Norman C. Rose
Texas A&M University College Station
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I
Hydration of an Alkyne U n d e r g r a d u a t e organic chemistry experiment
I n order t o introduce students to the chemistry of alkynes, most laboratory manuals include an experiment on the preparation and properties of acetylene. This experiment, however, has some major drawbacks. The preparation of other alkynes is not feasible for beginning students of organic chemistry. Commercially available alkynes which have no functional groups other than the triple bond are usually considered too expensive for use in the demonstration of the properties of alkynes. 2-methyl-3-butyn-2-01 (CH3)2C(OH)CECH,is available from commercial sources, is inexpensive, and displays properties typical of a terminal akyne. It readily decolorizes a dilute bromine solution and a dilute potassium permanganate solution. Reaction of ammonical silver nitrate with the alkynol yields the silver acetylide as a precipitate. The alkynol reacts with concentrated sulfuric acid, but the reaction is so vigorous that the reaction should not be included as part of an experiment. The hydration of 2-methyl3-butyn-2-01 to yield 3-hydroxy-3-methyl-2-butanone is a very suitable reaction for undergraduates who have had little prior experience in the organic laboratory.
The procedure of Scheihler and Fischerl was modified in order to make it possible to complete the experiment within three and one-half hours. The addition of all of the alkynol a t one time leads to a reaction which is too vigorous to control; thus it is better to add the alkynol in two portions. Since the exothermic reaction of the alkyuol in acidic medium begins a t about 70°C, it is best to have the temperature below 70°C when the alkynol is added. As both the starting alkynol and the adding molecule are unsymmetrical, two products are possible. Although the isolation of the aldehyde from the reaction mixture has not been reported, the student can be directed to prepare a derivative in order to determine if an aldehyde or ketone is formed. The recorded melting point2 if the semicarbazone of the ketone, (CH& C(OH)C(CH3)NI\'HCONH2, is 162.Fr163.5°C. The aldehvde. (CH2LC(OHXH.CH0. undereoes a d e " , - ~ , - ~ -, hydration reaction in the medium used to form the
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SCHEIBLER, H., AND FISCAER, A., Chem. Be?., 55,2915 (1922). 'DAVIS,R. B., AND ERMAN, W. F., 3. Am. Chem. Soe., 76, 3478 (1954).
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semicarba~one.~The recorded melting point3 of the semicarbazone of the unsaturated aldehyde, (CH3),C= CHCHNNHCONH2, is 222-223°C. The preparation of the semicarbazone takes an additional hour. The Experiment
Hydration of the Alkyne. Add 18 ml of concentrated sulfuric acid to 115 ml of water contained in a 500m1, round-bottom flask. Dissolve 1 g of mercuric oxide in the warm acid solution. Cool the contents of the flask to 50°C. Equip the flask with a reflux condenser. Add, all a t once, 14 ml of 2-methyl-3-butyn-2-01through the condenser. A precipitate, a mercury complex of the alkyne, forms. Shake the flask to mix the contents. An exothermic reaction begins, the precipitate dissolves, and the solution turns a light brown. After the reaction is allowed to proceed by itself for two minutes, heat the reaction mixture to reflux. (Even if the exothermic reaction does not begin by itself, heat the solution until it begins to reflux.) As soon as the reflux begins, stop the heating and cool the reaction mixture to 50°C. Add through the condenser a second 14 ml (a total of 26 g, 0.3 mole) of 2-methy13butyn-2-01. After a precipitate forms, mix the contents of the flask. After a few minutes, heat the contents of the flask to reflux, and reflux for 15 minutes. Transfer the reaction mixture to a 500 ml distilling flask, and add 100 5 of water to the reaction mixture. Distill the reaction mixture. Exercise care during the collection of the first 75 ml of distillate, since the distilling solution foams badly. Collect a total of 150 ml of distillate. Add approximately 100 g of potassium carbonate sesquihydrate to the distillate to salt out the p r o l . .\frrr sepnrati~~g Inyers, e x t n c t the aqueous h v n . w i t h 35 ml of b e ~ ~ e 311d u c ndd the benzene lover to the organic layer previously separated. Dry the benzene solution with approximately 8 g of anhydrous potassium carbonate. Filter and distill. The d e sired product distills between 138 and 141°C. Average yield is 10-15 g of a clear, colorless liquid. Preparation of Smicarbazone. The semicarbazone is prepared from 1 ml of the hydration product and a solution of 1 g of semicarbazide hydrochloride and 1.5 g of sodium acetate dissolved in 5 ml of water. To recrystallize the semicarbszone, add 5 5 of benzene to about one-third of the derivative initially formed. Heat the mixture and add 95% ethanol until all of the crystals dissolve. Cool and remove desired crystals by filtration.
~FIBCHER, F. G.,Chem. Ber., 76,735
(1943).
