The Catalytic Hydrogenation of Methyl Oleate by In Situ
Ben Plummer Trinity University, San Antonio. TX 78284 A pedagogically useful heterogeneous, catalytic hydrogenation experiment transforms methyl oleate into methyl stearate,'
Students are required to add sulfuric acid to zinc a t a coutrolled rate and pass the hydrogen gas through the methanolic solution of catalyst and reactants, which is sealed from the air hv a mineral oil bubbler. We have modified this experiment as shown in the figure. The apparatus is patterned after a commercial design suggested by Brown for in situ hydrogen g e n e r a t i ~ n . ~The .~ basic requirements are the availability of a student-prade magnetiE stirrer and the purchase of-commonly available "helium-grade" balloons, which retard helium loss. When a n alkaline solution of sodium borohydride is neutralized slowly with acid, molecular hydrogen can be generated a t a controlled rate by the decomposition of sodium horohydride as i t reacts with the hydroxylic solvent. The reaction can be visualized as a stepwise process to generate four molar equivalents of molecular hydrogen, NaBH4
+ ROH
-
NaOR
+ Hz + BH3
(2)
where R = H or CHs. As the reaction proceeds the solution can become basic, and i t is important to have enough ptoluene sulfonic acid vresent so that all of the available hydrogen will be liberaied.~hestudentshouldadd the borohvdride solution at a slow. drovwise rate surh that the ballion is kept barely inflatkd. Leaks in the system can be determined by generating hydrogen while the stirrer is turned off. If the balloon remains partially inflated for 30 s, leaks will not be simificant. The student must always turn the stopcock off &er each addition because outgassing through the liauid in the buret can occur. The buret must be tightl; clamped or the stopper tightly wired because gas pressure can cause i t t o pop up instead of inflating the balloon. Students should be cautioned that noble metal catalysts enhance air oxidation of alcohols and hydrogen and there is the ootential for a fire or e x n l ~ s i o nStirrine .~ should be moderate so that no catalyst is scattered onto tce upper surface of the flask. If this h a ~ v e n s occasional . "suarkina" phase ieactions can he observed as rapid cat&ic'vapor occur. This reaction should be run in the hood with the
safety window down and all safety precautions rigorously enforced including the use of safety goggles and laboratory aprons. Experimental Stabilized sodium horohydride is prepared by dissolving slowly the required amount of powdered sodium horohydride in a previously prepared sodium hydroxide solution. This solution may he stored in a oolvethvlene . - , container (not tiehtlv .. . sealed) for several weeks with only slight deterioration. For a class of 50 student5 19 g (0.5 mol) of sodium Ilorohydride should be dissolved in 1000 mL of an aqueous solution containing 3 g (0.075 mob ofsodium hydroxide. Each student should normally use no more than 20 mL of this Assemble the apparatus as shown uarng a 25.00-ml. graduated bum inserted through a rubber stopper that will fit tightly in the mouth of a 25Wmi. vacuum filtration flask. Arterh a 1-m.-long segment of standard rubber tubing tightly to the side arm of the flask, and slip the mouth of the balloon over this mbher tuhing so that the mouth of the balloon also surrounds the side arm. Seal the the sidearm hv twlstine conner mhher hnlloon and tnhine" tiehtlvto . . . . . ~~"~~~ , .. wire around the seal. Place a magnetic stirring hnr in the lruttum of the filter flask,but donot turnw themagneticstirrer.Sr1 theentire apparatus on a ring stand so that both buret and flask can he tightly &mped while mounted on the magnetic stirrer. Carefully weigh a 10-mL graduated cylinder, and add sufficient 70% methyl oleate so that about 4.0 g of methyl oleate will be obtained. Pour this quantity of methyl oleate into the 250-mL filter flask. Using 80 mL of methanol measured in a graduated cylinder, rinse with several small portions of methanol the residual methyl oleate out of the cylinder in which it was measured into the 250-mL filter flask. To the mixture of 80 mL of methanol and the dissolved methyl oleate contained in the reaction vessel, add quickly, in one hatch, between 0.1 and 0.2 g of previously weighed 10%Pd/C catalyst. Do not turn on the stirrer. Next, add to the methanol 3.0 g ~
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Pavia, D. L.; Lampman. G. M.: Kriz. G. S. lnircduction to Organic Chemistry Laboratow Techniques: 3rd ed.: Saunders: New York, 1987; pp 102-106. Brown. H. C. hydmbwation: Benjamin: New York, 1962. Brown. H. C.; Brown, C. A. J. Am. Chem. Soc. 1962,84.1495. 'Brown, H. C.; Sivasankaran, V.: Brown, C. A. J. Org. Chem. 1963, 28~ . , 214~ - . ..
Rylander, P. N. Hydrogenation Methods; Academic: New York. 1985.
518
Journal of Chemical Education
Apparatus for catalytic hydmgenation using in balloon ass pressure control device.
sit" hydrogen generation and a
(0.015 mol) of p-toluenesulfonic acid manohydrate. (Caution: This is a strong acid and should be handled with care.) Now tightly insert the buret and rubber stopper into the mouth of the filter flask, and clamp the buret and flask firmly together so that they cannot separate during the pressure increase that will occur. Carefully pour into the buret (caution: he sure the stopcock is turned off) about 20 mL of stabilized sodium horohydride that is 0.5 M in NaBHn and 0.075 M in NaOH (caution: this caustic solution will cause burns to skin and should be handled carefully). Turn on the stirrer to medium speed so that excessive splashing does not weur. Carefully add several drops of 0.5 M NaBHl from the buret, being sure to turn the ~tnncockoff after each addition. Watch for the balloon to inflate sl"wlv. Trim off the. .... .,~ ~~-stirrer. If it takes loneer than 30 s for the balloon todeflate, your nppara1u.i is rrasonnhly gas tight. If the hallwn will not :nflate,ask yrur instructor to help youcheck for leaka.C'untinur mndd dropaise with stirring the NaBH, solutim durinl: a prriod of 1h. If, after the addition of 2 or 3 mL of solution from the buret, the balloon does not deflate, you may have to stop the experiment and add a fresh batch of catalyst. (Confer with your instructor before vou do this.) Catalvst can occur if sulfides or grease have . oaisonine . EontaminaGd the r ~ t ~ l yor s tf i t is an old hatch. based upon your calculation of the moles of hydrogen needed for the hydrogenatim of methyl oleate, you should be able to ralrulste the ml. of 0.5 M
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NaBHl required to generate the correct quantity of hydrogen. I t may require an additional 2 or 3 mL of borohydride solution beyond the stoichiometric value to compensate for loss of hydrogen gas during the experiment. After the reaction is complete, turn off the magnetic stirrer, carefuUy disconnect the buret and stopper from the flask, remove the balloon, and return it to the instructor. Prepare a large Buchner funnel with a layer of Celite (Filter Aid), depositing it from a methanol slurry. Discard the liquid in the filter flask collected as a result of creating the slurry pad. Reassemble the filtration apparatus and rapidly suction filter the reaction mixture through the Celite pad to remove the catalyst. Rinse the pad carefully with several smdl portions of warm methanol (combined volume no larger than 10 mL). Some product may crystallize in the filtrate at this ooint. Remove the Buchner funnel, and warm the filtrate gently on a hot plate or steam hath. When the filtrate is bomogneous, carefully transfer it to a 250-mL Erlenmeyer flask, and slawly add dropwise warm water until the solution becomes cloudy. Cool the resulting mixture in an ice-water hath until complete crystallization of the methyl stearate has occurred. Collect the product by vacuum filtration using a small Buchner funnel. After allowing the product to dry, weigh it, determine its melting point (literature value 3g°C), and calculate the per cent yield based upon the amount of methyl oleate calculated to he present in the 70% mixture.
Volume 66
Number 6
June 1989
519
