going beyond. going further
N. H. edited ETTINGER by G m g e Washington High School
Bustleton Ave. and Verree Road Philadelphia. PA 19116
Chemical Properties of Commonly Available Hydrocarbons Ivo Perina Laboratory of General and lnorsanic Chemistw, Facultv of Science, University of Zagreb, P.O. Box 153, 41001 Zagreb, . Yugoslavia Chemistry teachers in some developing countries have been complaining lately that many chemicals are very expensive or unobtainable. Therefore thev have to turn to those drawn from natural materials or their derivatives. The topic of saturated hydrocarbons can be presented in class by using substances drawn from natural sources or their derivatives, i.e., natural gas, liquefied petroleum gas (LPG), and certain petroleum derivatives (113). The study of saturated hydrocarbons is based on the synthetic ore~arationof methane and the observation of its properties; Synthetically methane is most often obtained by application of heat toamixture of anhydrous sodium acetate and soda lime or through the action of water on aluminium carbide. Alkanes ohtained from natural sources (natural gas. liquefied petroleum gas) are easily available, simple to ;se; not much preparation is needed (e.g., preparing anhydrous sodium acetate, synthesis of aluminium carbides, etc.), unlimited quantities are available, and the experiments are of shorter duration. Natural gas is used in our experiments. Its composition is shown in the table. Current regulations stipulate that odorant agents such as merca~tans.and in oarticular ethvlmercaotan. which will facilitate detection oE gas leaks, be added. $ortukately, they are in such low concentrations that this does not interfere with the experimental studies of methane properties. In places where natural gas is not readilv available the liquefied petroleum gas ( ~ ~ ~can a be . 4used? Many laboratories use this kind of gas as a source of energy. Liquefied petroleum gas is propane or butane or a mixture of both in various proportions. Today most of the liquefied petroleum gas is ohtained from natural gas rich in propane and butane. In studying methane, i.e., saturated hydrocarbons, students are usually first made aware of some of the main properties of this group of compounds, and these are: combustion, explosiveness, saturation, substitution, and pyrolysis. The Experiments
The Burning of Natural Gas When natural gas burns, carbon dioxide and water are produced and this can be demonstrated in the usual way (through the condensation of water on a cold glass surface
' Available from gas distributing companies. 884
Journal of Chemical Education
Composltlon ol Natural Gas Compo.It1on carbon dioxide
VOl. %
Cpentane
0.16 2.70 94.04 2.57 0.39 0.04 0.05 0.01
rrwntane
~ilca
nilrwn
memans ethane propane
&butane *butane
and the introduction of carbon dioxide into lime water). In this way, we have shown that natural gas contains carbon and hydrogen. Explosion of the Mixture of Natural Gas and Oxygen One-third of the volume will consist of natural eas introduced from the gas mains into a polyethylene cylinder (optifilled w ~ t hwater and olaced in a mal dimension 100-200 m l ~ pneumatic tub; then oxygen is introduced, two-tcirds of the volume, from a small gasometer for oxveen. .. The immersed cylinder is closed by meansofan acrylicglass plate, it is then takenout of the tuband shaken thoroughlv. Asa orecautionary measure, an explosion shield is placed betwein the cylinder and the auditorium, and the experimenter puts a protective shield over his face. The plate is then removed and a long lighted wooden stick is placed near the opening of the cvlinder. An exnlosion follows. - rec cautionary Measures. Experiment is carried out with small volumes of gases. Plastic (polyethylene) cylinder, protective shield, and face shield are an absolute must. The gas mixture is lit with a long wooden stick. A Test for Unsaturated Hydrocarbons in Natural Gas
The apparatus is made up as shown in Figure 1.Bromine water (w = 4%) is added to one gas-washing bottle and potassium permanganate (w = 1%)containing some sodium carbonate solution (w = 1%) to the other. Natural gas obtained from thegas mains is passed alternately ineach of the gas-washing bottles. No changes are observed; thus, the natural gas does not contain unsaturated compounds. The exhaust natural gas is lit. In rare cases, with prolonged introduction, bromine water
can turn colorless. due to the oxidation of mercantan to the corresponding disulfide. This is not, however, thk case with notassium Dermaneanate. so that bromine water in this exberiment cannot betakenas the only test for the presence of unsaturated compounds. The same experiments can be carried out with the liquefied petroleum gas, only the experiment with the explosion is carried out with a different proportion of gases (116 C3Hs and 516 0 2 by volume) in accordance with the equation of combustion. Substitution of Higher Saturated Hydrocarbons Contained in Kerosene with Bromine Even thoueh substitution in schools is usuallv carried out with methane and chlorine in the presence of l&ht, our own investigations have shown that the same tvDe of exoeriment may be successfully performed with na&al or iiquefied petroleum gas. However, in order to avoid the use of chlorine in the class we suggest that this experiment be performed with kerosene and bromine. If kerosene is used, then it should first be freed of unsaturated compounds (4). The apparatus shown un Figure 2 is assembled. Ten milliliters of purified kerosene is put intt~the suction tube, then 10 drops-of elementary bromine is added. The components are gently mixed. Distilled water is poured into the test tube. The lower Dart of the suction tuhe is w r a ~ n e din a hlack paper (appioxunately half the height of tie' reaction mixture) and placed in a tin can. The black paper is placed so as to protect the reaction mixture from light, and the tin can is positioned to protect i t from the heat emitted by the bulb. The reaction mixture in the suction tube is then exposed to the light of the bulb of an overhead oroiector. After one to two minutes the reaction mixture beiomes colorless and, a t the same time, the se~arationof gas is observed. After removing the black pap& and the t& can, we see that part of the reaction mixture has not become colorless. The students can then draw their own conclusions about the catalytic effect of light on the substitution of saturated hydrocarbons with halogen elements. The next stage in the experiment is to expose this section to the light. We see that i t too soon becomes discolored. In order tljprovc that hydngen bromide isa by-produrt of the suhstitu~ionreartion, a rubber teat is used to flush out gaseous hydrogen bromide into the test tuhe containing some water (Fig. 2). After two to three minutes of flushing we can prove that water in the test tube turns blue litmus paper red. An addition of a silver nitrate solution produces a pale yellow precipitate of silver bromide. The presence of bromine can be proved in the halogenated hydrocarbons in the suction tube by means of Beilstein's test (5). A similar experiment using n-hexane was described in the literature (6, 7). ~
Flgure 1. Testing the natural gas far unsaturatedhydrocarbons.A S~ODMC~.
~
= three-way
Warnine: In dealing with bromine maximum caution should he exercised. ~ l i e x ~ e r i m e nare t s t o he carried out in the hood. Rubber gloves should be worn at all times and a bottle of glycerol b u s t be ready a t hand. After use, the dropper for bromine is immersed in diluted sodium hydroxide solution. Pyrolysis of Saturated Hydrocarbons Automobile gasoline can he successfully used for this experiment after being purified by the same method as described in the kerosene experiment above. At the same time this procedure removes tetraethyl lead present in the gasoline. An amaratus is assembled similar to that described in literature (81, the only difference being that an additional suction tube is added to the series. Five milliliters of purified gasoline is poured into the flask and copper filings-are put into the reaction tube. About 2 ml of bromine water is poured into the first suction tuhe, while the same quantity of potassium permawanate solution, alkalized with sodium carbonate, is poured into the other tuhe. The end of the tube is immersed in the pneumatic tub filled with water. The experiment starts off by heating the copper filings. When they become red hot, a rubber teat is used to push gasoline vapors through the tube. During pyrolysis the saturated hydrocarhons contained in gasoline (heptane, octane, etc.) decom~oseinto lower saturated and unsaturated hvdrocarbons Lnder the influence of the copper catalyst. he unsaturated hvdrocarbons discolor the bromine water and change the color of the potassium permanganate solution. The surplus vapors are caught in atest tube in the pneumatic tub and lit. The next step is to show that the purified gasoline, with which we started the experiment, neither discolors the bromine water nor changes the color of the potassium permanganate solution. The pyrolysis experiment is a model reaction for the cracking process. Note: I t is verv. i m.~ o r t a n tthat the whole anoaratus is .. proprrly and rightly assembled. Very small quantity of gasoline hiahlv redures the danger of an ~xvlosinnur fire. Sur" gestedbiantity is 5 ml. " The reactions described herein utilized commonly available materials that students will encounter daily regardless of their career choices. 11) Pfeifer, I MsuUmer, , F., and Reitlingeb 0..J. Pmkf. Chom., 99,239 (1919). (2) Claf, N. Z..Khim. Shk., No. 4.70 (1960).
(3) Conklin, A. R.,and Kramme,A,. J. CHBM.EDUC.,60.597 (1983). (4) Cvetkav, L. A.. "E*perimont po o~aniteskoihimii: 4th ed., Prosveaenic, MO8h.a.
1966, P. 145. (5) Vogel, A. I., '"PmctiealOrganic Chemistry," 2nd ed., Longmans. Green and Co, London, 1954,~. 283. ( 6 ) Alyea, H. N.. J. CHEM EDuc., 44. AS3 (1967). 17) Koludarou,J.A..Khim.Shk.,No.4.80(1971). (8) Mayer,G., Chem. Schuk,30.98(1983).
Figure 2. Substitution of hlgher saturated hydrocarbonscontained in kerosene with bromine. A = light source; B =rubber teat; C =tin can.
Volume 62
Number 10 October 1985
885
