The Wittig Reaction

Written by Michael

Purpose:  The purpose of this experiment was to learn about the use of carbanions and stablilized carbanions in synthesis through performing a synthesis that constructs a carbon-carbon bond.  Another purpose of this experiment lies in the purification of an organic base using an acidic aqueous extraction.

Experimental: All experimental procedure, observations, and data can be found in Appendix D.

Results and Discussion:

In the first step, we prepare triphenyl(4-pyridinylmethyl)phosphonium chloride.riphenyl(4-pyridinylmethyl)phosphonium chloride.  After the reaction ran to completion, the product was cystallized and recovered by vaccuum filtration.  A melting point determination provided evidence that the desired product was formed.  The experimental melting range was greater than 215ºC which agrees with the literature value for the melting point of triphenyl(4-pyridinylmethyl)phosphonium chloride, not only does this help us to identify our product, but to estimate the purity of our product as impurities would cause this number to deviate from the literature value.  The percent yield of this step was 14.3%.  This is a significant amount of product, though it appears that there is room for omptimization in our experimental procedure.  The limiting reagent in this reaction is triphenylphosphine.  The product formed appeared a pale brown-orange color; this is an expected property of the desired product.

 

The second step of the Wittig Reaction involves the transformation of  triphenyl(4-pyridinylmethyl)phosphonium chloride into trans-4,4’-bpe (cis-4,4’-bpe is also formed but does not precipitate out of the reaction mixture).  During the course of the reaction, the reaction mixture turned a purple-black color.  As the acidic product mixture was neutralized with base, the color changed to a fleshy pink that had a bubbly texture.  After seperation, our product was recovered by vaccuum filtration then purified by acidic aqeous extraction.  We then isolated our solid by means of a rotory evaporator.  A melting point determination provided evidence that the desired product was formed.  The experimental melting range was 143ºC-144ºC.  This range is relatively close to the literature value of 151ºC. This melting range also speaks on the relative purity of the compound, suggesting that we have a relatively pure product. The percent yield of this step was 8.80%. This suggests that a significant amount of product was formed, though there are inefficiences that can be improved upon yet.  The crude yield of this step was 32.8%.  This suggests that the purification was effective and that the purified product is relatively free of by-products.  In this experiment, we performed an extraction by treating the crude product with 10% HCl.  This was done to protonate the organic product, causing a salt to form which would then dissolve in the aqeuous layer.  From there, our product could be precipitated out of solution and collected.  NMR spectra of both the crude product and the purified product were provided.  The data is summarized in the following tables:

1H NMR of Crude Product
Proton Integration (number of protons) Position (Chemical Shift) (ppm) Multiplicity
Ha 2 7.85 1 (singlet)
Hb 4 7.96 2 (doublet)
Hc 4 8.69 2 (doublet)

 

 

1H NMR of Purified Product
Proton Integration (number of protons) Position (Chemical Shift) (ppm) Multiplicity
Ha 2 7.49 1 (singlet)
Hb 4 7.61 2 (doublet)
Hc 4 8.54 2 (doublet)

*peaks are labelled on the 1H NMR spectrum which can be found in Appendix B

The structure of trans-4,4’-bpe with each proton labeled is drawn here:

 

 

 

 

It is important to note that the 1H NMR spectra show an increase in purity from the crude spectrum to the pure spectrum.  One can see that the small peaks (related to the cis-4,4’-bpe) in the crude spectrum dissappear in the pure spectrum, indicating that we have succesfully isolated the trans-4,4’-bpe with high purity.

The overall yield for this experiment was 1.26%.  This seemingly low yield may be explained by procedural factors that may be toyed with in the future in order to produce a better result.

 

Conclusion:  Despite a relatively low yield, the desired product, trans-4,4’-bpe was succesfully isolated with high purity as is evidenced by the melting point determination, a comparison of crude and pure 1H NMR spectra, as well as a comparison of the crude yield to yield of the purified product. The acid extraction was effective as only the trans-4,4’-bpe was precipitated out of solution; this also satisfies the purpose of our experiment.