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Substituents On Aromatic Aldehydes And How They Affect E/Z Selectivity In The Wittig Reaction

4956 words - 20 pages

Abstract:The Wittig Reaction is a nucleophilic addition in which an alkene is formed as a product. Both the E and Z isomers of the alkene result. Substituents on the aromatic aldehyde affect the E/Z ratio of products that form. In this experiment, a nitro group was used as the substituent in the ortho, meta and para positions, with benzaldehyde as the control. Each of the four aldehydes reacted with (carbethoxymethylene) triphenylphosphorane to produce ethyl cinnamate, ethyl-3-(2-nitrophenyl)acrylate, ethyl-3-(3-nitrophenyl)acrylate, and ethyl-3-(4-nitrophenyl)acrylate. The prediction was the closer the substituent was to the aldehyde, the greater the ratio of E/Z isomers; benzaldehyde was predicted to result in the most similar ratio of E/Z isomers. The ratios of E/Z isomers were determined by using the NMR spectra obtained. A percent conversion was also obtained. Ethyl-3-(2-nitrophenyl)acrylate had a ratio of 9.03:1, ethyl-3-(3-nitrophenyl)acrylate had a ratio of 5.2:1, ethyl-3-(4-nitrophenyl)acrylate had a ratio of 1:1 and ethyl cinnamate had a ratio of 9.3:1. The results for the compounds except ethyl cinnamate supported the hypothesis that the ratio would be closer to 1:1 as the substituent moved further from the carbonyl.Introduction:The purpose of this experiment is to determine how a nitro group on benzaldehyde affects the stereoisomer formation of the alkene formed in a Wittig Reaction. The nitro group is substituted in the ortho, meta, and para positions, and benzaldehyde serves as a control. The ylide used is (carbethoxymethylene) triphenylphosphorane. Through the utilization of NMR spectroscopy, the products of the reactions will be analyzed to gain a ratio of E /Z isomers.Reaction Equations:Ethyl cinnamate from BenzaldehydeEthyl-3-(2-nirophenyl)acrylate from O-nitrobenzaldehydeEthyl-3-(3-nitrophenyl)acrylate from M-nitrobenzaldehydeEthyl-3-(4-nitrophenyl)acrylate from P-nitrobenzaldehydeIn 1954, Georg Wittig, a German chemist, discovered the Wittig Reaction. In 1979, Wittig won a Nobel Prize in Chemistry due to the Wittig Reaction's synthetic potential to form a variety of different alkenes. Today, this reaction is used to synthetically alkylate many different compounds that may otherwise be impossible to complete due to steric hindrance.The Wittig Reaction is a specific type of nucleophilic addition to carbonyls to form an alkene. This reaction is very important and powerful because a C-C bond is formed and numerous combinations of starting materials produce an extremely diverse number of products. The starting materials for a Wittig Reaction are a compound with a carbonyl group and a phosphorus ylide. The carbon of the carbonyl is a strong electrophile because the oxygen pulls electron density away from the carbon, due to the inductive effect. A phosphorus ylide is a neutral compound that has a separation of formal charges. The atom in the ylide that contains the negative charge is the nucleophile that attacks the electrophilic...

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