HYDROGENATION OF CYCLO~CTATETRAENE AND AROMATICS
Aug. 5, 1957
The total olefin obtained in this way, after recrystallization from acetone and from ether-methanol, melted at 85.887", I ~ ) D+6". The infrared spectrum indicated contamination by small amounts of cholest-&ene, which could not be removed by repeated crystallization." T h e product was finally rechromatographed on alkaline alumina and, after ~ free several passes, a sample, m.p. 86-87', [ a ] +11.7', of cholest-6-ene was obtained. Cholest-8: 14-ene, m.p. 53.5-55', [ a ] +20° ~ ( c 1.0, chloroform) was prepared from cholest-7-ene by isomerization over palladium (hydrogenation conditions) as de,cribed by Eck and H o l l i n g s w ~ r t h . ~ ~ T h e J~hydroxy-A1~-cholenic acid employed in this investigation was a purified sample, m.p. 165-166', of niaterial prepared some years ago a t the laboratories of Merck and Co., Rahway, XI J. Acid Isomerization a t %'.-A solution of the olefin and an equal weight of p-toluenesulfonic acid in a mixture of acetic acid and cyclohexane (4: 1, 6 ml. of solvent for 30 mg. of olefin) was heated to reflux temperature in an electrically heated oil-bath in a flask fitted with a thermometer extending below the surface of the solution. At the end of the selected heating period, the solution was poured into dilute sodium hydroxide, and the aqueous layer was extracted three times with petroleum ether. The combined organic fractions were then washed with dilute alkali, water, saturated sodium chloride and filtered through anhydrous magnesium d f a t e . After evaporation of the solvent, the product was twice dissolved in carbon disulfide and taken t o dryness in order t o ensure removal of the last traces of hydrocarbon solvent prior to infrared analysis. ~
(38) Cremlyn a n d Shoppee obtained pure cholest-7-ene directly from the alumina column.
[CONTRIBUTION FROM THE
4127
Acid Isomerization at 120°.-The procedure employed for isomerizations at the higher temperature resembled that described above, except that after the reflux temperature had been reached, about 30% of the solvent was distilled off. The cyclohexane and water passed over as an azeotrope, and the temperature of the solution rose t o about 120'. When reactions were carried out in acetic acid alone, without removal of small amounts of water that are present under these conditions, the results obtained were erratic. At the end of the reaction period, the products were isolated as before, and the acetate esters were removed by chromatography on alumina before infrared analysis of the hydrocarbon fractions. Hydrogenation of Product XI.-A sample of product XI was dissolved in 40 ml. of acetic acid-cyclohexane (4: 1) dnd stirred in a hydrogen atmosphere with pre-reduced platinum oxide catalyst. N o hydrogen was absorbed, and the recovered material proved to be unsaturated in the tetranitromethane test. The experiment was then repeated with 200 mg. of XI in 35 ml. of acetic acid-cyclohexane (4:1), to which 4 drops of 12 N hydrochloric acid was added. After 42 hr. at GO0, 110% of the calculated amount of hydrogen had been absorbed, and the product gave no reaction with tetranitromethane. The infrared absorption spectra of the reduced material showed no detectable amounts of cholestane or of coprostane. Hydrogenation of Product X1V.-Hydrogenation was carried out a t 60" with added hydrochloric acid as described in the preceding experiment. The reaction product showed a negative tetranitromethane test for unsaturation, and the infrared spectrum gave evidence of the presence of small amounts of cholestane.
HOUSTON, TEXAS
DEPARTMENTS O F CHEMISTRY OF THERICEINSTITUTE AND OF YALE UNIVERSITY .4ND FROM HICKRILL CHEMICALRESEARCH FOUNDATION]
THE
Heats of Hydrogenation. 111. Hydrogenation of CycloSctatetraene and of Some Seven-membered Non-benzenoid Aromatic Compounds1 B Y RICHARD
B.TURNER, w.R.MEADOR,w.VON E. DOERIIL'G, L. H. KNOX,J. R.~MAYER AXD D. M.'. WILEY RECEIVED FEBRUARY 25, 1957
The heats of hydrogenation of several potentially aromatic substances have been measured in solution, and resonance energies have been estimated by comparison of the heats of hydrogenation of these compounds with heats calculated for reasonable models. The compounds t h a t have been examined together with t h e AH values and calculated resonance energies are as follows: cyclooctatetraene (-AH 97.96 f 0.05 kcal./mole, R.E. 2.4 kcal./mole), 1,3,5-cyclooctatriene ( - A H 72.36 i 0.26 kcal./mole, R.E. 0.9 kcal./mole), azulene ( - A H 98.98 =I= 0.13 kcal./mole, R.E. 28 kcal./mole), heptafulvene ( - A H 92.63 =!= 0 41 kcal./mole, R.E. 13 kcal./mole), heptafulvalene ( - A H 130.77 Z!Z 0.31 kcal./mole for six molar equivalents of hydrogen, R.E. 28 kral./mole), dihydroheptafulvalene (-AH 138.81 f 0.20 kcal./mole) and tropone ( - A H 67.58 f 0.30 kcal./mole t o cycloheptanone).
Since the heats of hydrogenation of benzene, and of various related substances including styrene, hydrindene and furan, were measured by Kistiakowsky and his associates2some twenty years ago, no further studies of heats of hydrogenation in the aromatic series have been reported. In the meantime several potentially aromatic compounds of considerable theoretical interest have been prepared in sufficiently pure condition for hydrogenation studies. Of these substances cyclooctatetraene, azulene, methylenecycloheptatriene (heptafulvene), cycloheptatrienylidenecycloheptatriene (1) This work was supported by the Eli Lilly Co., Indianapolis, and by the Office of Ordnance Research, C o n t r a c t DA-19-059-ORD1562 with Yale University. (2) (a) G. B. Kistiakowsky. J . El. Ruhoff. H. A . Smith and W. E. Vsughan, TH!S JOURAAL, 68, 146 (le%); (b) M. A. Dolllrcr, T. L. Gresham, G. B. Kistiakowsky a n d W. E. Vaughan ibid., 19, 831 (1937); ( c ) M. A. Dolliver, T.L. Gresham, G. B. Kistiakowsky, E. A. Smith a n d X. E. Vaughan, a b i d . , 6 0 , 440 (1938).
(heptafulvalene), 7-(7'-cycloheptatrienyl) -cycloheptatriene (dihydroheptafulvalene) and cycloheptatrienylium oxide (tropone) have now been examined in the hydrogenation calorimeter.
Experimental T h e technique for measurement of heats of hydrogenation in solution described in Paper I 3 was employed in the present investigation with the following modifications. I n the cases of heptafulvene and heptafulvalene, Diethylcarbitol was used as the solvent, since these substances are subject t o rapid polymerization in acetic acid. Despite extensive purification of t h e Diethylcarbitol, reduction of the platinum oxide catalyst gave higher heat values in this solvent (-32.94 =!= 0.21 ca1./100.0 mg.) than in acetic acid (-30.95 f 0.13 cal./lOO.O mg.) and consumed one cc. more hydrogen (e.g., 16.25 cc./lOO.O mg. in 275 cc. of Diethylcarbitol us. 15.12 cc./lOO.O mg. in 225 cc. of acetic acid). The results were nevertheless consistent, and no discrepancies traceable t o t h e solvent have been noted. All reductions in (3) R.B. Turner, W.R. Meador and R. E. Winkler, ibid., 79, 4116 (1957).
4128
TURNER, MEADOR, DOERING, KNOX,MAYERAND WILEY
Diethylcarbitol were carried out in volumes of 275 ml.; hydrogenations in acetic acid were conducted in 225 ml. of solvent. The hydrogenation of tropone over platinum-in-acetic acid proceeded with the absorption of 92% of the theoretical amount of hydrogen, calculated on the basis of 4 molar equivalents. However, the clean absorption of 3 molar equivalents of hydrogen was observed over palladium supported on barium sulfate, and the results reported for tropone were obtained in acetic acid with this catalyst. All runs in which palladium was used were carried out in the following manner. The calorimeter containing the solvent, c,italyst and an evacuated ampoule containing tropone was flushed with hydrogen according to the previously described procedure.3 After absorption of hydrogen by the catalyst IKIS complete and thermal equilibrium had been established, the hydrogenation reaction was initiated by breaking the ampoule of tropone. The final results were corrected for the heat of solution of tropone (-0.14 & 0.01 kcal./mole), which was determined separately, and thus are referable t o the dissolved state. Materials.-A sample of cyclooctatetraene obtained from the General Aniline and Film Corp. was purified by conversion into the silver nitrate complex, m.p. 173' dec., as described by Cope and H ~ c h s t e i n . ~Regeneration and fractionation afforded a sample boiling a t 141-141 -5" (19 mm.), n * 5 ~ 1.5350. Examination of the infrared spectrum of this material for the styrene band a t 11.08 p gave no evidence for contamination by the latter substance. 1,3,5-CycloOctatriene was prepared by the procedure of Cope and Hochstein'; n " ~1.5248. Azulene was synthesized as described by Doering, Mayer and DePup.6 The sample employed for hydrogenation, u1.p. 99-loo", was purified by chromatography on alumina. Solutions of heptafulvene in Diethylcarbitol were freshly prepared for each run by the method of Wiley.e T h e heptafulvene was collected in a trap containing small amounts of Diethylcarbitol cooled to -40'. The resulting solutions were then made up to 275 ml. and were hydrogenated a t once. The stability of heptafulvene in Diethylcarbitol was established by the negligible decrease of the extinction coefficient of the absorption band a t 405 Inp over a period of 24 hr. a t room temperature. Heptafulvalene, m.p. 121-122', was prepared by the method of Mayer.7 Dihydroheptafulvalene, m.p. G l " , was obtained by the procedure of Doering and Knox.8 Tropone was synthesized a s described by Doering and DetertQand was purified by recrystallization and evapora1.6155, m.p. -5'. tive distillation; n * 6 ~ For reductions in Diethylcarbitol (diethylene glycol dicthyl ether) the solvent was purified by (a) distillation a t 10 mm. (b.p. 85-86'), (b) treatment with lithium aluminum hydride a t 50' for 4 days and distillation a t 2-3 mm. (b.p. ti.5-66') and (c) distillation through a 24-inch helix-packed column under nitrogen a t 37 mm. (b.p. 100'). T h e product n x s collected in 350-ml. fractions which were sealed a t once urider nitrogen in Pyrex flasks, m.p. -46 t o -45'.
Vol. 79
TABLE I HEATSOF HYDROGEXATION I N SOLUTION' Catalyst, - IH(2.i'.
Compound
Nrnoles
mg.
kml,,'rnole
9.;
tane,",?' the heat of hydrogenation of cis-cyclo- cyclooctatriene deviates from the standard ~ a l i i c octene is considerably lower than that of cyclo- of -27.1 kcal./'mole (solution). The quantity c a hesene (-27.10 k c a l . / m ~ l e )which ,~ furnishes a re- culated for this transformation from the heats of duction product in which non-bonded interactions hydrogenation of cyclooctatetraene and of 1,3,5are minimized.28 Since the repulsions that are cyclooctatriene to cyclooctane is - 25.6 kcal. introduced in the last of the four hypothetical steps mole. The discrepancy in this case is hence only in the hydrogenation of '(non-resonating" cyclo- 1.5 kcal./mole, a value that is comparable ill m a y octatetraene (Chart I) are undoubtedly greater nitude to the resonance energy of simple conjugated dienes. CHARTI With reference to the triene problem it is interesting to note that the heat of hydrogenation of 1,3,.5cyclooctatriene ( - 72.1 kcal. 'mole) is higher than that of I ,3,5cycloheptatriene (tropilidene) ( - i0.5 non-resonating non-resonating non-resonating kcal./mole) despite the fact that the heat of hydroAIT? /-' AHa I/ 7 genation of cis-cyclooctene is lower than that o f cycloheptene by almost 3 k ~ a l . ~The , ~ ~resonance energy of 1,3,5-cyclooctatriene (0.9 kcal. I' than those introduced in the earlier stages, the mole) is obtained directly as the difference beuse of AI94 to represent the heat of all four steps is tween the resonance energy of cyclotjctatetraene a poor choice, which leads to an unreasonably low (2.4 kcal./mole) and the contribution of the fourth prediction. In order to correct for this effect, the double bond (1.5 kcal.d/mole). The stabilization cyclohexene value (-27.1 kcal./mole) has been of tropilidene is 9.8 kcal./mole based on Kistiaassigned to AH1 on the grounds that the heat of kowsky's data" or 9.0 kcal. 'mole based on the soluhydrogenation of the first double bond of the model tion data,3 both figures being referable t o a "nonshould closely approximate that of a representative, resonating" model in rrhich cyclohesene is used cis-disubstituted olefin. An average of this figure for the first double bond, cycloheptene for the and that obtained experimentally for AH4 (-23.0 third and an average of the two for the second. kcal./mole) gives an average heat of hydrogenation The negligible resonance energy of 1,3,L5-cyclofor all four double bonds of -25.1 kcal./mole. octatriene probably is due to its non-planar strucThe resonance energy calculated for cyclooctatet- ture. By analogy one can assume that a nonraene on the basis of these assumptions is 2.4 kcal,' planar cycloheptatriene structure for tropilidene mole, a result that agrees reasonably well with the would also lead to little or no resonance energy. lower value5 derived from combustion data. The fact that tropilidene has a relatively high deAs a further check on the magnitude of the cyclo- gree of stabilization is in better accord with a octatetraene resonance, the heat of hydrogenation planar c~nformation.~"It should be mentioned of cyclooctatetraene has been compared with that that such an arrangement involves considerable of 1,3,5-cyclooctatriene in analogy with the proce- angular strain, of which no account is taken in iordure followed by Kistiakowsky and his asso- mulation of the comparison standard. This iriciatesZain their treatment of the resonance energy of adequacy of the model serves to lower the calbenzene. By subtracting the heat of hydrogena- culated resonance energy.31 tion of 1,3-cyclohexadiene to cyclohexane from that Azu1ene.--In view of widespread interest in the of benzene to the same product, the heat of the reac- propertiesof non-benzenoid aromatic compounds, we tion benzene HS + 1,3-cyclohexadiene can be have measured the heat of hydrogenation of azulene. calculated. This change is endothermic to the ex- This substance undcrgoes rapid reduction in acetic tent of 5.6 kcal./mole, as compared with the hydro- acid with clean absorption of five molar eyuivalellts genation of an unconjugated, cis-disubstituted of hydrogen and furnishes a A H value of -!fY.DS double bond, exothermic, - 28.6 kcal.!mole (gas 0.13 kcal. 'mole. Simple calculations based 011 phase). The reduction in heat evolution of 31.2 benzene is based o n a model consisting o f three molecules of cyclokcal./mole observed in this case represents the in- hexene d differs from the figure given above b y the resoniincc crease in stabilization resulting from introduction energy oafn1,3-cyclohexadiene (1.8 kcal./mole). of a third double bond into the 1,3-cyclohexadiene (30) For additional discussion of this problem see 15'. von E. Docrmolecule and is definedl0as the resonance energy of ing, G. Laber, R. Vonderwahl, S . I;. Chamberlain a n d I
