[CONTRIBUTION FROM IPATIEFF HIGHPRESSURE AND CATALYTIC LABORATORY, DEPARTMENT OF CHEMISTRY, KORTHWESTERN UNIVERSITY ]
STUDIES IN THE TERPENE SERIES. VI. CYCLIC ISOMERIZATION OF LIMONENE (1)’ V. N.IPATIEFF, HERMAN PINES, VLADIMIR DVORKOVITZ, R. C. OLBERG, A N D MICHA4EL SAVOY
Received April 8, 19@
During the study of the polymerization of limonene with silicophosphoric acid catalyst “Solid Phosphoric Acid’’ (2) at an elevated temperature under atmospheric pressure, it was found that the products formed consisted, to a large extent , of isomeric bicyclic monoterpenes CloHlaand tricyclic &terpenes C20H32. Based upon the experimental evidence to be cited, it was suggested that the unsaturated bicyclic terpene is composed largely of a new terpene containing a five- and a six-membered ring (Formula I). CH3
C H3
I
HC
CH
YC\
/
H2C-CH
1 ‘ I
1
H3CCH
H2\
\
CHz
/’
CH (1) 2 ,6-Dimethyl-bicyclo(3,2,l)-Boctene
HZC
/
/ \
\
HZC-CH
1 ‘ I H2CI\\ 1 / / HBCCH
CHz
CH (11) 2,6-Dimethyl-bicyclo(3,2,1)-octane
The cyclic isomerization of limonene was accompanied by side reactions which greatly complicated the investigation of the structure of the bicyclic terpenes. When limonene was passed over silicophosphoric acid at 200” and a liquid hourly space velocity of two, 35y0 of polymeric and 65% of monomeric hydrocarbons were obtained. The monomeric hydrocarbons were composed mainly of unreacted limonene, p-cymene, dihydrolimonene, and bicyclic terpenes with some admixture of bicyclic dihydroterpenes. The presence of p-cymene indicated that a hydrogen transfer reaction similar to the one reported previously (3) had occurred. A complete conversion of limonene was achieved when the monomeric fraction was repassed over the same catalyst; this seems to be an effective method of freeing the reaction product of monocyclic terpenes. For the removal of p-cymene, a portion of the monomer was selectively hydro-
* This work was financed in part
by Universal Oil Products Company. Presented in part by V. N. Ipatieff and V. Dvorkovitz before the Division of Organic Chemistry of the American Chemical Society, .4tlantic City, September 8-12, 1941. 34
STUDIES IN THE TERPENE SERIES,
VI
35
genated under pressure with nickel-kieselguhr catalyst (4)a t 40-50". Under these conditions, only the olefinic hydrocarbons underwent hydrogenation, while the aromatic hydrocarbons remained unchanged. The aromatic hydrocarbons were removed from the saturated monomer by sulfur dioxide extraction, followed by treatment with fuming sulfuric acid. It was found that the monomer contained 30% of p-cymene, which was identified by ultraviolet absorption spectra and solid derivatives.* The aromatic-free hydrogenated product consisted of a mixture of bicyclic dihydroterpenes (Formula 11) and 1-methyl-4isopropylcyclohexane. The latter was separated from the mixture by selective dehydrogenation ( 5 ) to p-cymene, followed by extraction with fuming sulfuric acid. By this procedure, it was established that the hydrogenated product contained 25% of 1-methyl-4-isopropylcyclohexane. The product that remained after dehydrogenation and extraction consisted of bicyclic dihydroterpenes and was distilled on a fractionation column of an efficiency of fifty theoretical plates a t a reflux ratio of 50:l. The distillation curve is given in Graph 111. The various fractions were analyzed and their physical properties were investigated. The data obtained showed that the products consisted of bicyclic hydrocarbons. According to infrared analysis, the two largest fractions boiling a t 162-164", and 164-168", and constituting sixty per cent of the bicyclic terpenes, were composed principally of the same compoud, each, however, mixed with small amounts of some other hydrocarbons, possibly stereoisomers. The validity of the proposed Formula 11, which was suggested for the hydrogenated bicyclic terpene obtained from the isomerization of limonene, was determined by destructive hydrogenation. For that reason an investigation of the action of hydrogen (1) upon compounds of bicyclic dihydroterpene type of known dxuctures was undertaken. The results of this investigation indicated that bicyclic dihydroterpenes containing rings of five and six members are cleaved by hydrogen less readily than those of three- and four-membered rings. Thus, isocamphane and isobornylane required a temperature of 210" to effect ring fission. The destructive hydrogenation of the bicyclic dihydroterpenes obtained from limonene likewise required a high temperature indicating the absence of three- or four-membered rings. The products of the reaction consisted mainly of alkyl cyclopentanes and a small amount of alkyl cyclohexane. In order to show the existence of a five-membered ring in the bicyclic dihydroterpene obtained from limonene, the product was treated with dilute aqueous magnesium chloride solution. It was reported previously (6) that while isocamphane and isobornylane are stable in the presence of dilute aqueous magnesium chloride even at 425", pinane, a bicyclic dihydroterpene containing a
* The formation of p-cymene in this reaction caused us t o investigate the purity of limonene and dipentene used in this study. It was found that limonene did not contain any traces of aromatics while dipentene contained 22% of p-cymene. These results caused us t o use only limonene in this investigation even though silicophosphoric acid catalyst yielded the same products with dipentene as with limonene; p-cymene present in dipentene did not take part in the reaction; only the concentration of p-cymene in the two cases was different.
36
IPATIEFF, PINEY, DVORKOVITZ, OLBERG, SAVOY
four-membered ring, readily underwent ring cleavage at 280-350" and resulted in the formation of monocyclic olefinic hydrocarbons. Application of this technique t o the unknown bicyclic dihydroterpenes lent further evidence t o the proposed five- and six-membered rings. No reaction occurred even at 400". The presence of a six-membered ring in the unknown bicyclic dihydroterpene was further substantiated by dehydrogenation over platinized asbestos. At 290-300" a small amount of the total product consisted of aromatic hydrocarbons. p-Cymene was identified in the reaction product. In order to determine the position of the double bond in the unsaturated bicyclic terpenes obtained from limonene, some of the fractions were oxidized either by ozone or by dilute potassium permanganate. In each case, a good yield of a keto acid was obtained, which on further oxidation with bromine in alkaline solution gave a quantitative yield of carbon tetrabromide and a dicarboxylic acid. The oxidation of the lorn-boiling isomers with selenium dioxide yielded an aldehyde, indicating again that the methyl group was adjacent to the carbon atom containing a double bond. A complete study of the degradative oxidation of the isomer is planned and this investigation will be described later. An attempt was made to condense the various unsaturated monomeric fractions, obtained from the isomerization of limonene, with benzene in the presence of sulfuric acid. The lowest-boiling isomers did not react with b e n ~ e n e . ~According t o molecular weight and analysis it consisted of bicyclic hydrocarbons C9HI4. In view of the small amount of the hydrocarbon available, it was impossible to study it any further. The fraction boiling above 162" reacted with benzene in the presence of sulfuric acid to form tricyclic hydrocarbons which, on the basis of the proposed Formula I, may have the following structure:
On the basis of all the evidence presented in this paper, it can be concluded that the proposed structure for the bicyclic isomer formed from limonene (Formula I) is the most plausible one. Further study of the structure of the bicyclic isomer produced is in progress. It was decided to repeat the experiments of Carter, Smith, and Read (7) who treated limonene with 85% orthophosphoric acid for 128 hours a t room temperature. These authors found that besides polymerization only isomeric monocyclic terpenes were formed. It vTas also found by us that polymerization wfts the chief reaction; only 12y0 of a product, distilling at 168-175", was obtained; the latter contained a mixture of monocyclic terpenes and 22y0 of p-cymene. 3 A similar phenomenon mas noticed previously by us (6) when an attempt was made t o cause benzene to react with 1-ethyl-2-isopropylcyclopenteneobtained through ring fission of pinane.
STUCIES IN T E E TERPENE SERIES.
37
VI
The presence of bicyclic terpenes mas not observed. The monocyclic terpenes on hydrogenation were converted to p-menthane; this indicated that cycloisomerization of limonene did not occur. EXPERIMEKTAL P.4RT
+
A. Once-through passage. Freshly distilled limonene, 1.4722, [a]2: 123.1°, was passed over 75 g. of silicophosphoric acid ("Solid Phosphoric Acid") catalyst of 4-8 mesh size at 200" and at atmospheric pressure at a rate of 150 ml. per hour. The monomeric hydrocarbons were separated from the polymers by steam distillation, which was carried out in the presence of sodium hydroxide in order t o decompose any phosphoric acid ester which might have been formed during the reaction. The steam distillate, consisting of 65YG by weight of the total hydrocarbons, was dried over calcium chloride and distilled on a column of an efficiency of fifty theoretical plates. The boiling point curve and the index of refraction of the distillate is given in Graph I.
G4PIQH.f
Analuses of the monomeric fractions, b.p. i6S-i?'S0. The percentage of unreacted limonene was determined by hydrogenating 30 g. of the product in the presence of a coprecipitated copper oxide-aluminum oxide catalyst (657, Cu and 35% A1203). The hydrogenation was carried out in a 450-ml. capacity rotating autoclave a t 90" and under an initial hydrogen pressure of 100 atmospheres. Under these conditions, only the double bond in the side chain underwent hydrogenation. The hydrogenation was considered terminated when the pressure remained constant for 1.5 hours and when 2 ml. of the hydrogenated product dissolved in 4 ml. of benzene t o which a few drops of 96% sulfuric acid were added, gave a yellow coloration to the acid layer. Where small amounts of limonene were present, the color of the acid became dark brown. From the amount of hydrogen absorbed, it was calculated that the monomeric product contained 19.5y0of unreacted limonene. The amount of olefins in the monomeric fraction was determined by selective hydrogenation of an aliquot portion by using nickel-on-kieselguhr catalyst with 50 atm. of hydrogen pressure a t 40-50". No hydrogenation of the benzenoid ring occurs under these conditions. From the total hydrogen absorbed and from knowledge of the previously determined diolefin content, the amount of monoolefins present could be calculated. The hydrogen absorbed corresponded t o a monoolefin concentration of 10%.
38
IPATIEFF, PINES, DVORKOVITZ, OLBERG, SAVOY
The amount of aromatic hydrocarbons was determined by extracting the product, obtained from the selective hydrogenation, with fuming sulfuric acid. Twenty-six grams of the monomeric fraction dissolved in an equal volume of n-pentane was hydrogenated in a 450-ml. capacity rotating autoclave, using 2.5 g. of nickel-kieselguhr catalyst (4). Twenty-six ml. of the hydrogenated product, which was free of olefins as determined by a dilute potassium permanganate test, was treated a t O'with 52ml. of fuming sulfuric acid which contained 15% of sulfur trioxide. The 18ml. of saturated hydrocarbons that remained after two such treatments indicated that 30% of aromatic hydrocarbons was present in the original sample. This analysis checked with that obtained by ultraviolet absorption; the latter also indicated that the aromatic hydrocarbons formed consisted of p-cymene only. I n order to determine and separate the dicyclic hydrocarbons from the menthane produced by hydrogenation, 12 g. of the saturated hydrocarbons, free from p-cymene, was selectively dehydrogenated by passing i t thrice over 35ml. of 8-14 mesh platinized alumina at 250" and at a rate of 7 ml. per hour. From the amount of hydrogen liberated and from the amount of aromatic hydrocarbons formed by catalytic dehydrogenation, it was found that the saturated product contained 55% of dicyclic hydrocarbons, CloHla, which boiled a t 164-165' and had ng 1.4507. AnaE. Calc'd from CloH18:4 C, 86.87; H, 13.13. Found: C , 87.24; H, 12.82. On the basis of the results obtained, i t was calculated that the monomer fraction consisted of 21% limonene, 10% dihydrolimonene, 30% p-cymene, and 39% bicyclic terpene. B. The recyclic procedure. The monomeric fraction separated from once-through passage of limonene over silicophosphoric acid catalyst, as described above, was again passed over the same catalyst at 200". The benzene-sulfuric acid color test did not reveal any unreacted limonene in the resulting product which was distilled on a column of an efficiency of 50 theoretical plates and a t reflux ratio of 50:l. The boiling range and the index of refraction of the various cuts are given in Graph 11. The distribution of the various types of hydrocarbons in the different fractions was determined by selective hydrogenations, extractions, and dehydrogenations as described in detail in the investigation of the product of once-through passage. The results of this investigation are given in the following table.
I Bicyclic terpenes, %. .................... Cyclic monoolefins, %.. . . . . . . . . . . . . . . . . . p-Cymene, % ' ............................
BOILING RANGE,
"c.
160-165
165-170
170-155
63 22 15
46 17 37
25 28 47
Reaction with benzene. The condensation of the various fractions with benzene was carried out in a 300-ml. capacity three-necked flask provided with a dropping-funnel, mercury-sealed stirrer, and a reflux condenser. One part by volume of 96% sulfuric acid and one part of benzene were placed in the flask which was cooled t o 4". To this was added, dropwise, a solution consisting of one part of benzene and one part of a monomeric fraction obtained from the cycloisomerization of d-limonene. After all the hydrocarbons were added, the mixture was stirred for an additional one-half hour. Two layers were formed; the upper layer, composed of hydrocarbons, was separated, washed with water, aqueous sodium hydroxide, water, dried over calcium chloride, and distilled. Fraction b . p . 156-159', n: 1.4568. Twenty-six grams of the product was treated with benzene. Eleven grams of bicyclic terpene boiling at 42-46' a t 12 mm., n? 1.4528 was separated from the upper layer after the removal of benzene. This product of hydrogenation 4
Microanalyses were made by Dr. T. S. Ma, University of Chicago.
STUDIES IN THE TERPENE SERIES.
39
Vi
at 50°, in t,he presence of a nickel-kieselguhr catalyst yielded bicyclic dihydroterpenes which distilled at 155-158", nz 1.4506, d: 0.8485. It did not contain any aromatics. Bn'al. Calc'd for ClaHl8: C, 86.87; H, 13.13. Found: C, 87.02; H, 12.76. The higher-boiling hydrocarbons, boiling at 128-135", at 4 mm., 1.5222 consisted of product of reaction between benzene and the bicyclic terpene. Fraction 6 . p . 164-167°, n: 1.4613. Twenty-five grams of the product was treated with benzene in the presence of sulfuric acid. The upper layer contained 7 g. of hydrocarbons boiling at 166-168", n: 1.4665. This hydrocarbon after selective hydrogenation and removal of p-cymene, which was originally present, with fuming sulfuric acid distilled at 162-165", n: 1.4515 and consisted of bicyclic dihydroterpenes. The product of the reaction of benzene with the bicyclic terpene distilled at 160-170" a t 27 mm., n: 1.5270, d," 0.9555; i t consisted of tricyclic hydrocarbons.
nt
1.4900
t7s
1.4800
$10
1.AlOO
\G5
l.4.SOo E
IC0
1.4506
r, $0 d
d
3
4 d
u
0
5'
I. 4400
I55
I50 IO
'LO
50
60
50
GO
70
80
90
GRAPH. E
A n a l . Calc'd for C16H22: C, 89.65; H, 10.35. Found: C, 89.45; H , 10.36. Investigation of bicyclic dihydroterpene. I n order t o prepare a large amount of bicyclic dihydroterpenes, 2155 g. of monomeric fraction was selectively hydrogenated with a nickelkieselguhr catalyst; by this method the olefinic hydrocarbons were converted into saturated product while p-cymene remained unchanged. After the extraction of p-cymene with fuming sulfuric acid, the p-menthane present was removed by a n exhaustive dehydrogenation a t 260" followed by extraction with fuming sulfuric acid; 839 g. of bicyclic dihydroterpenes was obtained which indicated that the original monomeric fraction contained 39.8% of bicyclic terpenes formed by cycloisomerization of limonene. The bicyclic dihydroterpenes were distilled on a column of a n efficiency of fifty theoretical plates and at a reflux ratio 5: 1 . The distillation and the index refraction of the product is given in Graph 111. Some of the fractions were analyzed and the data are given in table on p. 40. Treatment with aqueous mugnesium chloride solution. Fifty-seven grams of the bicyclic dihydroterpenes, boiling a t 164-166", ng 1.4527, was heated a t 400" for two hours in a rotating autoclave of 850-ml. capacity with an equal volume of a 201, aqueous magnesium chloride solution. A very small amount of gas was liberated by the reaction. The hydrocarbon layer was separated, dried, and the resulting fifty-two grams was distilled. The material
40
IPATIEFF, PINES, DVOREIOVITZ, OLBERG, SAVOY
FOUND*
FRACTION
2 5 13 17 21
* Calculated
B.P.
156 162 165 167 173
%
C
H
S6,95 86.76 86.99
13.16 12.95 13.05 13.16 12.77
86.95
87.26
1.4389 1.4485 1.4539 1.454s
1
for CIOHU:C, 86.87; H, 13.13.
remained unchanged as it distilled within the same range as the original product; its index of refraction after treatment was 1.4529. A n a l . Calc'd for ClaHls: C, 86.87; H, 13.13. Found: C, 87.03; H, 13.00.
4Q4Pn
-m
Destructive hydrogenation. Two fractions of the bicyclic dihydroterpenes were subjected separately t o destructive hydrogenation. 9.Fraction b . p . 168'. Thirty grams of the product was hydrogenated in an autoclave of a 450-ml. capacity with a nickel-kieselguhr catalyst and at a n initial hydrogen pressure of 100 atmospheres. .4t 180-200' hydrogenation did not occur. By raising the temperature t o 230"and maintaining the autoclave a t this temperature for two hours, destructive hydrogenation took place; the final pressure a t 25" was 80 atm. Thirty-two liters of gas was obtained composed of 63% hydrogen and 37% methane. It was calculated that 3.3 moles of hydrogen per mole of hydrocarbons charged entered into reaction. Twenty-two grams of liquid product was recovered which distilled as follows: (a) 50-75", 7.3%; (b) 75-90", S.9%, n! 1.3908; (c) 90-looo, 15.0%, nfP 1.4027; (d) 105-120', 22.29?.,, nfP 1.4100; (e) 120-135', 20.l%, ng 1.4206; (f) 135142", 19.97,, n: 1.4343.
STUDIES IN TEE TERPENE SERIES. VI
41
Cut e obtained from the destructive hydrogenation was dehydrogenated by passing i t over platinized alumina catalyst at 240'. One hundred fifty ml. of hydrogen was liberated; the liquid product obtained had n: 1.4242, and i t contained, according to absorption with 15% fuming sulfuric acid, 5% of aromatic hydrocarbons. B. Fraction b.p. 167". Thirty-three grams of the bicyclic dihydroterpenes was destructively hydrogenated at 230" under conditions described in A. Thirty liters of non-condensable gas was obtained which was composed of 55.47, hydrogen and 42.3% methane. Twenty-three grams of liquid hydrocarbon was recovered which distilled as follows: (a) 50-85', SI%, nz 1.3850; (b) 85-100°, IS%, n: 1.4042; (c) 100-120", 25% n: 1.4138; (d) 120136", 23'%, n: 1.4223; (e) 136-148", 25$,, n t 1.4367. Cut c was dehydrogenated by passing it over platinized alumina at 250". Three hundred eighty nil. of hydrogen was liberated. The liquid product, after the removal by fuming sulfuric ,acid of the 5% of aromatic formed, gave the following analysis. Anal. Calc'd for CTH,~: C, 85.63; H, 14.37. Found: C, 86.12; H, 14.22. These data show that Fraction c consisted of hydrocarbons containing mainly a pentamethylenic ring, with some alkylcyclohexanes. Cut d analyzed as follows: A n a l . Calc'd for CsHls: C, 85.63; H, 14.37. Found: C, 86.65; H, 14.11. Four and one-half grams of the product was dehydrogenated by passing i t over platinized alumina three times a t 240". One thousand twenty ml. of hydrogen was liberated. A liquid product distilling at 130-136O, n: 1.4360, was obtained; according t o ultraviolet absorption analysis it contained 11.7y0 of o-xylene; 5.17, of m-xylene, and 1.9% of p-xylene? No ethyl benzene was present. A nitro derivative was prepared melting a t 174' corresponding t o dinitro-m-xylene. Dehydl.ogenation. Twenty grams of bicyclic dihydroterpenes, b.p. 163', n t 1.4530 was passed over platinized alumina at 300" with an hourly liquid space velocity of 0.1. Two hundred forty ml. of gas was formed, the index of refraction of the resulting liquid product was 1.4558; i t showed a positive test for aromatics and a negative test for olefins. I n order to separate the aromatic hydrocaybons, the product was passed over silica-gel according to the procedure described by RIair and E'orziati (8). Two and one-half ml. of aromatic hydrocarbons, 7~: 1.4900, was separated which, according t o infrared analysis, consisted of p-cyincne. The action of liquid phosphoric acid on limonene. Three hundred thirty-seven grams of d-limonene was stirred with 360 g. of 857', phosphoric acid at room temperature for 128 hours. The upper layer was steam distilled in the presence of sodium hydroxide, dried, and fractionally distilled. Forty-eight ml. of hydrocarbons distilling at 166-175" was obtained, which did not contain any diolefins 3s determined by benzene-sulfuric acid color test, A part of the hydrocarbons was selectively hydrogenated at 45" using nickel-kieselguhr catalyst t o saturate the olefin. According t o ultraviolet absorption, the product contained 2 2 7 of p-cymene. This was removed from the saturated hydrocarbons by extraction with fuming sulfuric acid. The aromatic-free hydrocarbons distilled a t 166-168", n: 1.4395. On dehydrogenation, p-cymene was obtained.
Acknowledgment. The authors are indebted to Drs. M, J. Murray and W. S. Gallaway of the Universal Oil Products Company for the ultraviolet and infrared analyses, respectively. 5 The presence of the various isomeric xylenes in the dehydrogenation product is not a n indication that the corresponding dimethylcyclohexanes were present in the product of the destruct,ive hydrogenation, inasmuch as xylenes may probably undergo isomerization when passed over platinized alumina.
42
IPATIEFF, PINES, DVORKOVITZ, OLBERG, SAVOY SuMMtLRP
By passing d-limonene over silicophosphoric acid at 200°,a mixture of hydrocarbons was obtained consisting of &yoof polymers and 4570 of monomers. The monomeric fraction consisted of bicyclic terpenes, monocyclic dihydroterpenes and p-cymene. The probable structure for the bicyclic terpenes, resulting from the cyclic isomerization of limonene is given as 2,6-dimethyl-bicyclo-(3,2 ,l)-Z-ocfxne; this conclusion being based upon the evidence obtained by degradative oxidation, destructive hydrogenation, treatment with dilute acid-acting salts, and dehydrogenation. EVANSTON, ILL. REFERENCES (1) For Paper V of this series see IPATIEFF, PINES,AND OLBERG,J. Am. Chem. SOC.,68, 1709 (1946). (2) IPATIEFF (to Lniversal Oil Products Co.) U. S. Patents, 1,993,513 (March 5, 1935); 2,081,065, 2,018,066 (October 22, 1835) ; 2,020,649 (November 12, 1935); 2,057,433 (October 13, 1936); 2,060,871 (November 17, 1936). (3) IPATIEFF, PINES, AND OLBERG, J . Am. Chem. SOC., 67,694 (1945). AND CORSON, Znd.Eng. Chem., 26,1938 (1934). (4) IPATIEFF (5) PINES AND IPATIEFF, J . Am. Chem. SOC., 61,1076 (1939). (6) IPATIEFF AND PINES, J. Am. Chem. SOC.,67, 1931 (1945). (7) CARTER,SMITH, AND READ,SOC. Chem. Ind., 44, 5431' (1925). (8) MAIRAND FORZIATI, Bur. Standards J . Res., 32, 151, 165 (1944).
