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Wittig Reactions

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Amy Thai

on 7 May 2015

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Transcript of Wittig Reactions

Wittig Reactions
Explanation:What is a Wittig Reaction?
A Wittig reaction is when an aldehyde or ketone reacts with a triphenylphosphine ylide to create an alkene and triphenylphosphine oxide byproduct.
-Hypothesis is rejected because the E to Z ratios were inaccurate for 4-nitrobenaldehyde and 4-chlorobenzaldehyde

-Products of Wittig Reactions are not very affected by the reactant but rather the ylide
Theory and Principles
The presence of an electron withdrawing group on the aldehyde / ketone reagent leads to electrons being pulled away from the carbonyl carbon. This leads to a faster reaction and the formation of the Z alkene. The opposite holds true when there is an electron donating group.

- All of the reactants contained more E alkenes to Z alkenes
Instrumental Error
Impurities in the HNMR
Conversion percentage : Amount of Aldehydes fully converted into product.
= E + Z Integration / ( integration of aldehyde + E + Z Integration)

E to Z Ratio:
Amount of E alkenes in contrast to Z alkenes
= E or Z integration/ E + Z Integration
4- nitrobenzhaldehyde
E to Z ratio = 92 to 8
Conversion = 93 %
E to Z Ratio = 90 to 10
Conversion = 100 %
P -tolualdehyde
E to Z ratio = 83 to 17
Conversion = 97.5 %
- Hypothesis is rejected because the data does not support hypothesis
Why the hypothesis is rejected ?

Reasoning Behind Data
-The contributing factor that affects whether the product will be E isomers or Z isomers is the ylide.
- When an ylide is stabilized by other electron withdrawing groups then it will result in more E alkenes and a slower reaction
-An ylide is unstable when it does not have groups withdrawing electrons from the carbon, which results in a majority of Z alkenes and a faster reaction.
An ylide is a molecule that contains both an atom with a positive formal charge adjacent to another atom with a negative formal charge.

The objective of the experiment was to synthesize an alkene product from a Wittig reagent and an aldehyde to determine whether electronic characteristics present on the para position of the aldehyde affects the formation of E or Z product. From the synthesized product, a NMR spectra will be taken and be used to identify whether the Z, E, or both products were formed. The ratio of Z to E product will then be calculated.

There should be more Z alkene formed from the aldehyde containing the strongest electron withdrawing group, meaning 4-nitrobenzaldehyde should produce the most Z alkene.
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