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71. Preparation of Ethylene from Alcohol (Section 29). - Weigh directly into an 8-inch test-tube 4 grams of phosphorus pentoxide. Connect the test-tube by means of a closely fitting cork with a reflux air-condenser; immerse the tube in cold water, and pour 5 cc of ethyl alcohol slowly into the condenser. The alcohol should be added cautiously in small portions and the test-tube shaken under water, as much heat is evolved when alcohol comes in contact with phosphorus pentoxide. Support the test-tube at an angle of about 45° with the table by means of a clamp, and connect it with a delivery-tube arranged to collect a gas over water. Heat the tube carefully until the mixture becomes homogenous; then more strongly until a steady stream of gas is evolved. Collect three 250 cc. wide-mouthed bottles of the gas, and prepare an explosive mixture of ethylene and air. This can be done by placing 20 cc. of water in a 250 cc. wide-mouthed bottle, covering the latter with a glass plate, inverting in a pneumatic trough, and displacing the water by ethylene. See the following experiment.
Note. - When sulphuric acid, which is commonly used as a dehydrating agent in the preparation of ethylene, is employed, the reaction takes place at a temperature very near that at which the alcohol chars; as a consequence, carbon often separates, the mixture froths badly, and sulphur dioxide and carbon dioxide are formed. The apparatus for the preparation in this way is described under ethylene bromide (experiment 137, page 103).
72. Properties of Ethylene (Section 30). - (a) Inflammability of ethylene. - Throw a lighted match into a bottle of ethylene. In order to facilitate the removal of the gas, a stream of water should be poured into the bottle as the gas burns. Note the color of the flame, and compare with results obtained when methane was burned. (Explain.)
(b) Explosive mixture of ethylene and air. - Throw a lighted match into the bottle containing the mixture of ethylene and air.
(c) Ethylene and bromine. - Hood. - Add 2 drops of bromine to a bottle of the gas. (See experiment 66d, page 44.) Replace the glass cover and shake. When the color of bromine has disappeared, observe carefully the contents of the bottle. Are there drops of an oil present? (Eq.) Test the gas in the bottle with moist blue litmus paper. Compare the results with those obtained with methane.
(d) Ethylene and oxidizing agents. - Dissolve a small crystal of potassium permanganate in about 20 cc. of water, and add a little of the solution to a bottle of the gas. Replace the cover and shake. Explain the value of the test, and compare the results with those obtained with methane. (See experiment 66e, page 44.)
(e) Test for unsaturated compounds in coal gas. - Collect over water a bottle of illuminating gas. Add a dilute solution of potassium permanganate in portions of 5 cc. to the bottle and shake. What conclusions can be drawn as to the presence of unsaturated hydrocarbons in illuminating gas? What are the chief constituents of coal gas? Of water gas? Of what value are the unsaturated hydrocarbons in these gases?
73. Preparation of Ethylene from Ethylene Bromide (Section 29). - Into a 6-inch test-tube place about 2 cc. of ethylene bromide, 2 cc. of alcohol, and about 0.5 gram of sheet zinc. Connect the tube with a delivery tube, warm gently until reaction begins, and collect a test-tube full of the gas over water. (Eq.) Burn the gas. (Eq.)
74. Properties of Unsaturated Hydrocarbons (Section 35). - (a) Amylene and bromine. - Dissolve 1 cc. of amylene in 5 cc. of carbon tetrachloride, and add gradually, as long as an evident reaction takes place, a solution prepared by dissolving 2 cc. of bromine in 50 cc. of carbon tetrachloride. (Eq.) Test for evolved hydrobromic acid by breathing across the mouth of the tube.
(b) Amylene and oxidizing agents. - Shake a few drops of amylene with about 20 cc. of a dilute solution of potassium permanganate prepared by dissolving a crystal of the salt in one-half a test-tube full of water. (Eq.)
75. Preparation and Properties of Acetylene (Sections 38 to 40). - (a) Support a dry 200 cc. distilling flask by means of a clamp, and connect the sidearm of the flask with a delivery-tube arranged to collect a gas over water. Fit a dropping funnel into the neck of the flask by means of a cork. Place in the flask about 10 grams of calcium carbide, and let water fall very slowly, drop by drop, from the funnel onto the carbide. (Eq.) Fill four 250 cc. bottles with the gas generated, rejecting the first bottle-full collected. Why? Prepare an explosive mixture of acetylene and air by putting 15 cc. of water into a wide-mouthed bottle, covering the bottle with a glass plate, inverting the bottle in the pneumatic trough, and displacing the water by acetylene. Caution. - Do not let acetylene escape freely into the air; the gas has an unpleasant odor and is poisonous.
(b) Inflammability of acetylene. - Throw a lighted match into a bottle of the gas. Is much soot deposited? Note the luminosity of the flame and compare your results with those obtained when methane and ethylene burned.
(c) Explosive mixture of acetylene and air. - Throw a lighted match into a bottle containing the mixture of acetylene and air. How does the explosion compare in intensity with those obtained with methane and ethylene?
(d) Acetylene and oxidizing agents. - Apply the potassium permanganate test for unsaturation. (See experiment 66e, page 44.)
(e) Acetylene and bromine. - Add to a bottle of the gas about 5 drops of bromine. Replace the cover of the bottle and shake. If the color does not disappear in a minute, add to the bottle a few cubic centimeters of a dilute solution of sodium hydroxide and shake. When the color has disappeared, observe the contents of the bottle carefully. (Eq.) Note the odor of the product formed. How could acetylene be obtained from this compound? (Eq.)
(f) Test for the triple bond. - Test a bottle of the gas for a compound containing a triple bond, using an ammoniacal solution of cuprous chloride which can be made as follows: Heat together in a test-tube over a flame, a few pieces of copper oxide and metallic copper with about 10 cc. of dilute hydrochloric acid (sp. gr. 1.1). When the solution becomes colorless, cool, decant off about 5 cc. of the liquid and add ammonia until the solution is alkaline. Add this solution to a bottle of the gas; cover the bottle and shake. (Eq.)
Notes. - (a) The acetylene prepared in this way from commercial calcium carbide contains impurities, such as hydrogen sulphide and phosphine. If the gas is to be used to prepare other compounds, it should be purified by passing it through a wash-bottle containing a solution of mercuric chloride in hydrochloric acid.
(c) It is easy to obtain an explosive mixture of air and acetylene as the proportion of acetylene in such mixtures may vary between wide limits, namely, from 3 to 82 per cent by volume. In the case of methane and ethylene, the limiting volumes are from 5 to 13 per cent of the former and from 4 to 22 per cent of the latter.
(f) This test applies only to compounds containing the C-=CH group; the compounds must contain a hydrogen atom linked to a carbon atom which is joined to a second carbon atom by a triple bond.