WEEK 14: HYDROLYSIS OF METHYL BENZOATE PURPOSE: This experiment will use microwave energy to cause a hydrolysis of the ester methyl benzoate to benzoic acid using a 5% aqueous sodium hydroxide solution. After the reaction mixture cools, the solution will need to be acidified to cause the precipitation of benzoic acid. IMPORTANT REACTIONS: COOCH3 1) 5% NaOH COOH 2) conc. HCl Methyl Benzoate Mol Wt 136.16 bp 198-199oC Benzoic Acid Mol Wt 122.12 mp 121-123oC BACKGROUND INFORMATION: The hydrolysis of an ester is an easy and important reaction in organic chemistry. One can use an acid or a base to shorten the time needed for the reaction to occur. The acid catalyzed hydrolysis of an ester gives the product in the free acid form without further pH adjustment. While this may seem preferable, it is seldom done as this reaction is reversible and the equilibrium process significantly lowers the yield. The base promoted reaction forms the sodium salt of the carboxylic acid which cannot enter into an equilibrium reaction so the reaction can proceed to completion giving a high yield of product. Note that the base, sodium hydroxide, is not a catalyst in this reaction as it is consumed in forming the sodium salt. The free acid form is recovered from the sodium salt quite easily by a simple pH adjustment. In this reaction, concentrated hydrochloric acid will be used to acidify the basic reaction product. Since benzoic acid is moderately water soluble, it is necessary to limit the amount of water in the solution. Microwave heating will be used in this experiment. As mentioned in an earlier experiment, the use of microwave energy for heating dates back to the 1960’s and became widespread in the 1980’s. Since microwave energy can penetrate deep within a molecule, the entire molecule can be heated at the same time. Conventional heating is generally done by conduction and heats from the outside to the inside of the sample. Therefore, microwave heating can be faster than conventional heating. However, for heating to occur, the microwave energy needs to be absorbed by the bonds in the molecule. Absorption of energy is better in polar bonds and poor in nonpolar bonds. Hence, water heats rapidly in a microwave oven. It is said that alkanes do not heat well with microwave energy. The design of a microwave oven presents some challenges to the designer. It is hard to have a uniform distribution of microwave energy in the cabinet. Further, if no absorption of microwave energy occurs a reflection of energy in the unit can destroy to apparatus. Some manufacturers warn that operating a microwave oven when empty can ruin the oven. Microwave ovens can also cause fires. Many people who have made popcorn in a microwave oven have seen burnt popcorn kernels at one time or another. It is important to remember that three things are needed for a fire to start: a source of ignition such as microwave energy, a combustible substance such as many organic compounds, and a source of oxygen such as air. It is important that one follows the experimental details carefully to work in a safe manner. Microwave energy can penetrate the human body and can interfere with certain electronic devices that may be implanted for health reasons (such a cardiac pacemakers). If you have implanted electronic devices, let the Instructor know and avoid the lab while the microwave unit is turned on. Please make careful observations during this experiment and comment on any success or failure in your report. EXPERIMENTAL PROCEDURE: You will use the laboratory microwave in this experiment. Be especially careful when handling the reaction tubes as they are delicate and expensive to replace. Place methyl benzoate (1.00 ml) in a microwave reaction tube and add 5 % aqueous sodium hydroxide (10 ml). Add a magnetic stirrer bar, then screw on the cap and place your tube in the microwave turntable. Record the position number of your reaction tube on the turntable. When all of the tubes have been added, your instructor will place the turntable into the microwave unit, close the door and select the correct filesetting for your reaction (Ester.rot). The program will take 12 minutes to run, followed by a 5 min. vent and cool down program. On completion, your instructor will open the microwave door and remove the turntable. Carefully remove your reaction tube, and empty the contents into a beaker. Cool the contents to room temperature, and acidify the solution with about 2 ml of concentrated HCl. As acidification proceeds, solid benzoic acid will precipitate. Be sure to check that the final solution is acidic by using pH paper. Cool the mixture in an ice bath and suction filter the benzoic acid. After the benzoic acid has dried sufficiently, weigh the solid, take its melting point and calculate the percent yield. IMPORTANT INFORMATION ABOUT THE REPORT: The report for this experiment will follow the usual format for synthesis reactions. Be sure the percent yield calculation is carefully done. Also, record the melting point range of the final product and compare that melting point to the reported melting point of acetylsalicylic acid. Using these data, discuss the relative success on the experiment. Also, be sure to critique the effectiveness of using microwave heating to cause the reaction to occur. END OF EXPERIMENT. © 2007 STEPHEN ANDERSON AND ROBERT SHINE