CHM230

CARBONATION OF A GRIGNARD - PREPARATION OF BENZOIC ACID

INTRODUCTION The Grignard is one of the most versatile reactions in organic chemistry. It can be used to produce 2-methyl-2-hexanol from bromobutane and acetone. Carboxylic acids may also be prepared by a Grignard reagent. The “Grignard” is an organometallic compound that contains one the most powerful nucleophiles. This carbanion is capable of reacting with something that is only slightly basic like carbon dioxide. This reaction will convert bromobenzene to benzoic acid. The bromobenzene is converted to the Grignard and then added to solid carbon dioxide. The carbon dioxide acts as a reactant and as the means to keep the reaction cold. The Grignard is prepared by the process of reflux with addition. In this reaction the glassware and reactants must be kept absolutely dry, as the presence of water will inhibit the reaction.

Reactions:

Part One PROCEDURE (Day 1)

A. PREPARATION OF THE GRIGNARD REAGENT 1. Add 0.1 mole of Mg metal (2.43 grams ) to a 100 ml round bottom flask and add 30 ml of anhydrous ether to cover the metal. Clamp the flask to the grid at a height that will allow enough room for heating and cooling. 2. Add 0.1 mole of bromobenzene to a separatory funnel. Attach a Claisen adapter to the round bottom flask and fit the adapter just above with a water cooled column (vertical, not set up like a distillation). Attach the separatory funnel to the other connection. 3. Prepare an ice/water bath and position it so that the reaction mixture may be cooled by raising the bath to the round bottom with a lab jack. 4. Initiate the reaction by adding one third the volume of the bromobenzene to the flask containing the ether and magnesium metal. The evidence for the reaction will be a cloudy appearance to the reaction mixture with the eventual evolution of sufficient heat to cause it to boil. If the reaction does not start, add a crystal of Iodine and heat. When the reaction has started add the bromobenzene at a rate to keep the reaction going. Make sure to use all of the bromobenzene.

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B. REACTION OF THE GRIGNARD REAGENT WITH CARBON DIOXIDE 1. Crush at least 0.1 moles of dry ice (carbon dioxide, 4.4 grams) using a towel and hammer. Add it to beaker and slowly add the Grignard reagent (step A) with stirring. The beaker should be in the ice bath from the beginning of the reaction. Keep the reaction mixture cold. The dry ice is a reactant and the source of cooling. The reaction is complete when all the dry ice is consumed and there is no more obvious activity.

C. ACIDIFICATION 1. Add 10 ml of concentrated HCl to a beaker containing 20 grams of crushed ice. 2. Slowly add the ice/acid mixture to the cooled mixture from step B. 3. The reaction is complete when all the activity in the beaker has ceased. There should be two layers in the beaker. Add more ether to dissolve any solid that is suspended in the ether layer. 4. Store the mixture in a closed container in the lab refrigerator.

Part Two PROCEDURE (Day 2) D. RECOVERY OF THE BENZOIC ACID 1. Transfer the stored mixture to a separatory funnel. Separate the aqueous layer, leaving the ether layer in the funnel. 2. Wash the ether layer with 30 ml of saturated sodium bisulfite solution. This is done to remove any color due to traces of iodine. Discard the washes. 3. Wash the ether layer with two 25 ml portions of RO water. Discard the washes. 4. Extract the benzoic acid from the ether layer with three 10 ml portions of 3 M NaOH. Collect these extracts in 250 ml beaker. Dispose of the ether layer in the waste ether container. 5. Acidify the basic extract with 6M sulfuric acid. Check with litmus to insure that the solution is acidic. 6. Set up a vacuum flask and use a Buchner funnel with a #4 filter paper. Filter the suspension and dry the product with stirring while applying vacuum. 7. Transfer the product to a clean storage vial. Weigh the vial minus the cover and the product to the ten thousandth of a gram. 8. Optional: If your product seems wet, heat the vial and contents in the oven for 5 to 10 minutes. Make sure the oven temperature is set below the melting point of benzoic acid. Weigh them again. If the weights differ by no more than a few thousandths of a gram the product is sufficiently dried. 9. Determine the actual yield, melting point and run the IR of your product (see next page for reference spectra). 10. For your lab report, don’t forget to include a detailed mechanism of the reaction.

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Reference Spectra