ISOMERIZATION OF CY- AND B-PINENE
Dec., 1941
a mixture of equal amounts of the 1,2,3,5- and 1,2,4,5-isomers. Prehnitene is converted into a mixture of 83% isodurene and 17y0 durene.
3517
All of the hydrocarbons yield varying amounts of lower and higher alkylbenzenes in addition to the principal product. COLUMBIA, MISSOURI
RECEIVED AUGUST 8, 1941
[CONTRIBUTION FROM NAVAL STORES RESEARCH DIVISION,BUREAU OF AGRICULTURAL CHEMISTRY AND ENGINEERING 1
Vapor Phase Thermal Isomerization of
CY-
and B-Pinene*
BY L. A. GOLDBLATT AND S. PALKIN Thermal isomerization studies of a- and 8- thermal isomerization of 8-pinene,' the known difficulties pinene in the past have been confined largely to frequently encountered in obtaining the initial terpenes in pure state, and the consequently confusing conclusions, the liquid state (below 160') or to the vapor phase aThis included repeated fractionation in vacuo in efficient a t relatively high temperatures (above 450°).' columns. The purified pinenes for this investigation (aInterest in the intermediate range was stimulated cept for preliminary trials in which the pinenes were of by Arbuzov's discovery2 that pyrolysis of a- somewhat lower purity) had the following characteristics pinene yielded, in addition to dipentene, the chain a-pinene [a]D +32.06'; nZDD 1.4645; dzD* 0.8600 triolefin allo-ocimene (2,6-dimethyl-2,4,6-octa- @pinene [a]D -21.81'; T Z ~ O D 1.4782; d 2 0 p 0.8712 triene), the best yield of about 26% being obtained Apparatus.-The reaction apparatus was a n all-glass a t 340-350'. Later, similar results were reported* (Pyrex) assembly (Fig. l), consisting essentially of a chargwith respect to 8-pinene (20% yield of allo- ing flask (C), preheater (F), jacketed reaction tube (I) ocimene). This finding, as will be shown pres- and water condenser (N1), backed up by a packed condenser (NJ, dry-ice trap (P) and receivers (0). Hot ently, is erroneous. pinene was vaporized with the aid of nitrogen gas as a Dupont and Du1o1.1,~ repeating the work of carrier which was admitted through a fritted glass disArbuzov on a-pinene but using a copper tube tributor (D), and the rate of vaporization was varied by filled with copper gauze, obtained in addition to changing the rate of flow of nitrogen or the temperature of allo-ocimene and dipentene two new monocyclic the liquid pinene or both. Procedure.-The pinene vapors were passed through the conjugated diolefinic terpenes (up to 50%) which preheater where the temperature of the vapor (measured they designated as a-pyronene (1,1,2,3-tetraat Ql) was raised to 250-300 " and were then passed through methyl-3,5-cyclohexadiene)and p-pyronene (l,l,- the reaction tube. For the range 338-375" the tempera2,3-tetramethyl-2,4-cyclohexadiene). However, ture in each case was maintained relatively uniform along Charlton and Day6 operating chiefly in the range the entire length of the tube by a jacket of mercury vapor 380-425", both with and without a thorium cata- (J) from the mercury boiler (K). With the mercury vapor lyst, did not find allo-ocimene or any of its deriva- jacket in operation, the temperature along the entire length of the reaction tube was found to vary less than 5" and tives in the pyrolysate of a-pinene.6 usually less than 2", despite the fact that the reaction is
Experimental Special precautionary measures were taken t o purify the pinenes used, in view of the apparently contradictory findings of Charlton and Day, the erroneous character of the conclusions of Arbuzov with respect to the products of
*
Not copyrighted. (1) More than 50 literature citations on thermal reactions of the pinenes and turpentine @-pinene constitutes about a third of American gum turpentine) are given in Chem. Rzv., 14, 349-352 (1934). (2) Arbuzov, J . Gen. Chem. (U.S. S. R.), 3, 21 (1933); Bcr., 8'78, 563 (1934). (3) Arbuzov, ibid. (U. S. S.RJ,6,297 (1936). (4) Dupont and Dulou, Compl. rend., 901, 219 (1935). (5) Charlton and Day, I n d . E n g . Chcm., 99, 92 (1937). (6) I b i d . , p. 95. They state: "It was not possible t o isolate such a hydrocarbon [ah-ocimene], and it was extremely doubtful that the physical constants for the various fractions gave much indication of its presence. When the pinene was passed through the Pyrex tube without the catalyst, other conditions being the same, there was no indication of the presence of an unsaturated open chain compound."
strongly endothermal. The boiling point of the mercury was varied by varying the pressure [gaged by the manometer (L)J from 0.5 to 1.5 atmospheres. Temperature was measured a t four points Ql to Q,, by thermocouples leading to a potentiometer through ice junctions. For temperatures above 375" the outer jacket was heated electrically. In such cases, however, the temperature could not be kept uniform along the entire length of the tube, extreme variations sometimes reaching 30" or more. The maximum temperature indicated by any thermocouple was in each case recorded as the temperature of the react ion. (7) Arbuzov's findings were probably due to working with impure &pinene, the allo-ocimene resulting from a-pinene present as an impurity. This is indicated by the physical constants reported: "b. p. (10.5 mm.) 44.5'; dlr 0.8670; n% 1.4740; a D -13.50; a v / a j 1.09.'' The low rotation and high dispersion indicate the presence of a considerable proportion of an impurity which, from the characteristics cited, may well be suspected of being a-pinene.
::518
I,. A. COLDBLATT AND S. PALKIN
A-NEEDLE VALVE 0-FLOWMETER C-CHARGING FLASK D - 0 1 5 DISTRIBUTOR E- ELECTRIC HEATER
f-PREHEATER G-ASBESTOS WPER LINING W-B5!4 MAGNESIA I -REACTION TUBE J-JACKEl(AlnoRurr;ur v m )
K-MERCURY BOILER P-DRY ICE TRAP L - MERCURY MANOMETER O1,Q.Qa.-MfRMOCOURE M-VENT N, .&-CONDENSER 0-RECEIVER
Vol. 03
P
l l O V A.C.
Fig. 1.-Apparatus
for thermal isomerization.
Isomerizaticm of a-Pinene.-Since allo-ocimene, one of the chief products formed, has a very high refractive index ( T L ~ ~1.544), D change in the index was used as a guide in following the extent of isomerization of a-pinene. The effects of changes in temperature and throughput on refrac-
0
Fig. 2.-Effect
50
tive index for a few selected isotherms are shown in Fig. 2. In general, for any fixed temperature the refractive index increases with increase in throughput to a maximum and then decreases; similarly, for any fixed throughput, the refractive index increases to a maximum and then decreases
150 200 250 300 350 400 Throughput in grams per hour. of temperature and throughput on refractive index of isomerization product from a-pinene. 110
ISOMERIZATION OF a- AND P-PINENE
Dec., 1941
3519
as the temperature is increased. The effect of a n increase 1.7 cc. of "oil" (b. p. (20 mm.) 50-53", shown to be largely in throughput is more critical a t lower temperatures than a-pinene) and 2.65 g. of an adduct (b. p. (8 mm.) 195"). The adduct, after washing with 10 cc. of pentane and two a t higher temperatures. Maximum change in refractive index occurs a t a temperature of about 375" and a through- crystallizations from methanol, melted a t 91-92'. a-Pinene (unreacted).-1.5 cc. of the unreacted ''oil'' put of 260 g. per hour. This throughput was obtained by passing a stream of 500 ml. per minute of nitrogen through mentioned above was again treated with maleic anhydride the liquid a-pinene maintained at 146'. Under these conditions the yield of easily condensable product 1.545C is practically quantitative, 100 g. of a-pinene consistently giving more $5" 0.8600 1.540C than 97 g. of product with practically no liquid condensate in the dryice trap. 90 f4" 0.8520 1.530C Fractionation and Chemical Examination.-One hundred grams of isomerizationproduct c cy]^ +1.42'; 13% 1.5025; d20d 0.8283) obtained 84 f3" 0.8440 1.52OC under optimum conditions for d o - '. * ocimene formation was separated 6 into 33 fractions a t 20 mm. pressure 2 +20 , 0.8360 sg l.jl0C in an efficient all-glass fractionating S -* .-F: B column (equivalent to about 35 % Q 2 s .theoretical plates). The refractive d, "a i 72 +1 ' OS280 1.500c index, optical rotation and density8 4of the individual fractions (except i? for a few densities) were deterL mined. These are plotted together 66 00 0,8200 1,4900 with the distillation temperature in ;j $ Fig. 3. Examination of these curves indi6o 0.8120 1.4800 cates that only about 15% boils in the range where the pyronenes might be expected (48-70' a t 20 mm.).g The remainder apparently 54 0.8040 14700 is nearly equally divided between dipentene (b. p. (20 mm.) 71.4') and allo-ocimene (b. p. (20 mm.) 48 1.4600 88.4 The following tests were used for
4
h
8
6
-
3
3
4
''
.E k
2
0"
0).
nents of the various fractions.
a-Pyronene.-The first three Fig, 3.-Fractional fractions (9 g., b. p. (20 mm.) 48.656.0") were combined and a portion ( 5 cc.) of the mixture was heated with 3.5 g. of maleic anhydride on a steam-bath. Complete solution occurred in about onehalf hour but heating was continued for another three hours. The product was fractionally distilled, and in addition to unchanged maleic anhydride there was obtained ( 8 ) The densities were determined by hlr. S. A. Hall. (9) The pyronenes apparently have never been obtained pure. Graphs indicating the physical characteristics of the fractionated apiuene pyrolysate were published by Dupont and Dulou, Compl. F r i i d . , 201, 319 (1935), and also in Affi X o Conor. Intern. Chim., 3, 125 (1939). Roth graphs indicate practically the same values for density and refractive index, but in the former the values are indicated as for 15'' and in the latter for 25'. The constants for the purest a-pyronene are given in the latter reference as b. p. (11 mm.) .Go; n% 1 4665: d*6 0.8347, and for 5-pyronene as b. p. (8 mm.) 48-50'; n% 1.4747; d*s 0.8401. Hultzsch, B e y . , 72, 1187 (1939), without giving other physical characteristics, indicates that from the pyrolysate of a-pinene there was obtained an a-pyronens fraction, b. p . (16 mm.) 52-53', and a 5-pyronene fraction, b. p. (16 mm.) 56-57', which gave adducts in goad yield with maleic anhydride.
Per cent. by weight. distillation of isomerization product from a-pinene. as before, but there was only a slight yellowing and the melted maleic anhydride and oil remained as two separate layers. The mixture was fractionally distilled and after washing the distillate with water and drying with sodium sulfate there was recovered 1.0+ cc. of oil (b. p. (20 mm.) 52.0"; ? ~ * O D 1.4675) which formed a nitrosochloride. melting with decomposition a t 106-107 ', and apparently was essentially unreacted a-pinene.10 6-Pyronene.-The next three fractions (6.7 g., h. p. (20 inin.) 56.0-70.1 ") which should be rich in ,8-pyronene, wen' combined and a portion (5 rr.) was condensed with 3.5 g . of maleic anhydride. This mixturc reacted more readily than did the a-pyroneiirt; at 60-70" a vigorous reactioii set in and complete solution occurred immediately. However, heating on the steam-bath was continued fot(10) Dupont and Dulou, Bul. Z m l . P i n , 20, 173 (1936); C o n o f . r e n d . acad. sci., 22, 1861 (1936), state that the nitrosochloride of apyronene melts at 103'.
L. A. GOLDBLATT AND S. PALKM
3520 0.8175
I ,
0-
%
I
0 8125----
I
I
I
L -
----
_ _ _ _ 150 250 Throughput in grams per hour
-
-
350
Fig. 4.-Effect of throughput a t 403 ' in density of isomerization product from P-pinene. another hour. The product was worked up in the same fashion as was that obtained from a-pyronene. I n this way there were obtained 1 4 g of colorless "oil" (b. p (20 mm ) 57-70") and 2.8 g of adduct boiling a t 185-190" a t 7 to 8 mm which, after washing with pentane and crystallization from methanol, melted a t 163-164". Fractions 7-9 (7.7 g , b. p. (20 mm.) 70 1-71.2"), similarly combined and treated with maleic anhydride, reacted only very slightly The major portion was recovered apparently unchanged (b. p. (20 mm.) 65-70"; n 2 0 ~ 1.4730) and 70 +lo" 0.8450 only a few drops of adduct boiling a t 185-195' a t 8 mm. were obtained. -10" 0.8350 Dipentene.-3.5 cc. of a fraction boiling a t 71.4' (20 mm.) was treated with 2 cc. of bromine ac" -300 0.8250 cording to the method of God1ewski:l After evaporation of the ether, the crystalline mass was re.E crystallized from methanol, yield- % on -50" 0.8150 h c1 ing 2.3 g. of product which melted d .* E -E a t 124.5-125.5' and showed no de- y pression in melting point when 2 6o .,O 2 -70" 0.8050 mixed with an authentic sample of 'J + 4 dipentene tetrabromide. .- e2 do-0cimene.-Three fractions .-$ boiling at 88.4" (20 mm.) were E -90" 0.7950 combined, and 5.9 g. of the mix0 ture was treated with 4.4 g. of maleic anhydride. An exothermal 55 0.7850 reaction occurred as soon as the maleic anhydride melted, but heating on the steam-bath was continued for another hour. The product was then distilled, yielding 8.8 g. of adduct boiling a t 188" a t 8 mm. This was washed with 50 pentane. After two recrystallizations from hexane it melted a t 83-84'". Fig. 5.-Fractional From these data, rough calculations indicate that the product obtained by the isomerization of a-pinene a t 375" contains about 3% unchanged a-pinene, about 12% of a- and 6-pyronene. about 42% dipentene and 40% allo-ocimene.
