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Chem 143d Experiment 1: Wittig Reaction

Post lab data analysis

Part a: analyzing the NMR spectrum of ethyl cinnamate
Before starting to analyze the spectrum think about the following aspects of the Wittig
reaction of a stabilized ylides. How many products can be obtained in a Wittig reaction?
Are stabilized ylides special with respect to the product distribution? What is the
byproduct of the reaction? Is it present in the spectrum?
The Wittig reaction yields a mixture of E and Z isomers. Stabilized ylides (carrying
electron withdrawing groups) usually show preference for the formation of the E isomer.
In this spectrum we have major and minor isomers. Annotate each peak that belongs to
the major component with the letter a, and each peak of the minor component with the
letter b. First try to assign the major component. The key point for assigning the 1
H spectrum is identifying the two alkenyl protons. Where would you expect to find these
protons? Does the integration fit? Notice that this is a non-symmetric double bond and
therefore the two protons will have different chemical shifts. Once you located one
proton, measure the coupling constant and use it to locate the second proton.
Which isomer is the major product? How do you assign the configuration based on
coupling constants? Now locate the alkenyl protons of the minor component and assign
the configuration of this isomer. Is it clear which isomer it is based on coupling
constants?
Once you identified both isomers, calculate the ratio between E/Z based on integration.
Analyze the spectrum of E-3-(9-Anthryl)-2-propenoic acid ethyl ester using the same
strategy.

Part b: factors influencing the chemical shifts
You noticed that the chemical shift of the alkenyl protons is different from shifts typically
observed for alkenyl protons. You need to explore the origin of this difference.
1. Summarize in a table the chemical shifts of the alkenyl protons for the three
compounds synthesized in the experiment.
2. Write various resonance structures for Ethyl trans-Cinnamate. Pay attention to
structure that places different charges on the α and β alkenyl protons. Can
resonance explain the difference between the chemical shifts of the α and β
protons?
3. If resonance provides a reasonable explanation, why is there a difference between
the trans and cis isomers?
4. Compare the trans-cinnamate to the anthracene derivative. What caused the
difference in chemical shifts? Use the following NMR data for comparison

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