Joseph Casanova, Jr. 10s Angeles State College LOS Angeles, California
Student Preparation and Manipulation of a Gas-Methyl
In an effort to incorporate some newer techniques and reactions into an undergraduate laboratory program, we have studied some experiments which we feel are of general interest. I t is the purpose of this report to note one such experiment. The experiment described here has been successfully employed early in a second semester organic chemistry laboratory and has been found to provide a useful gauge of the student's technique and ability. Standard laboratory textbooks usually include under the subject "preparation of ethers" a synthesis involving either acid catalyzed dehydration of an alcohol to give a symmetrical ether such as diethyl ether or a nucleophilic displacement of an alkoxide on an alkyl halide (Williamson synthesis) to give a symmetrical or unsymmetrical ether such as diethyl ether, dibutyl ether or anisole. The preparation of methyl ethyl ether (boiling point 6.6") ( 1 ) by a Williamson synthesis offers an excellent opportunity to demonstrate by
Ethyl Ether
analogy some of the principles of high vacuum l i e manipulation of gases without the actual employment of a vacuum or any elaborate or expensive equipment. The boiling point of methyl ethyl ether is sufficiently high to permit condensation and temporary storage a t ice-brine temperatures; but it is sufficiently low that i t is a gas a t room temperature, thereby presenting problems which the student would not usually encounterproblems such that a student who fails to anticipate in advance the requirements of the experiment may fail to obtain the ether. A high purity of product will result only when strict adherence to appropriate bath temperatures and distillation rates is maintained during the series of four successive bulb-to-bulb distillations used for purification. The instructor may verify weights or boiling points while the experiment is in progress or may elect to defer these observations until a later date, if this is done, student preparations may be stored in stoppered test
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tubes in a freezer for a long period of time. Quantitative analysis of student samples by GLC is recommended and under the conditions suggested here require only 3 or 4 min per sample for a complete analysis. The reaction of sodium methylate and ethyl iodide rather than the alternative sodium ethylate and methyl iodide was chosen for several reasons: the boiliig point difference between product and starting materials is greater in the former, favoring a purer product; CH30-Na+
-
+ C2HsI CHaOH CHIOC2HI+ NaI
(1)
availability of dry sodium methylate as a commercial preparation does away with student-handling of substantial quantities of sodium in an experiment requiring the use of several water baths; and the sodium methylate-ethyl iodide combination is more economical, for the total cost of the experiment is estimated a t about $0.65 per student. Safety Precaution. The product of this experiment i s volatile and highly flammable. For these reasons the experiment should be conducted in a well ventilated area i n which no flames or electrical apparatus are in use. Methyl ethyl ether has been frequently studied ( I , $ ) and prepared (3) but has seen little chemical use. In laboratories where student preparations would result in a substantial amount of sample this ether might he considered for research use in the extraction of exceedigly volatile substances. I n addition, the presence of two different simple alkyl groups on a basic oxygen atom suggests possible application to proton magnetic resonance studies of bonding in certain Lewis acidbase complexes. The Experiment
The apparatus shown in the drawing is assembled and dried by flaming. The flask is 200 ml; and four 2.5 X 20-em test tubes, each containing a boiling chip, are needed. A n additional safety precaution would be to insert a safety tnbz extending from the drying tube to the floor. When the flask has cooled to room temperature, a 6.0-grn (0.11 mole) portion of sodium methylate is placed in it. The flask is then quickly covered with 40 ml of anhydrous methanol and replaced in the apparatus. (Sodium methylate (sodium methoxide) is extremely hygroscopic. It is advisable to preweigh the appropriate amount of this reagent into test tubes which are then corked and stored in a large screw cap
Position No. 1
Position No. 2
Position No. 3
The Rask ir 200 mi; four 2.5 X 20-cm test tube,, each containing a boiling chip, are needed.
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bottle over Drierite. As a substitute 2.3 gm of sodium metal may he dissolved in 45 ml of anhydrous methanol, using external cooling. Due t o the hazards of handling sodium metal and the flammability of the generated hydrogen, this procedure is not recommended.) The heat of solution is dissipated by swirling the reaction vessel in an ice bath during the addition. When the sodium methylate has dissolved and the solution is cool, the flask is removed and 15.6 gm (0.1 mole) of reagent-grade ethyl iodide is added rapidly in one portion, to prevent the introduction of moisture. From this point onward, it i s essential that alljtamesin the laboratory be extinguished. Because of possible sparks. no electrical apparatus should be near the area i n which this preparation i s being conducted. The test tubes at positions No's. 2 and 3 are cooled by an ice-salt freezing mixture (-5 to - 10°C) and the reaction is swirled intermittently for 15 min. The reaction flask is then heated by a gentle current of steam until gas evolution becomes smooth. The rate of heating is adjusted so that slow refluxing occurs about half-way up the column.' After about 45 min, gas evolution will cease. The reaction flask and column are removed and replaced by a clean tared test tube. The freezing hath a t position No. 2 is then replaced by a warm water bath (45°C). The product will distill into the test tube a t position No. 3, leaving behind a substantial amouat of unreacted ethyl iodide and methanol. The test tube at position No. 2 is removed and replaced by a clean test tube. I n a smooth operation the test tube a t position No. 3 is removed with its freezing bath and placed in position No. 1,while the clean tared test tube which was a t position No. 1 is placed a t position No. 3. An ice-salt freezing mixture is placed around the test tuhe a t position No. 3; the freezing mixture is removed from the product a t position No. 1, placed around the test tube a t position No. 2, and the test tube a t position No. 1 is immersed in warm water bath (40°C). The product will distill smoothly into the test tube a t position No. 2, leaving behind a small amount of higher boiling material. When the distillat,ion is complete the cold hath is removed from position No. 2 and replaced by a warm water bath (35'C). Practically all the material will now distill into the test tube a t position No. 3. The drying tube is removed from the apparatus and inserted in a one hole stopper. The freezing bath and test tuhe a t position No. 3 are then removed, and the test tube is capped with the drying tube. This test tube containing the product may be wiped dry and weighed quickly without the drying tube to determine the yield. Two to four grams of methyl ethyl ether, a water-white liquid should be obtained. The boiling point may be observed as follows: Into a two-hole stopper a thermometer is inserted so that it will be partly immersed in liquid when placed in the ether, but will not touch the sides of the tube. The test tube is then removed from the ice-salt bath, wiped dry and insulated a t the bottom with several thicknesses
' We recommend a. simple distilling column packed loosely with stainless steel sponge, as described in F ~ e s ~ L. n , F., "Experiments in Organic Chemistry," 4th ed., D. C. Heath and Co., Boston, 1957, p. 14.
of dry paper towel. The tube is held so that it is not heated by the hands and is agitated continuously under a hood while the temperature is observed a t 30 sec intervals until constant for a t least 3 min. This value should be recorded as the boiling point of the liquid. If the sample is to be turned in to the instructor, the thermometer is removed and replaced by the drying tube; then the sample is placed in a dry-ice bath provided for this p u r p o ~ e . ~
% T othe instructor: Methyl ethyl ether may be quantitatively analyzed for purity by gas-liquid chromatography. All components are rapidly and cleanly separated by a column 18 X I/, in of tricymoethoxypropane, TCEP, (4-6), on firebrick at 25". Retention volumes are as follows: methyl ethyl ether, 24.8 ml; ethyl iodide, 135 ml; methanol, 315 ml. Careful regard far detail in this experiment will produce a product 92-95% pure (by GLC) at s. yield of 50% of theoretical. A Perkin-Elmer model 154D Vapor Fractometer was employed for these analyses.
Questions
1. Why is it necessary to exclude moisture from the reaction? Illustrate your answer with an equation. 2. What is the side reaction product which would result from elimination rather than substitution? Write the equation. How would you detect this material? Literature Cited (1) IPATIEFF, V. N., AND BURWELL, R. L., J . Am. Chem. Soc., 63,969 (1941). 12) . . BERTAOUD. . A.,. A N D BRUM, . R.,. J. Chim. Phys., 21, 143-60 (1924). (3) A. W.. Ann.., 81.77118521: . , WILLIAMSON. . . ., DoBRINER.P.A~~., 243,~ (1888). ' (4) BRUSON, H. A,, A N D RIENER,T . W., J. Am. Chem. Sac., 65,23 (1943). (5) CASANOVA, J., JR., A N D COREY,E. J., Chem. and Ind., 1941, 1664. (6) MCNAIR,A. M., A N D DEVRIES, T., Anal. Chem., 33, 806
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