Background: The ester functional group is quite common and is present in many naturally occurring compounds. A number of simple esters are volatile liquids that are known for their pleasant, fruity aromas. Esters are also found in fats and oils, as well as many other biologically important molecules. In this basic synthetic experiment you will attempt to prepare a simple methyl ester. Since this particular ester is a solid, you will be able to assess the purity of the product by determining its melting point. The overall effectiveness of the reaction can also be evaluated by determining the percent yield of the product.
Reaction: You will use a general esterification reaction for your synthesis. The Fischer esterification is discussed in McMurry (pages 781-782). You will use a modified version of the procedure discussed in Williamson (pages 486 and 495-6). Sulfuric acid will be used as the acid catalyst. (Most organic chemistry texts and laboratory manuals will include this reaction.)
To help drive the reaction towards ester formation, you will use an excess of alcohol versus the carboxylic acid.
Procedure: Weigh approximately 0.2 g p-bromobenzoic acid and add to a 100-mL round bottom flask along with magnetic stirrer. Add 25 mL methanol to the flask and stir to dissolve the p-bromobenzoic acid. Attach a reflux condenser. Add about 20 drops of concentrated sulfuric acid through the condenser and heat gently with a heating mantle while stirring for 30 minutes.
Caution: Sulfuric acid is toxic and corrosive, so avoid skin contact. p-Bromobenzoic acid is an irritant and should be handled carefully.
After the reaction mixture has boiled for 30 minutes, carefully transfer the mixture to a 50-mL beaker. Wash the reaction flask with a few mL of alcohol and add this rinsing to the beaker as well. Add a few boiling chips and place the beaker on a hot plate with a low setting. Evaporate the solvent until only a few mL remain (bubbling will likely cease) but not to dryness. (This step should be done in a hood if possible. To avoid evaporating to dryness, only about one half to two thirds of the methanol should be removed here. Any residual methanol should be removed in the subsequent steps. Excessive heating may char the product.) After the beaker has cooled, add 10 mL of ether to dissolve the residue that remains. Transfer the mixture to a centrifuge tube. Add 1 mL of distilled water to the centrifuge tube, shake to mix, and then and allow the layers to separate. Remove the (bottom) aqueous layer using a Pasteur pipet. Additional extractions will be performed by carefully (to avoid spattering) adding 10% aqueous NaHCO3 to the flask (three separate 1 mL portions) to neutralize any acid catalyst (bubbling may occur) that may remain. Once the bubbling has ceased, carefully remove the (bottom) aqueous layer using a Pasteur pipet. Add 1 mL distilled water to the test tube and again remove the aqueous layer.
To dry the ether layer, add several calcium chloride pellets (until they no longer clump together) and wait a few minutes. Using a Pasteur pipet, transfer the ether solution to a clean, dry evaporating dish. Place the dish atop a beaker of warm water to evaporate the ether. Once the ether has been removed, the remaining material will crystallize to yield the product methyl p-bromobenzoate. Scrape the solid from the dish, dry on paper towels (if necessary), and weigh the product. Determine the percent yield. Check the purity of the product by obtaining a melting point (literature value 81 C).