Roger 1. Weichmon College of Lake County 19351 W. Washinaton st. Grayslake, Illinois 60030
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hob+ion of Benzoic Acids from Listerine A n introductory organic chemistry experiment
An organic chemistry experiment which introduces a number of laboratory techniques in a meaningful, integrated, but simple way has been developed a t the College of Lake County. The initial laboratory periods are devoted to the isolation and identification of some of the components of Listerinel antiseptic, a commodity already familiar to the students. The separation scheme introduces the students to fractional distillation, extraction, decolorization with activated carbon, hot gravity filtration, vacuum filtration, crystallization, recrystallization, melting point, mixed melting point, refractive index, gas chromatography, and infrared spectroscopy. The Experiment The label on a bottle of Listerine lists its components as "Thymol, Eucalyptol, Methyl Salicylate, Menthol, Benzoic Acid, and Boric Acid. Alcohol 25%" In order to design a satisfactory scheme for isolating the benzoic and boric acids one must know something of the structure, chemistry, and physical properties of these ingredients. A single sheet giving the formula, structure, molecular weight, melting point, boiling point, refractive index, and solubility characteristics of each of the components is attached to the instructions given each student. While some instructors may prefer that the students search out these data themselves, I have found it prudent to pmvide them. One of the purposes of this experiment is to remove as much "busy work" as possible from their first encounter with the organic chemistry laboratory. There will he ample opportunity to search the handbooks in later experiments when the students have gained more familiarity with nomenclature and structure. While the students are not exnected to he able to desien the separation procedure which'is used, it is required tI& they understand the rationale for each step once it has been discussed with them. Students are required to prepare a flow-scheme of the procedure to be followed before starting the experiment. They are also expected to read the pertinent material in the laboratory manual before using a particular technique. This experiment will occupy three to four 4-hr laboratorv neriods. It beeins with a fractional distillation to re&o;e mostly eth;nol and water. About 25% of the original Listerine samnle is removed in this manner and 5-ml fractions from the beginning, middle, and end of the distillation are retained. These fractions lend themselves nicely 'Listerine is a registered trademark of the Warner-Lambert Company.
t o an introduction to gas chromatography, refractive index, and infrared spectroscopy. Gas Chromatography and Refractive Index Before analysis of the distillation cuts by gas chromatography, the students are shown commercially prepared slide-casette presentations describing the basic theory of gas chromatography, column selection, and indicator characteristics. These presentations are made during the first half-hour of each of the first three laboratory sessions. Gas chromatographs with both thermal conductivity (TC) and flame ionization (FID) detectors are used in this experiment. The chromatograms of the distillations demonstrate that the TC detector "sees" both ethanol and water; while the FID detector "sees" only the ethanol. The retention times of authentic ethanol and water samples are also determined. If desired, all three distillation fractions can be run through the TC instrument and, from the areas under the peaks (corrected for detector sensitivities), the relative proportions of alcohol and water can he determined. The values so obtained can be compared to those determined from refractive index measurements. A chart showing refractive index versus composition for the ethanol-water system is provided. Insight into azeotropic distillation also can be ohtained from this data. Infrared Spectroscopy After the distillate is dried its infrared spectrum can be obtained and compared to that of a sample of absolute ethanol. Alternatively, the infrared spectrum of the benzoic acid obtained later in the experiment can he compared to that of an authentic sample. Here the emphasis is on proper operation of the instrument with no attempt to go into detail regarding specific functional groip frequencies. At this point it is sufficient to state that pure oceanic compounds have spectra whichamount to finserprints for idkntification pu>poses. One may also point "out some of the important features of the spectra and assure the students that these features will become meaningful to them later in the course. This brief introduction to vpc and ir followed immediately by some "hands-on" experience has been well received by the students. They appreciate learning something of the capahiliti6s and limitations of these instruments early in the game without feeling any need to he experts in their use. The distillation residue is extracted with ether and the aqueous phase is retained for subsequent treatment. Ex-
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traction of t h e ether layer with aqueous sodium hicarbona t e followed bv acidification a n d concentration vields henzoic acid. he melting point of product is obtained a n d a mixed meltina point is taken with authentic benzoic acid. An importan< feature of the extraction procedure is that it provides a n excellent demonstration of conjugate acid-base theon. and .iolubilitv relationshius. Particulnrlv useful is t h e explanation of t h e use of sodium bicarbonate rather t h a n sodium hydroxide-the carhoxylic acid reacts with sodium bicarbonate t o form a soluble salt, whereas t h e weakly acidic phenols d o not. T h e aqueous phase from extraction of t h e distillation residue contains horic acid. This phase is decolorized with activated carbon followed by hot gravity filtration and concentration t o about one-fourth of its initial volume. T h e boric acid so obtained is recrystallized using t h e minimum amount of boiling water. T h e weight percents of both t h e benzoic a n d horic acids are determined a n d t h e final products are submitted t o t h e instructor. Experimental Assemble the components of your glassware kit for fractional distillation, using a 250-ml single-neck distillation flask. Introduce 150 ml of Listerine into the distillation flask, determining the weight in addition to the volume. Fractionally distill the Listerine, removing a total of about 40 ml of distillate. Isolate separate 5-ml fractions from the beginning, middle, and end of the distillation and store in stoppered, labelled vials. Place the distillation residue in a separatory funnel and extract five times with 20-ml portions of ether. Hazard. Do this in the h w d and away
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from any open flames. Save both the aqueous phase and the combined ether extracts. During the extraction store the eamhined ether extracts in a stoppered container to minimize losses due to evaporation. Place the combined ether extracts in a separatory funnel and extract four times with 15-ml portions of 5% sadium bicarbonate solution. Transfer the organic phase to an evaporating dish and allow the ether to evaporate in the hood. Describe the residue and comment as to its probable composition. To the combined sodium bicarbonate washes in a 250-ml beaker, add 6 M HCI slowly (Care. Foaming.) until a positive test is obtained with litmus paper. Evaporate the resulting solution to approximately 20 ml total volume and coal to ice-water temperature. If no crystals appear, the solution is probably not acidic enough, so add more 6 M HC1. Recover the crystals by vacuum filtration. When the product is thoroughly dry, determine its weight and obtain the following melting points: (1) the product, (2) pure henzoic acid, and (3) a 50:50 mixture of the product with benzoic acid. Decolorize the aqueous phase from the extraction of the distillation residue with activated carbon (use hot gravity filtration) and evaporate the filtrate to approximately 25 ml. Cwl the solution to ice-water temperature and recover the solid by vacuum filtration. Recrystallize the solid, using a minimum amount of boiling water. Dry and weigh the product. Determine the weight percents of both the boric and henzoic acids. Per instructions to be given, determine the composition of the three distillation fractions by gas chromatography and from refractive index measurements. Per instructions to he given, obtain the infrared spectrum of the benzoic atid and of the initial distillation fraction (after drying) and compare same with the spectrum of authentic benzoic acid and the spectrum of absolute ethanol. Submit your boric and benzoic acid samples to the instructor in properly labelled vials. Yields of benzoic and horic acids, respectively, should be on the order of 0.15 and 1.5 g.
