Question

WITTIG REACTION USING A STABILIZED YLIDE: THE SYNTHESIS OF (E)-ETHYL CINNAMATE Background The Wittig reaction, named after Georg Wittig, is employed to convert the carbonyl group of aldehydes and ketones into an alkene group. This reaction is of great importance in organic chemistry and led to a Nobel Prize for Wittig in 1979 that was shared with H.C. Brown who worked on organoborane chemistry. In a typical Wittig sequence (Scheme 1), triphenylphosphine (1) reacts with an alkyl halide (2) to form a phosphonium halide (3). The subsequent addition of a strong base eliminates the hydrogen halide to produce an alkylidenephosphorane, otherwise known as an ylide (4) due to the opposing charges on adjacent atoms. A. Scheme 1 The carbon of the ylide acts as a nucleophile, see Scheme 2, and adds to the carbonyl group (5) resulting in the production of triphenylphosphineoxide (6) and an alkene (7). The reaction conditions can be manipulated to favor the production of either the E or the Z isomer by varying a number of parameters such as temperature, solvent, stabilizing salts, excess base, etc. B. Scheme 2 Ylides such as (4) in which R' = electron withdrawing groups are particularly stable and are commercially available. In this experiment, you will use one such stabilized ylide, (carbethoxymethylene)triphenylphosphorane (4, R^1 = CO2Et), to prepare (E)-ethyl cinnamate from benzaldehyde. You will also need to carry out molecular modeling using Spartan. Procedure In a clean and dry 50-mL round-bottomed flask equipped with a magnetic stir bar, weigh 0.05g benzaldehyde. Add (carbethoxymethylene)triphenylphosphorane (0.197g, 1.2 equivalents) to the falsk and a magnetic stirrer. Stir the mixture for 15 minutes. Next, add 3 mL of hexanes to the reaction flask and stirr for another 10 min. Let the suspension settle for 5 minutes and filter it through a pasteur pipet equipped with a piece of cotton in the bottom end. Evaporate the hexanes in the erlenmeyer using air and a heating plate. Cap the erlenmeyer with a rubber septum and place the resulting oil under vacum for 5 minutes to achieve complete dryness. Register a 1H-RMN and a 13C-NMR spectra of the product. Hazards and Disposal Hexanes are flammable and both the ylid and benzaldehyde are potential irritants. The residue from the filtration step must be disposed in the specially marked solid waste container kept under the hood.

WITTIG REACTION USING A STABILIZED YLIDE: THE SYNTHESIS OF (E)-ETHYL CINNAMATE

Background
The Wittig reaction, named after Georg Wittig, is employed to convert the carbonyl group of aldehydes and ketones into an alkene group. This reaction is of great importance in organic chemistry and led to a Nobel Prize for Wittig in 1979 that was shared with H.C. Brown who worked on organoborane chemistry. In a typical Wittig sequence (Scheme 1), triphenylphosphine (1) reacts with an alkyl halide (2) to form a phosphonium halide (3). The subsequent addition of a strong base eliminates the hydrogen halide to produce an alkylidenephosphorane, otherwise known as an ylide (4) due to the opposing charges on adjacent atoms.

A. Scheme 1

The carbon of the ylide acts as a nucleophile, see Scheme 2, and adds to the carbonyl group (5) resulting in the production of triphenylphosphineoxide (6) and an alkene (7). The reaction conditions can be manipulated to favor the production of either the E or the Z isomer by varying a number of parameters such as temperature, solvent, stabilizing salts, excess base, etc.

B. Scheme 2

Ylides such as (4) in which R' = electron withdrawing groups are particularly stable and are commercially available. In this experiment, you will use one such stabilized ylide, (carbethoxymethylene)triphenylphosphorane (4, R^1 = CO2Et), to prepare (E)-ethyl cinnamate from benzaldehyde. You will also need to carry out molecular modeling using Spartan.

Procedure
In a clean and dry 50-mL round-bottomed flask equipped with a magnetic stir bar, weigh 0.05g benzaldehyde. Add (carbethoxymethylene)triphenylphosphorane (0.197g, 1.2 equivalents) to the falsk and a magnetic stirrer. Stir the mixture for 15 minutes. Next, add 3 mL of hexanes to the reaction flask and stirr for another 10 min. Let the suspension settle for 5 minutes and filter it through a pasteur pipet equipped with a piece of cotton in the bottom end. Evaporate the hexanes in the erlenmeyer using air and a heating plate. Cap the erlenmeyer with a rubber septum and place the resulting oil under vacum for 5 minutes to achieve complete dryness. Register a 1H-RMN and a 13C-NMR spectra of the product.

Hazards and Disposal
Hexanes are flammable and both the ylid and benzaldehyde are potential irritants. The residue from the filtration step must be disposed in the specially marked solid waste container kept under the hood.

WITTIG REACTION USING A STABILIZED YLIDE: THE SYNTHESIS OF (E)-ETHYL CINNAMATE

Background
The Wittig reaction, named after Georg Wittig, is employed to convert the carbonyl group of aldehydes and ketones into an alkene group. This reaction is of great importance in organic chemistry and led to a Nobel Prize for Wittig in 1979 that was shared with H.C. Brown who worked on organoborane chemistry. In a typical Wittig sequence (Scheme 1), triphenylphosphine (1) reacts with an alkyl halide (2) to form a phosphonium halide (3). The subsequent addition of a strong base eliminates the hydrogen halide to produce an alkylidenephosphorane, otherwise known as an ylide (4) due to the opposing charges on adjacent atoms.

A. Scheme 1

The carbon of the ylide acts as a nucleophile, see Scheme 2, and adds to the carbonyl group (5) resulting in the production of triphenylphosphineoxide (6) and an alkene (7). The reaction conditions can be manipulated to favor the production of either the E or the Z isomer by varying a number of parameters such as temperature, solvent, stabilizing salts, excess base, etc.

B. Scheme 2

Ylides such as (4) in which R' = electron withdrawing groups are particularly stable and are commercially available. In this experiment, you will use one such stabilized ylide, (carbethoxymethylene)triphenylphosphorane (4, R^1 = CO2Et), to prepare (E)-ethyl cinnamate from benzaldehyde. You will also need to carry out molecular modeling using Spartan.

Procedure
In a clean and dry 50-mL round-bottomed flask equipped with a magnetic stir bar, weigh 0.05g benzaldehyde. Add (carbethoxymethylene)triphenylphosphorane (0.197g, 1.2 equivalents) to the falsk and a magnetic stirrer. Stir the mixture for 15 minutes. Next, add 3 mL of hexanes to the reaction flask and stirr for another 10 min. Let the suspension settle for 5 minutes and filter it through a pasteur pipet equipped with a piece of cotton in the bottom end. Evaporate the hexanes in the erlenmeyer using air and a heating plate. Cap the erlenmeyer with a rubber septum and place the resulting oil under vacum for 5 minutes to achieve complete dryness. Register a 1H-RMN and a 13C-NMR spectra of the product.

Hazards and Disposal
Hexanes are flammable and both the ylid and benzaldehyde are potential irritants. The residue from the filtration step must be disposed in the specially marked solid waste container kept under the hood.

Added by Guy N.

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