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Chapter II

General Processes: Hydrocarbons of the Methane Series

62. Calibration of a Thermometer. - Calibrate a thermometer at 0°, 100°, 183.7°, and 218.1° according to the method described in §48, page 31.

63. Determination of Melting-points. - Determine the melting-point of the substances furnished according to the directions given in §49-52, pages 32-35.

64. Fractional Distillation. - Read the description of fractional distillation in §21-23. Set up an apparatus for distillation like that shown in Fig. 2, page 9. Use a 200 cc. distilling flask and as receivers three 100 cc. flasks which are labeled, respectively, I, 78°-82°; II, 82°-95°; and III, 95°-100°. Place 50 cc. of alcohol and 50 cc. of water in the distilling flask, and heat the latter with a flame of such a size that the liquid distils at the rate of about 1 drop per second. Collect the part which distils up to 82° in the flask numbered I. When this point is reached replace this receiver by flask II, and collect the distillate in this flask until the thermometer registers 95°. At this point use flask III as the receiver. When nearly all the liquid has distilled, pour out the small residue from the distilling flask and dry it, or use a clean dry flask. Measure in a graduated cylinder the volumes of the contents of the three receivers at the end of the first distillation, and record the results in your notebook. (See the tabulation on page 14.) Place in the distilling flask the liquid in receiver I (the first fraction), and distil as before, collecting what boils up to 82° in receiver I. When this point is reached, allow the distilling flask to cool slightly, and then add to it the contents of flask II. Replace the thermometer and distil. Collect what passes over below 82° in flask I, and the part which boils between 82° and 95° in flask II. Remove the flame, and add to the distilling flask the contents of flask III. Replace the thermometer and collect the distillate in the several receivers according to the boiling-point. Measure and record the volumes of the three fractions at the end of this second distillation. Repeat the fractionation a third time and record the volumes of the fractions.

65. Qualitative Analysis of Organic Compounds. - Make analyses of the substances furnished, following the directions given in §58-61, pages 39-41.

66. Preparation of Methane from Sodium Acetate; Properties of Methane (Sections 17, 18). -(a) Grind and mix thoroughly in a mortar 8 grams of fused sodium acetate[1] and 8 grams of soda-lime[2]. Transfer the mixture to an 8-inch test-tube provided with a rubber stopper fitted with a short piece of glass tubing. Connect to the latter by means of a short piece of rubber tubing a delivery-tube arranged to collect a gas over water. Support the test-tube, by means of a clamp, in such a position that the end containing the stopper is slightly lower than the other end; this prevents any water given off during the heating from running back into the hot tube and cracking it. Heat the tube cautiously, keeping the flame in motion, in order to avoid local overheating. (Eq.)[3] Collect three 250 cc. bottles of the gas. Prepare a mixture of the gas with air by putting into a fourth 250 cc. bottle 25 cc. of water; cover the bottle with a glass plate, insert it in the pneumatic trough, and pass the gas into the bottle until the water has just been replaced. Caution. - Remove the delivery-tube from the water before the heating of the tube is stopped. If during the heating the water begins to go back into the test-tube, the breaking of the latter can be prevented by separating the test-tube from the delivery-tube where they are joined together.

(b) Inflammability of methane. - Drop a lighted match into one jar of the gas. Is soot produced? Why? (Eq.) What is the color of the flame?

(c) Explosive mixture of methane and air. - Light the mixture of methane and air. Explain the difference between the results obtained in this case and in (b). (Eq.) Calculate the relation between the volume of methane and the volume of oxygen in the gas exploded. State the volumes which interact according to the equation for the reaction.

(d) Methane and bromine. - Add one drop of bromine to a bottle of the gas. This can be done conveniently by placing a pipette, made from a piece of glass tubing drawn down to a small bore at one end, into a bottle of bromine and placing the finger over the end of the pipette, which is then withdrawn. Cover the bottle with a glass plate, and allow it to stand for a few minutes until the bromine has vaporized. Drop a lighted match into the bottle. (Eq.) When the reaction is complete breathe sharply across the mouth of the bottle, and test the gases in the bottle with a piece of moist blue litmus paper.

(e) Methane and an oxidizing agent. - Make a dilute solution of potassium permanganate by dissolving a crystal of the salt in one-half a test-tube full of water. Add the solution to a bottle of methane, replace the cover, and shake. Explain.

Notes. - (a)[4] Soda-lime is preferably used in experiments of this type rather than sodium hydroxide, as the latter rapidly attacks glass at the temperature at which the reaction takes place.

(b) The methane prepared in this way is not pure; it contains small amounts of substances (acetone, for example) which impart a yellow color to the flame. The reaction should be brought about at the lowest temperature at which it takes place, in order to reduce the amount of by-products formed. Pure methane burns with a flame that is only slightly luminous.

(c) The violence of the explosion is reduced by the fact that the oxygen which reacts with the hydrocarbon is diluted with nitrogen. A mixture of one volume of methane and two volumes of oxygen explodes with violence when ignited.

(e) This test is of value in distinguishing hydrocarbons of the methane series from other hydrocarbons which rapidly reduce an aqueous solution of potassium permanganate. If reduction occurs it is due to the fact that the methane is impure.

67. Preparation and Properties of Ethane: Grignard Reaction. - (a) Into a dry 50 cc. Erlenmeyer flask fitted by means of a cork to a return condenser, place 2 grams of magnesium powder and a small crystal of iodine. Add through the condenser 5 cc. of ethyl bromide and 10 cc. of ether dried over sodium. In about 15 minutes when reaction ceases (Eq.) replace the condenser by a two-holed stopper carrying a small separatory funnel and a delivery-tube to collect a gas over water. Add water to the flask drop by drop, through the funnel. (Eq.) Collect two 250 cc. bottles of the gas.

(b) Inflammability of ethane. - Drop a lighted match into one bottle of the gas and note the color of the flame.

(c) Ethane and oxidizing agents. - Test the second bottle of gas to determine whether ethane can be readily oxidized. (See experiment 66e, page 44.)

Note. - (a) A trace of iodine is usually added to the mixture of halide and magnesium in effecting a Grignard synthesis, in order to hasten the reaction. The reaction is similar to that by which methane is formed from methyl iodide and magnesium. (See end of Section 17.)

68. Preparation and Properties of Di-isoamyl: Wurtz Synthesis. - (a) To 20 grams of isoamyl bromide contained in a dry 100 cc. round-bottomed flask add 5 grams of sodium cut into about six pieces. Connect the flask with a piece of glass tubing about 3 feet long, to serve as a reflux condenser, and allow it to stand over night. Connect the flask with an air condenser, and distil with a smoky flame which is kept constantly in motion to avoid local overheating. (Eq.) Redistil slowly the distillate from a small flask, and save and weigh the fraction which boils at 158°-161° Isoamyl bromide boils at 118.6° and di-isoamyl at 159.5°. Caution. - Read carefully §41, page 27. The flask contains unused sodium, and water should not be put into it. Add portions of 5 cc. of alcohol from time to time until the evolution of hydrogen ceases, and there is enough of the liquid to make a thin paste of it with sodium bromide; then pour the contents of the flask cautiously into an open vessel containing water. Do not pour water into the flask. Great care should be taken in the disposal of sodium residues which should be always treated in the manner just described; serious explosions and accidents frequently happen as the result of a lack of proper precautions.

Calculate the theoretical yield from the isoamyl bromide used and the percentage obtained in the experiment.

(b) Di-isoamyl and bromine. - Add 2 drops of bromine to about 2 cc. of di-isoamyl. Breathe across the tube. Is there any evidence of substitution? Heat the contents of the tube to boiling and test for hydrobromic acid as before. (Eq.)

(c) Di-isoamyl and sulphuric acid. - Add 2 cc. of the hydrocarbon to about 5 cc. of concentrated sulphuric acid, and shake well. Allow the tube to stand a few minutes and observe the contents. Do the two liquids mix?

(d) Di-isoamyl and nitric acid. - Repeat experiment (c) above using concentrated nitric acid instead of sulphuric acid. Record the results obtained.

Notes. - (a) In carrying out the Wurtz synthesis the reaction is often brought about in a solvent; the halide, or mixture of halides, is diluted with about twice its volume of ether which has been dried over sodium. The preparation described above should yield from 7 to 8 grams of the slightly impure hydrocarbon.

(c and d) Concentrated sulphuric acid and nitric acid are valuable agents for distinguishing between the paraffin hydrocarbons and many other classes of compounds which dissolve in these reagents.

69. Composition of Kerosene (Section 24). - Distil about 25 cc. of kerosene, using a small flask and a condenser. (See Fig. 2, page 9.) Caution. - See that all corks fit tightly and that the receiver is not near a flame. Note the temperature at which the liquid begins to drop freely from the condenser and when the last part is distilling over. Compare the boiling-points obtained with those of the paraffin hydrocarbons, and state what compounds are present in the sample studied.

Note. - Crude petroleum is separated into various commercial products according to the specific gravity of the distillates. These products are not carefully fractionated and are mixtures. (See Section 28.)

70. Properties of Gasoline and Kerosene (Sections 26 to 28). - (a) Kerosene and acids and alkalies. - In separate test-tubes shake about 5 cc. of distilled kerosene with 10 cc. each of concentrated nitric acid, concentrated sulphuric acid, and a dilute solution of sodium hydroxide. Note if heat is evolved or if there is a change in color. Record your observations and conclusions. Gasoline behaves in a similar way.

(b) Solubility of kerosene. - Test the solubility of about 2 cc. of kerosene or 1 gram of paraffin in water, ether, alcohol, and ligroin or petroleum ether. Caution. - Volatile inflammable liquids such as ether, alcohol, and petroleum ether should not be heated over a free flame. They can be heated conveniently by immersing the vessel containing them in boiling water.

(c) Gasoline and bromine. - To 10 cc. of gasoline contained in a dry test-tube add 2 cc. of a 5 per cent solution of bromine in carbon tetrachloride. Divide the mixture into two portions; place one in your desk in the dark, and the other in direct sunlight. After a few minutes compare the two tubes. Breathe across the tubes. Explain the difference observed. (Eq.)

(d) Flash-point of gasoline. - Pass a burning match over a few drops of gasoline placed on a watch-glass. What statement can you make in regard to the flash-point of gasoline?

(e) Flash-point of kerosene. - This experiment should be carried out in a place where there is no draft. Support a plain beaker of 50 cc. capacity in a 100 cc. beaker by means of a triangle made of copper wire, or a piece of pasteboard in which a hole has been cut just large enough to allow the body of the small beaker to pass. Fill the smaller beaker with kerosene and the larger beaker with water. In each case the level of the liquid should be within one-half inch of the top of the beaker. Caution. - Have an asbestos board or a watch-glass at hand with which to smother the flame in case the kerosene takes fire during the experiment. Insert a burning match into the kerosene. What can you say of the flash-point of kerosene? Place the apparatus on a wire gauze, and heat cautiously with a flame, the tip of which should be about 3 inches below the gauze. Insert into the kerosene the bulb of a thermometer, which should be supported by a clamp. The temperature of the oil should rise not faster than 2° per minute. Attach by means of a rubber tube to the gas-cock a blowpipe or a piece of glass tubing drawn out to a fine opening. The gas should be regulated to produce a flame about one-half inch in length. When the temperature of the kerosene reaches 35°, pass the flame quickly over the surface of the oil, taking care not to touch it. Repeat the test for every degree rise in temperature. Record the temperature at which the vapor of the oil ignites; this is made evident by a slight flash which is more or less difficult to see. As soon as the flash-point has been observed, extinguish the flame under the apparatus. If the kerosene does not flash before 48°, turn out the flame and consult the instructor. Compare your results with the flash-point of kerosene required by law (Section 28).

Note. - (a) Owing to the cracking of the paraffin oils in their distillation, samples of the commercial products from petroleum often contain small quantities of hydrocarbons which react with sulphuric acid and with nitric acid. When these impurities have been removed by repeated shaking with concentrated sulphuric acid, the resulting paraffin hydrocarbons show their characteristic inertness.

[1] Commercial fused sodium acetate is generally not anhydrous; it is well to fuse it before use. If the compound is to be prepared from the hydrated salt (CH3COONa.3H20) proceed as follows: Heat cautiously about 15 grams of the crystalline salt contained in an iron pan over a small flame. Stir continuously with a glass rod. The salt melts at first in its water of crystallization; as the dehydration proceeds it solidifies, and, finally, when anhydrous, melts a second time. Care should be taken to avoid heating the dehydrated salt much above its melting-point, as it undergoes decomposition when strongly heated.

[2] The soda-lime can be replaced by an intimate mixture of 4 grams of powdered sodium hydroxide and 4 grams of quicklime.

[3] The abbreviation (Eq.) signifies that an equation for the resection which takes place should be written.

[4] The letters used in the notes refer to the paragraphs under these letters in the description of the experiments.

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