A Simple Experiment Illustrating the Preparation and Properties of Isobutylene (2-Methylpropene) CHARLES BARKENBUS and JERRY B. KELLEY University of Kentucky, Lexington, Kentucky
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LL laboratory manuals of organic chemistry have
an experiment illustrating the preparation and properties of alkenes and, in the majority of cases, the alkene made has been ethylene although a few manuals give directions for making petene-2. In the last few years isobutylene, because of the ease of polymerization and alkylation, has become one of the outstanding compounds of the petroleum industry. For this reason it bas been thought worth while to substitute this important alkene for the time-worn examples used in the average organic laboratory manuals. Isobutylene has other advantages, such as ease of preparation and greater reactivity, which make i t superior to ethylene. Isobntylene bas been made by the dehydration of tert-butyl alcohol with concentrated sulfuric acid.' Tertiary butyl alcohol is smoothly dehydrated with 30 per cent sulfuric acid a t 98°C. with no frothing, carbonization, or production of sulfur dioxide. Noms2 found that tertiary alcohols do not form alkyl acid sulfates with dilute sulfuric acid. Since tert-butyl alcohol boils a t 82.g°C. it is necessary to use a condenser on the generator to prevent the rapid volatilization of the alcohol. Many variations in the experimental procedure were tried in an attempt to eliminate the use of a condenser but in all cases the loss of tert-butyl alcohol was too great to conduct the experiment without one. The following procedure has been used on the lecture table and by classes in the laboratory, and offers no difficulty or hazard. The apparatus is simple and the experiment can be readily carried out in a minimum of approximately two hours. The wash bottles used were made from 8-inch test tubes fitted with rubber stoppers and were furnished the students. In a 200-ml. round-bottom flask attached to a reflux condenser were placed 10 ml. of tert-butyl alcohol and 20 ml. of approximately 30 per cent sulfuric acid made by diluting 7 ml. of 95 per cent sulfuric acid with 14 ml. of water. The flask is placed in a water bath and the end of the condenser connected by a bent glass tube and rubber tubing to a 125-ml. distilling flask which is surrounded with ice. The distilling flask acts as a safety flask and also condenses some tert-butyl alcohol which may come over. The side neck of the flask is con-
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~ K I ~ T I A K OG~. ~ B.,KBT Y ,AL., I . Am. Chem. (1935).
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nected by rubber tubing to an 8-inch test tube wash bottle containing 10 ml. of acidified dilute potassium permanganate solution. This solution is made by diluting 1 ml. of 2 per cent potassium permanganate solution with 9 ml. of water and adding 1 ml. of 40 per cent sulfuric acid. This wash bottle is then connected to another wash bottle surrounded with ice containing 3 ml. of bromine and 5 ml. of water, the water being added to cut down the volatilization of the bromine. A delivery tube runs from the outlet of this wash bottle to a pneumatic trough where samples of the gas can be collected if desired. While displacing the air in the apparatus a small amount of bromine vapors escape which are absorbed in the water of the trough. About 30 minutes are required to assemble the apparatus. The water bath is brought to gentle boiling and a steady flow of isobutylene is generated. The gas starts coming off slowly a t 50°C. and at the boiling point of water a steady flow results. No further attention to temperature control is necessary, which is a definite advantage. About 15 minutes are required to heat the bath, and a t that time most of the air has been displaced and complete absorption of isobutylene by the two wash bottles is observed. The potassium permanganate solution is decolorized in about four minutes and the bromine in seven minutes. When the bromine is decolorized, the test tube is replaced by one containing 3 ml. of 95 per cent sulfuric acid and the gas is allowed to bubble into it for five minutes. An insoluble layer of di-isobutylene and higher polymers separates which bas a pronounced gasoline-like odor. This tube is then replaced by one containing 3 ml. of 60 per cent sulfuric acid, and the gas is passed in for 5 minutes. The contents of this tube when diluted with water have a definite odor of tertbutyl alcohol. Sufficient isobutylene can still be generated to collect'several100-ml. bottles for running tests on inflammability, diffusion, and explosive mixtures with air. This experiment well illustrates the ease of dehydrating tertiary alcohols with dilute sulfuric acid. The smoothness with which the gas is generated eliminates all careful attention to details and makes the experiment especially free of difficulties. The reduction of the per(Continued on page 363)
~J. F. ~. AND ~ J.~M.~JOUBERT, , ibid., 49, 873 (1927). 35 6
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