Chem 3BL 403
November 3, 2011
Lab 21: Wittig Reaction
•Discussion
In this lab, a Wittig Reaction took place between methyl (triphenylphosphorane anylidene) acetate and 2-nitrobenzaldehyde. Two isomers (cis and trans) form in the product, and in this reaction trans is actually the favored isomer, contrary to the normal Wittig Reaction. Trans is favored because the ylide used is conjugated (very stable). Thus, if the product was cis, there would be a lot of steric hindrance between the phenyl and the carbonyl. The ylide also has an ester derivative that allows resonance to occur when the carbon in the ylide becomes nucleophilic. The coupling constant calculated is also 15.8 which would be classified as trans.
Adding silica to the reaction would facilitate the reaction better because since the ylide is attracted to the gel, it disperses around the solution. By dispersing the ylide around the solution, it comes into better contact with the aldehydes, thus maximizing the product yields.
The TLC plates consisted of samples of TPO and 6 factions collected during chromatography in 50:50 hexane: ethyl acetate. In the first plate, the TPO and factions 1-2 were present. There was only one spot in factions 1-2, suggesting that there was only the alkene product in the factions. However, in the 2nd plate, there was no product in faction 3, and there were 2 spots in faction 4 (alkene and TPO). Faction 5 had just the alkene product. In order to separate better, a longer column could be considered in order to allow the TPO to interact more with the silica. The factions could have also been run again through the columns to extract a purer sample.
•Questions
•The phosphorane used in this experiment is stabilized by resonance due to the ester derivative. The phosphorane is able to disperse the negative charge around while the Ph3=CH2 will form a single carbon atom with a negative charge (see attached).
•This procedure is important because by ensuring all of the products are adsorbed onto the silica gel. The silica is what separates the products. If the products did not adsorb onto the silica, then the products would not separate and we would obtain impure compounds (containing both the TPO and alkenes). This procedure is also important to ensure that none of the products dissolve above the solvent line. Otherwise, the products will not go down the column because there is no eluent to carry it down to the bottom of the column.
•Factions 1 and 2 have byproducts that the researcher does not want. Factions 3-5 have the desired product, but they are impure. Finally, factions 6-10 have the desired product. In order to maximize his isolation of the wanted product, the researcher should run factions 3-5 through the column and try separating the products even more. In addition, the researcher could also use a longer column because it would need more silica and more eluent. This would allow for more separation.
•The Horner-Wadsworth Emmons reaction has a phosphorate carbanion (which makes it more nucleophilic than the Phosphate anion used in the Wittig Reaction). It is also useful because the product is an E isomer instead of the Z preferred product in the Wittig Reaction. The E isomer is always preferred in order to avoid steric hindrance.
•By alkylating the phosphine, you obtain a salt. Then, by adding a strong base, you form the double bond and the product desired. (See attached)
•Conclusion
This lab allowed us to study the Wittig Reaction in detail. It also exposed us to another type of chromatography in column chromatography. In addition, chromatography techniques were also used to see our products.