Catalytic Dehydrogenation of Tetrahydrlocarbazole
James E. Van Verth and Simon W. Ulmer Canlslus College Buffalo, New York 14208
An organic laborato~yexperiment Although dehydrogenation is a process of considerable importance i n t h e synthesis of aromatic molecules ( I ) , exneriments i n v o l v i n ~its use a r e rarelv included i n oreanic iaboratory textboo!&. Presumably a major reason for &is is t h a t dehydrogenations at conventional laboratory temperatures usually require reaction periods which extend far beyond the few hours p e r week allotted to a sophomore laboratory. This paper describes a simple experiment i n catalytic dehydroeenation which h a s been modified so that it can he completkd within the limits of t h e usual 3 or 4-hour laboratory period. T h e reaction, conversion of 1,2,3,4-tetrahydrocarbazole to carbazole, is of additional value in t h a t it illustrates t h e synthesis of a common heterocyclic system.
In considering the experimental conditions needed for catalytic dehydrogenation, it is helpful to recognize the fact that the reaction is simply the reverse of hydrogenation
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hydrogenations, as a result of their endothermicity, have rather high activation energi??. Moreover, a t higher temperatures, owing to the increased equilibxium constant and the decreased solubility of hydrogen, the equilibrium concentration of hydrogen is more likely to be beyond the limit of its solubility in the medium used. Di- and trialkylhenzenes having hoiling points of 140'C or more are often employed as solvents. In some cases, the reagents are heated together without solvent to temperatures as high as 375%, or, in gas-phase processes, sometimes much higher. The amount of catalyst used can be an important consideration, since rates of hydrogenation and dehydrogenation are, as arule, denendent on the amount of catalvst. The reaction rate is also dependent on the activity of the catalysr, which may be a tuncfmn of how it :s ~ r r ~ a r eThe d . prprenee 131impuritw in the reaction mixture which paism the catalyst wll lower its activity; even n m k a l l s pure rengenU and solvents may contain traces of catalyst poisons. Homing, Homing, and Walker (2) found that a 95% yield of carhazole could be obtained after only 2 hr refluxing of a solution of tetrahydrocarhazole in redistilled trimethylbenzene (b.p. 168-172°C) with their awn preparation of 5% palladium-on-carbon. With commercially available catalysts and solvents, however, our experience is that, under similar conditions, yields approaching complete conversion can he olnained only aiter a much longer reaction time. Slmllnr results were indicated by Campaignr and Lake 131,who reportrd a 91% yield of rart,arde after I? h r in refluxing xylem t h p. ; 140°C). Asuhstantial reduction in the time can,-however, be achieved bv em~lovinea relativelv laree " amount of eatalvst. With a 1:2 catalyst-substrate ratio andp-cymene (b.p. 17IDC)as solvent, a reasonable eonversion(-75%) can be ohtained within 1%hr.
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gcnation, rht transition m a d s uf Group VIII, in panicular pallnd~um, platinum. rhodlum, and nickel, are also rffectivr for dehydrwgrnnt~on. Palladium, usually supported on carbon, has been most often used. Since addition of hydrogen to a carbon-carbon double bond is exothermic by about 28 kcal mole-', dehydrogenation is, in itself, a thermodynamically unfavorable process. The accompanying positive entropy change of approximately 30 cal male-' OK-' is, however, a mitigating factor, and, where only one double bond is needed to complete an aromatic r-system, the resulting increase in resonsnee energy may he sufficient to provide a negative AG. Nevertheless, in the most common applications where at least two new double bonds are introduced, the reactions are significantly endergonic. The success of these reactions, then, usually depends on displacement of an unfavorable equilibrium. This is accomplishedmost simply by allowing the liberated hydrogen to escape into the atmosphere. The use of a vigorously boiling reaction mixture aids the expulsion of hydrogen from the solution, and may he essential to the success of a dehydrogenation. Sweeping the atmosphere ahove the reaction mixture with a slow stream of nitrogen or other inert gas is also helpful, and the time required for complete dehydrogenation is often considerably reduced by the use of this technique. A high reaction temperature is usually necessary, since most de-
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Procedure Into a 25- or 50-ml round-bottomed flask2 are placed 1.00 g of 1,2,3,4-tetrahydrae~rh~z01e ( 4 ) , 0.5 g of 5% PdIC, and 10 ml of p cymene. Boilingstones are added and the mixture is refluxed gently for 1.5 hr aver a mierohurner flame. with occasional swirline to wash down carnlysr depmited on the flask walls.'l'he flask a thenallowed tu cool: 10 ml of erhyl ncetnw IS added and the mixturr is uarmrd gently, if necessary, to diswlve rhs preripitntcd carl,ardc. 'l'hr carnlyrt is removed hy vacuum tilrrat~on,usmg a rarefully fitted double (Present Addrers: Pioneering Hesearch Laboratory, E. 1. duPont deNemours and (.'ompany, Experimental Station, Wilmington. rielaware 19898. 2A 25-ml flask is perhaps preferable, hut the 50-ml size found in most standard-taper glassware kits was used in the studies reported here, and was found to he quite adequate. Since the flask is well under half full, the burner flame should be adjusted carefully to avoid superheating the flask walls ahove the surface of the liquid; the use of ashield of asbestos hoard with a small hole cut in the center may be advantageous, hut is not necessary if reasonable care is taken.
Volume 54, Number 6, June 1977 / 383
Table 1.
Dehydrogenation of Tetrahydrocarbarole (1.00 g l to Carbarole
Table 2. Solubilitv of Carbazole in 1 0 ml of Various Solvents Solventa
Catalyst,
Run
Solventa
Time, hr
g
XYlB"B xylene xyiene meritylene pcymene pcymene g-cymene I)-cvmene
Wash Liquid
Yield. %b
95% E t O H
95% EtOH 95% E t O H 05% EtOH
90% methanol
8546 methanol
0.120 0.070 0.100, 0.095C 0.070. 0.080C O.O1O.O.OIOC
OPercentager are by volume. ~ L O S Sin weight of excess pure carbarole after being allowed to stand for some time in contact with 10 ml of the rolvent, with occasional agitation. CDuPlicate determinations.
95% EtOH 85% MeOH 85% MaOH 85% MeOH
a10 ml. All reacttons were run at reflux. Solvent boiling points: xylene (isomer mixture). -140°c: meritylene. 1 6 5 " ~ ; ~ - c y m a n e . 17+" A,,
95% ethanol
85% ethanol 100% methanol
Carbazole Dissolved. eb
-.
b ~ h melting e points of the products from runr 1-4 were several degrees lower and of wider range than those of the products from runs 5-8, a l l o f which melted, within a range of 2- above 2 4 4 ' ~ . except for that of run 7 , which naa m.p. 243-z4sdc. CYieids reproduced within 3% in duplicate runr. d ~ i e i d 6reproduced precisely in duplicate runs.
filter paper.3 The reaction flask is rinsed with 2-3 ml of ethyl acetate, and the rinsings are poured through the filter. Ethyl acetate is removed from the filtrate by evaporation until it has been reduced to a volume of 10 ml or less.4 T o further reduce the solubility of the carbazole, 10 ml of low-boiling petroleum ether is added to the cooled mixture. The slush of crystals is filtered with suction, washed with petroleum ether, and dried by passing air through the filter for afew minutes. The resulting crude solid is removed t o a small beaker and stirred well with 10 ml of 85% ("1") methanol; the product is colleded, washed on the filter with a little more 85- methanol, and air-dried. About 0.75 g (77%) of earhazole melting above 2 4 4 T within a Z0 range, is obtained. I t may be recrystallized, if desired, from aylene or p-cymene. When this proeedure was originally employed in our undergraduate laboratory using xylene as a solvent, i t was found that a high catalyst loading could indeed provide enough carbmle in a short time to make it a useful stcdent experiment. The incomplete conversion proved not to be a serious difficulty, since the unchanged starting material could be removed by washing the dry product with ethanol. Student yields weresometimesquite low, however,and could, in fact, be reduced to zero by overzealous washing. A subsequent, more detailed study of the reaction, summarized in Table 1, revealed that, as expected, the use of higher-boiling solvents markedly increased the conversion, p-cymene being preferred to ~
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Whatman No. 1paper is suitable. If two fdter papers are tied, and care is taken that the edges are kept well sealed, contamination of the product with catalyst, which gives i t a grayish appearance, will be avoided. To prevent loosening of the paper, the vacuum should be kept on a t all times during the filtration and washing. The mixture should be poured slowly through the filter to minimize foamina of the filtrate. 'This evaporation can be accomplished very rapidly near room temoerature if the filter flask is fitted with a clean.. emntv funnel whose stem exlends to a point near the surface ofthe liquid. A No. 2 Buchner funnel and s 123-ml filter flask make a auitahle ambination. With the side arm of the flaskattached to the aspirator, air isdrawn in through the funnel while the flask is warmed gently on a water bath or hot plate. 5Ahout $0.3710.5 g a t January 1976 prices.
mesitylene on the basis of cost. The use of a wash solvent in which carbazole was less soluble was also advantageous; 85%methanol was chosen on the basis of the solubility studies shown in Table 2. For reactions on the scale described here, catalyst cost per student is not high.5 If desired, the amount of catalyst can, as shown in Table 1 (Run 7) be reduced almost in half without much sacrifice in yield. Run 8 shows that there is no great advantage in extending the reaction time beyond that recommended. The catalyst used in the presently described work was obtained from Englehard Industries. Yields of carbazole may vary somewhat with different sources or batches of catalyst, but our yields in repeated runs with the same catalyst were quite reproducible. When this experiment is assigned, additional interest may be generated by dividing the class into groups using differing amounts of catalyst andlor solvents of different boiling paints. Same students may wish to examine the effect of sweeping the apparatus with an inert gas. The starting material for the experiment, 1,2,3,4-tetrahydrocarbazole, is readily prepared by the reaction of cyclohexanone and phenylhydrazine in refluxing glacial acetic acid, as described by Rogers and Corson (4). Preparation of sufficient material for alarge class demands a minimum of time and skill from a laboratory assistant. The user of the Rogers and Corson procedure, however, should he alerted to a fact not noted therein. As pointed out by Campaigne and Lake (31, melting points taken in an open capillary are an unreowing t o their rapid liable guide to the purity of tetrahydr-bmoles, autonidation to hydroperoxides a t elevated temperatures (5). The melting point of tetrahydrocarbazole is reported (4) to vary from 113 to 118"C, depending on the rate of heating. In an evacuated capillary, however, the compound melts sharply (3) a t 118.5-119.5PC. The instructor may wish to assign to his class the synthesis of tetrabydrocarbazole, as well as its dehydrogenation. This provides an interesting two-step sequence in which a tricyclic aromatic compound is prepared from monocyclic starting materials. In addition, it illustrates an application of the Fischer indole synthesis, and the technique of sealed capillary melting points (6).
Literature Cited
.,
384 1 Journal of Chemical Education
Campaigne.E., and Lake,R. D.,J Org Chem., 24.478 (1959). (0 Rogers, C.U..and Corson. B.B.,"Orpanie Synthesaa? Coll. Vol IV. John Wilcy and I31
Sonn. he.. New York. 1963. o. 884
Ca.. Lexington. Mass.,1975, p. 41
