Synthesis of 4-vinylbenzoic acid using the Wittig reaction
The Wittig reaction is a reaction between an aldehyde/ketone with a species called a phosphorus ylide (also called a phosphorane) and the result is the formation of a carbon-carbon double bond. Phosphorus ylides are generated by reaction of an alkyl halide with triphenylphosphine and subsequent treatment with a base. In this session you will prepare 4-vinylbenzoic acid according to the sequence shown below. The halide 4-(bromomethyl)benzoic acid will be converted to a triphenylphosphonium bromide salt and subsequent treatment with base affords the phosphorus ylide. Note how the phosphorus ylide can be represented using resonance structures to highlight the nucleophilic carbon atom. Reaction of the ylide with formaldehyde produces 4-vinylbenzoic acid.
Experimental
Preparation of 4-carboxybenzyltriphenylphosphonium bromide
*Carry out the initial mixing for this step in the fume hood.
In a 50 mL round bottom flask, dissolve 4-(bromomethyl)benzoic acid (2.15g, 10 mmol) and triphenylphosphine (2.6 g, 10 mmol) in acetone (30 mL) and reflux the mixture for 45 minutes. Cool the reaction mixture and isolate the precipitated phosphonium bromide salt using vacuum filtration. Wash the solid on the funnel with diethyl ether (2 x 20 mL) and air dry it using the vacuum. Record the yield of the product, which is sufficiently pure to use directly in the next step.
Preparation of 4-vinylbenzoic acid
*Formaldehyde is a suspected carcinogen. Carry out the initial mixing in the fume hood.
In a 100 mL conical flask equipped with a magnetic stirrer bar, place 4-carboxybenzyl-triphenylphosphonium bromide (1.9 g, 4 mmol), aqueous formaldehyde (16 mL, a large excess) and water (8 mL). Clamp the flask, stir vigorously and add a solution of sodium hydroxide (1.25 g) pre-dissolved in water (8 mL) over 10 minutes. Stir the mixture for an additional 45 minutes and isolate the precipitate using vacuum filtration, washing it with water. Acidify the filtrate and washings with concentrated hydrochloric acid and isolate the resultant precipitate of crude product using vacuum filtration. Recrystallise the product from aqueous ethanol (50:50 water:ethanol). Record the yield and melting point of the material obtained.
Prepare a sample of the 4-vinylbenzoic acid for analysis by GCMS. Weigh out ~20 mg of the recrystallised product into a GCMS vial and completely fill the vial with dichloromethane.