CARBONATION OF A GRIGNARD PREPARATION OF BENZOIC ACID

INTRODUCTION The Grignard is one of the most versatile reactions in organic chemistry. It was used to produce 2-methyl-2-hexanol from bromobutane and acetone. Carboxylic acids may also be prepared by the 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:

Br MgBr Ether + Mg

MgBr COOH + H3O + CO2

Week One

PROCEDURE 1. PREPARATION OF THE GRIGNARD REAGENT

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. 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. Attach the separatory funnel to the other connection. 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. 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 on Iodine and heat. When the reaction has started add the bromobenzene at a rate to keep the reaction going.

PROCEDURE 2. REACTION OF THE GRIGNARD REAGENT WITH CARBON DIOXIDE

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 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.

PROCEDURE 3. ACIDIFICATION

Add 10 ml of concentrated HCl to a beaker containing 20 grams of crushed ice. Slowly add the ice/acid mixture to the cooled mixture from procedure 2. 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. Store the mixture in a closed container in the refrigerator.

Week Two

PROCEDURE 4. RECOVERY OF THE BENZOIC ACID

Transfer the stored mixture to a separatory funnel. Separate the aqueous layer, leaving the ether layer in the funnel.

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.

Wash the ether layer with two 25 ml portions of RO water. Discard the washes.

Extract the benzoic acid from the ether layer with three 10 ml portions of 3 M NaOH. Collect these extracts in 250 ml beaker. Return the ether layer to the waste ether container.

Acidify the basic extract with 6M sulfuric acid. Check with litmus to insure that the solution is acidic.

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.

Transfer the product to a clean storage jar. Weigh the jar minus the cover and the product to the ten thousandth of a gram. Heat the jar and contents in the oven for 5 to 10 minutes. Weigh them again. If the weights differ by no more than a few thousandths of a gram the product is sufficiently dried.

Determine the actual yield, melting point and run the IR of your product.