Experiment 3: Wittig Reaction
This experiment performs a modified Wittig reaction using a phosphorus-containing Hornes-Emmons-Wittig reagent to generate an enolate anion of trimethyl phosphonoacetate instead of a phosphorus ylide. The methyl trans-4-methoxy cinnamate produced is then analyzed using melting point and 1H NMR spectroscopy. Theory
The Wittig reaction prepares alkenes from carbonyl compounds by attacking a phosphorus ylide with a nucleophilic carbon atom stabilized by a triphenylphosphonium group. An ylide is a compound that contains two oppositely charged atoms bonded together with complete octets and is generated through phosphonium salts. This experiment is a modified Wittig reaction that uses a Horner-Emmons-Wittig reagent that is more acidic than the Wittig salts. This allows a weaker base to be used and gives an advantage when isolating and purifying the product because the final phosphorus byproduct is water soluble. The overall reaction can be best explained through the mechanism below:
The stability of the Wittig reagent dictates the stereochemistry of the reaction. Wittig reagents stabilized by a carbonyl group that share the carbanion’s negative form the E isomer as the major product and unstabilized Wittig reagentsform the Z isomer as a product. The E isomer has a melting point of 88°C while the Z isomer exists as an oil at room temperature. Results
The experiment produced 0.033 g of white crystal product, an 11.7 % percent yield. The melting point range of the product was 80.0°C-82.3°C. Small amounts of yellow oil was also produced before recrystallization. Attached are the 1H NMR spectroscopy of both the product and the oil side product from the reaction. Discussion
This reaction proved to be inefficient, yielding only 11.7% of product. Perhaps letting the reaction run longer than an hour would have helped increased the percent yield. The reagents used in this experiment were also exposed to the air for a long period...
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