Chester B. Kremer and Samuel H. Wilen The City College New
York
1 I I
Apparatus for the Friedel-Crafts Reaction -For the undergraduate organic chemistry laboratory
The Friedel-Crafts alkylation reaction is a standard experiment carried out in nearly all nndergraduate organic laboratory courses. The reaction and its modifications clearly illustrate electrophilic aromatic substitution. The standard set-up usually employed in FriedelCrafts alkvlations consists of a flask connected to a reflux condenser, with a two-hole cork inserted in the top of the condenser to provide for a dropping funnel and a gas trap takeoff. The heterogeneous reaction mixture is agitated by shaking the entire set-up or by partially breaking the reaction flask away from its connection with the reflux condenser and shaking the flask alone. The first method endangers the gas trap and dropping funnel atop the condenser; the second invites the escape of hydrogen halide gas. A simple and practical reaction apparatus has been designed which offers the following desirable qualities: compactness and stability, efficient stirring, ease of temperature control, moderate capacity, and an efficient gas trap. The apparatus consists of a 38 X 200 mm horosilicate elass test tube fitted with a cork holding a dronoiue funnel, a manual stirrer, and an attachment to a suction flask gas trap. The stirrer is made from 4-mm. glass rod in two pieces held together by a small section of tight-fitting gum rubber tubing. The location of the rubber tubing is such that it prevents the stirring circle from striking the bottom of the tube on a downward stroke. At its point of furthest descent, the stirring circle is within '/n inch of the bottom of the tube. The plane of the stirring circle is a t right angles to that of the rod shaft. The clearance of the stirring rod through the 7-mm glass tube bearing allows for a rapid up and down motion without the use of a lubricant. The suction flask serves as a trap for any hydrogen halide gas evolved. The inlet tube terminates about ' / r inch above the surface of water in the flask. When the apparatus is functioning, slight suction from a water aspirator produces a gentle sweep of hydrogen halide gas from the reaction tube into the trap. Xone of the gas escapes through the stirrer during a reaction due to this suction. In reactions where no hydrogen halide gas is evolved, as in the preparation of cyclohexylbensene from cyclohexene, benzene, and sulfuric acid, the gas trap is dispensed with. The dropping funnel is then set in an outside hole of the cork and a thermometer in a center hole if a more accurate measure of the reaction temperature is desired. Temperature control in many of the alkylations is effected simply by the cooling action of a beaker of ice u
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Journal of Chemical Education
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and water, brought up around the reaction tube when the temperature begins to exceed 30°. An approximate temperature of 30' can be readily noted without the use of a thermometer: it is the temperature that first imparts a sensation of warmth to the touch of the hand. Since most of the alkylations proceed readily below 30' there is no need for a reflux condenser. Refluxing a t any stage mould probably increase the amount of polysubstitut,ion. Reaction volumes up to 90 ml are readily accommodated in the large test tube. Good yields are obtained in various alkylations by employing reaction volumes varying between 60 and 90 ml. The described apparatus has been used successfully in these laboratories for Friedel-Crafts alkylations to prepare: Cyclohexylbcmzene from either cyclohexene or chlorocyclohexane reacting with benzene in the presence of either concentrated sulfuric acid or anhvdrous aluminum chloride as catalyst. Tertiary bzitylbenzene from either tert-butyl chloride or tert-bntyl alcohol reacting with benzene and using anhydrous aluminum chloride as catalyst. Diphenylmethane from benzyl chloride and benzene with anhydrous aluminum chloride. Tertiary butyltoluene from either tertbutyl chloride or tert-hutyl alcohol reacting with toluene in the
presence of either anhydrous aluminum chloride or concentrated sulfuric acid as catalyst. The preparation of tert-butylbenzene' is described here to illustrate the use of the apparatus and to present a Friedel-Crafts modification that is ideally suited for the undergraduate organic laboratory course. Prepare the apparatus as shown in the figure. The large test tube, the dropping funnel, and the stirrer assembly must be dry. Place 11 g of powdered anhydrous aluminum chloride and 60 g of dry benzene in the tube. Cork the tube, attach the gas trap, and apply slight suction to the flask from a water aspirator. Now place 12.5 g of dry tevt-butyl alcohol in the dropping funnel and add it dropwise t o the vigorously stirred mixture in the reaction tube. Test for approximate temperature by placing the hand against the reaction tube. If a.sensation of warmth is imparted
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HIISTON,R., Fox, W., (1938).
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BINDER, M., J. 079. Chern., 3,251
to the hand, cool the tube by surrounding it momentarily with a mixture of ice and water contained in a beaker. Repeat this operation when necessary during the course of the reaction. After all the alcohol has been added ( I s 2 0 minutes), allow the reaction mixture to stand a t room temperature, with occasional stirring, for one-half hour. I n a. hood, disconnect the reaction tube and carefully pour its contents, with stirring, into a. slurry of 50 g of concentrated hydrachlorio acid and 50 g of ice. After the ice has melted, separate the benzene and aqueous layers in a separatary funnel, extract the aqueous layer with 25 ml of diethyl ether, and combine the ether extract with the heneene layer. Wash the combined organic solutions with 35 ml of a saturated sodium bicarbonate solution (Caution! Pressure may develop in the separat a w funnel due to evolved carbon dioxide eas). - .. s e.~ a r a t ethe organic layer, and wash i t with an equal volume of water. Dry the hensene-ether solution aver 3 4 g. of anhydrous calcium chloride. Fractionate the dried liquid and collect the fraction hailing between 165-172" as tert-butylbenaene. Yield, 11-14 grams.
Volume 38. Number 6, June 1967
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