the ylide can attack from above or below the aldehyde furnishing either the E or Z isomer. If one  looks at the 4 membered betaine intermediate, there is only one conformation that has the least  steric hindrance (the Ph 3 P anti to the nitrobenzene). This is the one leading to the E isomer.  Attached to this lab is a NMR spectra of the product alkene and a table of all chemical shift  values. Coupling constants of the hydrogens on the internal alkene can tell us whether the  alkene is cis or trans. If the constant is greater than 13 Hz the alkene is trans. For constants  less than 12 Hz the alkene is cis. There are two constants pertaining to the hydrogen on the  internal alkene of the product, 15.811 Hz with 1.000 integral and 11.976 Hz with 0.110 integral.  This shows that there was indeed a mixture of E and Z isomers. However because the integral  of the 15.811 Hz peaks is much greater than the other the E isomer is observed to be the major 
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