A Wittig Reaction of trans-Cinnamaldehyde
Statement of Question: What is the stereochemistry in the Wittig product, and why is it formed?
Hypothesis: There were two possible stereochemical outcomes for this experiment, either cis, (E,Z), or trans, (E,E). The cis product, (E,Z), would form if the double bond and phenyl group added to the same side. The trans product, E,E), would form if the double bond and phenyl group were to add to the opposite sides. The product of desire for the Wittig reaction is the trans, (E,E), isomer. Since the trans isomer of cinnamaldehyde was used in the experiment, the addition of one of the two double bonds formed in the major product was expected to be trans (E). The purpose …show more content…
The Wittig reaction consists of a nucleophilic attack from a negatively charged carbon of the phosphorus ylide on to the carbonyl group carbon. Elimination then occurs to form the carbon to carbon double bond with the assistance of the Ph₃P=O reagent. After elimination is completed, the Ph₃P group and the oxygen of the carbonyl group are no longer present.
In this experiment, the phosphonium salt used was benzyltriphenylphosphonium chloride, which was provided by the instructor. During synthesis of the phosphonium salt, the ylide was formed in the presence of the phenyl group. The phenyl group provides stabilization. The cinnamaldehyde then reacts with the ylide to form a product of 1,4-diphenyl-1,3-butadiene. The ylide-forming reaction occurs in a set of two phases. Dichloromethane was used as the organic layer of the reaction, which was later evaporated. The benzyltriphenylphosphonium chloride helps to transfer the ylide from the aqueous layer to organic layer. After this occurrence, the ylide then reacted with the cinnamaldehyde. A majority of the major product stayed in the organic layer, but some was recovered from the aqueous layer. After trituration and recrystallization, the product formed was weighed (~0.025g) and a melting point and IR spectrum was recorded for further …show more content…
This value is extremely close to that of the literature value for the trans product, (E,E)-1,4-diphenyl-1,3-butadiene, which is 153°C compared to that of the cis product, (E,Z)-1,4-diphenyl-1,36-butadiene which is 88°C. With just that small amount of data, I was rather sure that I knew what my product was before doing any further analysis. An infrared spectrum was recorded as well and compared to that of the (E,E)-1,4-diphenyl-1,3-butadiene since the melting points were so close. Although I was unable to find an infrared spectrum for that of the cis, (E,Z)-1,4-diphenyl-1,3-butadiene, I was certain of my product after observing the IR of the trans product. There were multiple identical peaks from both my own recorded infrared spectrum and the researched infrared spectrum from the SDBS website (see attached). There were peaks shown to represent aromatic carbon to hydrogen bonds at 618cmˉˡ on both IR spectrums. Peaks were also shown at 3015cmˉˡ on my products recorded IR spectrum and 3016cmˉˡ on the SDBS IR spectrum that represented the carbon to hydrogen bond stretching. There were many other identical peaks which can be observed on the attached infrared spectrums that provide assurance of my formed product as