Preparation of Triphenylmethanol by a Grignard Reaction

Lab 2: Preparation of Triphenylmethanol by a Grignard Reaction

Introduction:

The Grignard addition of an organomagnesium compound to a carbonyl to form an alcohol is one of the classic reactions in organic chemistry. First, magnesium will be inserted into a carbon-halide bond to form nucleophilic phenylmagnesium halide Grignard reagent. This reaction is an oxidation of the metal converting Mg0 to Mg2+ carried out in anhydrous ether solvent. This process will be done with care due to the moisture sensitivity of the phenylmagnesium halide. Second, methyl benzoate is added to form a new carbon-carbon sigma bond. The intermediate magnesium alkoxide will then be hydrolyzed to form tertiary alcohol product.

Experimental Procedure:

Part A: Preparing the Grignard reagent

* I placed 1.98 grams of magnesium turnings into a cleaned 250mL round-bottom flask and then attached the calcium chloride drying tube into the round-bottom flask

* Then, I placed this flask onto a heating mantel and adjust the controller to setting '6'. This flask is heated until it was too hot to touch.

* After the flask was heated, I removed it from the heating mantel and allowed it to cool to room temperature.

* Next, I poured 15mL of absolute ether and 9mL of bromobenzene into the 250mL flask and waited 5 minutes for any reaction.

* There were no signs of reaction after 5 minutes; therefore the lab assistant used a glass stirring rod to crush the magnesium in order to get the reaction started. When this was done, the liquid became cloudy, rapidly bubbling, and the color turned rusty red.

* Next, I added 25mL of absolute ether and attached a reflux condenser to the flask. I turned on the water when I saw ether vapor had successfully wet the joint at the top of the condenser.

* Every 60 seconds, I swirled the flask and I continued to perform this action until I saw that the reaction stopped bubbling.

* When I noticed that the liquid level decreased below the marked level, I poured in approximately 5mL of absolute ether.

Part B: Reaction between Grignard reagent and methyl benzoate

* First, I cooled the flask containing the Grignard solution into an ice bath.

* Then, I mixed 5.00mL of methyl benzoate and 15mL of absolute ether into a cleaned separatory funnel.

* I removed the drying tube from the apparatus and inserted a Claisen adapter and the separatory funnel containing methyl benzoate and ether to the top of the condenser.

* With this apparatus set up, I ran the methyl benzoate solution drop wise into the 250mL round-bottom flask over 10 minutes. Then, I observed product salt began to separate.

* I continuously swirled the flask until the reaction settled down.

Part C: Hydrolysis

* Next, I poured the reaction mixture into a 250mL Erlenmeyer flask with 50mL of 10% sulfuric acid and approximately 27.26g of ice.

* In order to rinse the remaining of the flask, I used some ordinary ether and 10% sulfuric acid. Then, I poured these washings into the Erlenmeyer flask.

* Next, I swirled the flask to aid hydrolysis.

* Lastly, I placed a cork stopper into the Erlenmeyer flask and stored it over the week until next lab period.

Part D:

* First, I poured the mixture from the Erlenmeyer flask into a separatory funnel through a glass funnel with a small plug of cotton in order to filter out the liquid and crystals.

* With the remaining