Irving Allan Kaye
Brooklyn College City University of N e w York Brooklyn, N e w York 11210
II
Catalytic Hydrogenation of Ketones at Moderate Pressures An organic demonstration-experiment
Although catalytic hydrogenation at elevated pressures is used extensively in laboratory and industrial processes1 and many such applications are discussed in introductory organic chemistry lectures and texts, the operation seems remote to the student in the course since, for practical and economic reasons, it is usually not brought to life by a demonstration or laboratory experiment. The procedure described below requires a moderate pressure hydrogenator2 and can be performed in most universities and colleges where an apparatus of this type is usually available for use in an advanced organic chemistry laboratory course and/or research. In practice one or more students [but no more than about 8 per instructor] are asked to reduce any one of several ketones to a secondary alcohol on a scale much greater than that usually encountered in the introductory laboratory. Students with better laboratory skills and an interest in laboratory work are to be preferred for performing this exercise. Organic chemistry majors especially should profit from the experiment since training in this technique should be included in their background. For the purpose for which it was devised this procedure has several merits. The starting ketones, the 'GROGGINS,P. H. (Editor), '(Unit Processes in Organic Synthesis" [5th ed.], McGraw-Hill Book Co., Inc., New York, 1958, Chapter 10. For a description of a moderate pressure hydrogenator and for directions in using the apparatus, cj. ADAMS,R., AND VOORRam, V., "Organic Syntheses," Coll. Vol. I [2nd ed.] (Editor: GILMAN, H., A N D BLATT;A. H., John Wiley & Sans, Inc., New York, 1941, pp. 61-7, and "Shaker Type Hydrogenation Apparatus," Specification No. 3900, Parr Instrument Co., 211 53rd St., Moline, Ill. 61265. I t is advisable to place s. safety shield in front of the hydrogenator and to require the student performine the emeriment to wear safetv elasses. : H:int.y No. 2\ activr nickel ~:1131?.91 i n water may he ohC Cxtaly;t I)ivwon, W. Ii. (;rare end (:"., tninrd i n m I ~ L l:n~.cy 021 liilrniltm Sntiunzl Iluuk Ht~ilding, (:hn~tunmgn, 'l'cnn. :47t01 -.
4 K ~I. ~A,, ~AND, YUSKA,H., an experiment for the introductory organic chemistry laboratory to be submitted for publication shortly. If the alcohols are not needed for this purpose, the quantity of ketone employed in the reduction can be ss little as 0.1 mole. The amounts of catalyst, triethylamine, and methanol required in the hydrogenation can then he reduced to about '/&h of those given in the experimental section and the reduction will be completed in a.sharter time interval. 5Rubber stoppers which come into contact with the catalyst should not contain sulfur in free or combined form as this would poison the catalyst. To remove such catalytic poisons, heat the stopper in a strong aqueou solution of sodium hydroxide on a steam bath for about an hour and then wash well with water. V l r r s m , L. F., "Organic Experiments" [2nd ed.], Raytheon Education Ca., Lexington, Mars., 1968, pp. 30-31. 'K h m , I. A,, Chemist-Analpl, 54, 56 [1965].
Raney nickel catalysta and other reagents are inexpensive and readily available. The products, which are liquids readily separated from the reaction mixture and purified by distillation at atmospheric pressure using equipment which is usually found in the student's chemistry locker, are obtained in high yield. The fire and explosion hazards involved in handling a pyrophoric catalyst and a combustible gas under pressure are minimized by the manner in which the reaction is carried out. Reduction time is shortened considerably by displacing the water adhering to the catalyst with methanol prior to hydrogenation. Finally, the products may be used as unknowns in an experiment involving the identification of an unknown alcohoL4 Experimental Transfer 7 level teaspoons [about 35 g] of No. 28 Raney active nickel catalyst3 to a 500-ml Parr hydrogenation bottle9via a small powder funnel. Wash the catalyst into the bottom of the hydragenatian bottle with di'tilled water. Let the finely divided metal settle for a t least 13 min and then remove most of the water by cautious decantation into a beaker; remove as much of the remaining liquid as possible by aspiration with a glass dropper. Add 200 ml of methanol, stopper the bottle with a rubber stopper,6 and then suspend the nickel in the solvent by inverting the bottle twice. I h not sheke or invert the bottle more as the c a t d y d becomes deactivated on reaction with oxygen. Let the catalyst settle for a t least 10-15 min before removing as much methanol as possible by aspiration. Repeat this washing procedure twice more. Transfer 90-110 g of the ketone, weighed in a 125-ml Erlenmeyer flask, to the hydrogenation bottle containing the catalyst. Effect a quantitative transfer by washing it into the bottle with 20 ml of triethylamine. Set up the bottle in the hydrogenator and evacuate it until the liquid in the reaction-mixture starts to boil, then fill i t with hydrogen. Repeat this sequence a t least twice more and then shake the mixture, a t room temperature and an initial pressure of .59-60 psi, until hydrogen is no longer absorbed; ahout 2-5 hr is usually required. Stop the shaker, allow the catalyst to settle, shut the vslve to the hydrogen reservoir, evacuate the hydrogenation bottle and finslly replace its hydrogen atmosphere with air or nitrogen. Filter the mixture, using gentle vacuum, through a 2-3 mm layeyer of a. diatomaceous earth supported on a. piece of filter paper in a No. 2 Bnohner funnel, collecting the filtrate in a 500-ml r.b. Bark with a standard-taper joint. Donot a l l w the catalyst to besucked dry or it will eatehpe! Wash the hydrogenation bottle, funnel and catalyst Alcohols Obtained b y Hydrogenation of Ketones Al0ohol
2-heptsmol 3-heptad 4-heptad &methyl-2-hexanol 5-methyl-3-heptsnol 2-nonanon~l 2-undeoanol 2-trideohnol I-phenyl-2.propanol
BP. 'C
% Yield
15F158 153-155 151-153 151-153
90
192-195
89 93 93
161-165
228-229 2.56261
208-211
94
87 90 91
95
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several times with methylene chloride,8 exercising care during the filtration to keep the nickel moist with solvent a t all times. Distill the filtrate slowly, using a fractionating column in the distillation assembly'and heating with ashielded semimicro burner,' until the vapor temperature reaches about 75'C when the diitillation rate slows perceptibly. Continue heating with a Tirrilltype burner until the vapor temperature reaches about 115°C. Cool the liquid remaining in the distilling flmk to 40' or lower by immersing the flask in an ice bath. Then transfer the liquid, using methylene chloride to effect a quantit,ative transfer, to a. 2.50-ml distilling f l a ~ k . Distill slowly, using no fractionating column or condenser, collecting the product over 8. 2-4- range. Yields and boiling points of nine alcohols prepared by this method are listed in the table.
The Experiment as a Laboratory Demonstration
The author conducts this experiment in the following manner. The student preparing the alcohol is provided with a hydrogenation bottle and shown how to transfer the catalyst from its container to the reaction vessel. This operation, as well as the subsequent washing with methanol, is carried out on a desk which is covered with newspaper, to contain any spillage, and provided with a plastic washbottle containing water. The student is alerted to the fact that the catalyst is pyrophoric when dry and if spilled, the affected area should be sprayed with water and then transferred, with its underlying paper, to a beaker for subsequent solution of the metal in concentrated hydrochloric acid.a He is also advised to leave the hydrogenation bottle a t all times, until it is placed in the hydrogenator, in a tall tin can or beaker which is just wide enough to keep it upright. After the ketone and triethylamine have been mixed with the catalyst, the entire class is assembled around the hydrogenator. The instructor points out the essential features of the apparatus and then attaches the hydrogenation bottle, replaces its air with hydrogen and adjusts the pressure to 59-60 psi.% The student assigned the preparation then starts the shaking mechanism after noting the initial time, pressure, and temperature. The other students usually remain a few minutes longer, watching the movement of the pressure gauge pointer as hydrogen is absorbed by the system, before returning to their own laboratory work. Afterwards they are kept informed of the progress of the hydrogenation by means of a table, placed on a centrally-located blackboard, in which a record is kept of the elapsed hydrogenation time, pressure drop, and temperature. At the next meeting of the laboratory section, the class
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reassembles about the hydrogenator, and the student assigned the preparation, following the directions of the laboratory instructor, replaces the hydrogen atmosphere in the hydrogenation bottle by air or nitrogen and then removes the vessel from the hydrogenator. Following prior instructions, immediately thereafter he washes down its inner walls with methylene chloride8 using a glass dropper [to prevent any of the catalyst adhering to the sides of the bottle from becoming dry and possibly igniting] and then stoppers the bottle. The latter, in its protective tin can [or beaker] carrier, is taken to a desk which is covered with newspaper and provided with a plastic wash bottle containing water. At this point the instructor informs the class of the fire hazard attending the use of catalysts and demonstrates the separation of the catalyst by filtration. Emphasis is placed on insuring that the catalyst is not sucked dry nor spilled during the filtration. The students are told what steps are to be taken in the event of an accident and are shown the catalyst in its original wet, safe state. Then, arter the filtration has been completed they are treated to a pyrotechnic display which usually impresses them more than words of caution regarding the pyrophoric character of the catalyst. The Biichner funnel, containing the washed nickel, is placed atop another filter flask and the catalyst is washed two or three times with methanol [to remove the nonflammable methylene chloride] and then briefly subjected to the full force of a vacuum filtration, until no solvent drops are expressed from the metal. The instructor, wearing gloves, immediately thereafter dumps the metal into a small tin can whose inside diameter is somewhat larger than the outside diameter of the Biichner funnel. The can is placed inside a much larger can which is then left on the floor of the laboratory exposed to the air in full view of the students. Usually the nickel begins to smolder within 30 min and then bursts into flame. The display is quite drama ti^.^
aAlthough methanol or ethanol may he used for washing the catalyst, methylene chloride is generally a better solvent. Its use as a. wash solvent has the added advantage of reducing the fire hazard since its vapor is not flammable and, when mixed with air, is not explosive. Waste catalyst is usually disposed of by pouring i t into adilute mineral acid solution.
