Symposium on the Chemistry of Gaseous Hydrocarbons Presented before the joint meeting of the Divisions of Petroleum Chemistry and of Gas and Fuel Chemistry a t the 89th Xeeting of the
American Chemical Society, Ken. York,N.Y., April 22 t o 26, 1935.
Formation of Alicyclic Hydrocarbons from Free Radicals F. 0. RICE
AND
0. L. POLLY, The Johns Hopkins University, Baltimore, Md.
+
CH~CHZCHZCH~CHZCH~CH~ + CH~CHZCH~CH~CHZ CzH4 ( 5 ) + CH3CHzCHz 2CzH4 (5A) + CHa 3CzH4 (5B) CHa CH3 CH3CHz
HE cracking of straight-chain hydrocarbons of high molecular weight seems to be accompanied in greater or less degree by
+
the formation of naphthenic hydrocarbons. There are severa1 mechanisms through which this may occur, but this paper will review the evidence and discuss in detail only one of these mechanisms-namely, the rupture of the original hydrocarbon of high molecular weight into two free radicals which then decompose successively into cycloparaffins and smaller free radicals. Examples of such decompositions are:
I
I
1
CHsCHCHzCHzCH CHCHp + CH3CHz CHI
1
+
or
CH,
+
CH3hHCH,CH,CHCHCH,+CH3CHCH3
/
\ CHI zCH CH,CH
CHs CH,
CH3CHzCH=CHz
(6A)
Diheptyl mercury . was decomposed at about 350' C. and the products obtained were fractionated; cyclohexane and some other unidentified cyclic compound were found in the products. Since the diheptyl mercury was diluted to a great extent with an inert gas, carbon dioxide, it seems probable that the cyclohexane was produced by direct decomposition of the heptyl radical rather than by some polymerization process of ethylene.
3
CHgCH
-+
I
+ CHaCH CHzCH2CHCHs +
Previously published work on hydrocarbon cracking does not permit any decisive answer to these questions or even t o the question as to whether free radicals play any important role in the ordinary cracking operation. Ssachanen and Tilitschejew (7) have made an extensive study of the cracking process, using paraffin of 53" C. melting
CH2CHCH3 / I +CH2 (3)
/
or
(6)
CH,
(1)
CH2 (2)
CH, LH2
'
I
kCHZCHZ\ \CH2CH:
CH3 CHs
+ CH2 /,CHyCHz ,
+
c&
CHZCHCHzCHzCHCH=CHZ CH3 or
CH~CH~CHZCHZCH~CHZCHZ +CHaCHz
+
+ CgZ
'CHCH,CH,
(4)
\CH dH, I CH3 The relative extent of thi- method of decomposition of radicals as compared with the decomposition into olefins a* represented b y the following equations will presumably depend on the conditions: 915
916
INDUSTHIAL AND ENGINEERING CHEMISTRY
point, a cracking temperature in the range 425" to 450" C., and pressures of 140 to 400 pounds per square inch (9.8 t,o 28.1 kg. per sq. cm.). They found that the saturated product consisted chiefly of normal paraffins, and the largest fraction of these had half the molecular weight of the original hydrocarbon. Saphthenes were largely absent if the unsaturated hydrocarbons were removed rapidly from the reaction zone. Frolich, Sirnard, and White ( 5 ) were not able to synthesize cyclohexane with any appreciable yield from olefins a t atpheric pressure and in the absence of catalysts. They cate that aromatic substances formed in t,he cracking of paraffins perhaps come from a condensation bet'ween butadiene and olefins ( 1 ) rather than by dehydrogenation of naphthenic hydrocarbons. Petrov (6) has found that, when fatty acids are decomposed under 2500 to 3500 pounds per square inch (175.7 to 246 kg. per sq. cni.) pressure a t 400" C. in the presence of water and alumina, the fraction of products boiling in the range 100" to 150" C. has a high content of naphthenic hydrocarbons. Frey and Hepp (4) observed that in the decomposition of the loner paraffins a t about 850" C., butadiene and cyclopentadiene are the chief low-boiling hydrocarbons formed, other than benzene and toluene which predominate in the volatile oils. Tongberg and Fenske (8) have identified methyl cyclohexane in Pennsylvania straight-run gasoline, and their resuks indicate also the presence of other naphthenic hydrocarbons.
Experiments to Determine Origin of Alicyclic Hydrocarbons Since the origin of the alicyclic hydrocarbons that occur in petroleum and in various cracking processes is somewhat uncertain, the experiments described below were performed in an endeavor to throw some light on the problem. While theqe are only preliminary experiments, the authors believe they have obtained definite experimental proof that the nheptyl radical decomposei a t least to some extent according to the equation: CH2CHZ /
CH:CH::CHrCHpCHpCH2CH2
/
--+ CHz \
CH2
/
+ CHS
CH2CH2
I n carrying out this experiment, diheptyl mercury was first prepared "by the method of Frankland and Duppa (3). The material was di3tilled at 140" C. under approximately 25 mm. pressure. 4 small amount of decomposition occurred at t,hie temperature and no further purification of the product' was attempted. It had a straw-yellow color and contained approximatelv 96 per cent of diheptyl mercury. It decomposed very slowl$ even in the dark, with separation of metallic mercury. The decomposition was erformed by passing a slow stream of carbon dioxide into liquid) diheptyl mercury which was maintained at 100" C. The carbon dioxide, containing a small quantity of the diheptyl mercury vapor, then passed through a. Pyrex tube 1 cm. in diameter and 15 cm. long, msinbained at 310" to 360" C. The decompo'sition products were caught in two traps at -100" C., and the perminent. gases were collected over water. The material collected in the traps was fractionated in a heated, jacketed semi-micro column about 55 em. long contain-
ing a nickel spiral. It gave quantitative separation of binary mixtures with an 18' C. boiling interval hut only qualitative separation \Then the boiling interval was 3" or 4". However, the column was considered adequate for this preliminary experiment. In a typical run 3.21 grams of product were obtained in the traps; this product was distilled and separated into four fractions. FIRSTFRACTIOS. This comprised 0.18 gram or 5.6 per cent by weight of the total distillate. The boiling point taken by t,he Einich micronlethod ( 2 ) gave very irregular and inconsist'ent results (80" to 99" C.), indicating a mixture. The crit'ical solution temperature with a n equal volume of aniline (microaniline number) mas found t o be 30" C., while with nhexane or le-heptane the corresponding value; were 52" and 71" C. The refractive index nz: = 1.3940. Froin these results the fraction appears to be predominantly cyclic. The remainder of the fraction was treated with concentrated sulfuric acid. d red-brown color developed immediately, indicating the presence of unsaturated compounds. The acid was pipetted off and a second portion added. a f t e r mat'er-washing and centrifuging, the remaining oil gave n2: = 1.4270 which corresponds exactly to cyclohexane. SECOKDFRACTIOK-. This comprised 0.16 gram (5.1 per cent of distillate) and gave only a trace of discoloration when treated with concentrated sulfuric acid. The refractive index for the original sample n2j = 1.3920 rose to 71': = 1.3930 after treatment with concentrated sulfuric acid. The aniline point was 58.5" and the boiling point 96.5" C. From the boiling point and t'he inactivity with sulfuric acid, this fraction appears to be heptane. On the other hand, the high refractive index, 1.3930 against 1.385 for heptane, and the low aniline point indicate contamination by a cycloparaffin. THIRD FRACTIOK. This fraction, 0.163 gram (5.1 per cent), became deep red on treatment with concentrate -1 sulfuric acid. Its aniline point was below 20" C. The refractive index was n': = 1.4260. Boiling point measurements show the fraction to be impure but boiling in the range 145" to 155" C. This unexpected fraction is certainly cyclic but too impure to be charact'erized by its properties. FOURTH FR.ICTION. The last fraction, distilled under 3 mm. pressure, mas largely tetradecane as shown by it;: boiling point of 250" C. and refractive index, nZD5 = 1.4280. The exit gas decolorized aqueous permanganste and presumably contained ethylene. The gas has not been further examined.
Literature Cited (1) Davidson, J . G., J . IND. ENG.CHEM., 10, 907 (1915). ( 2 ) Eniich, F., "Microchemical L a b o r a t o r y M a n u a l , " p. 32, New York, John Wiley &- Sons, 1932. (3j Frankland, E., and Ddppa, B. F., J . Chem. SW., 1 6 , 4 1 4 (1863). (4) Frey, F. E., and Hepp, H. J.. E d . , 24, 282 (1932). ( 5 ) Frolich, P. K., Simard, R., and White, rl., ISD ESG.CHEU.,22, 240 (1930). (6) Petrov, d.D., Ber., 63, 75 (1930). (7) &achanen, A. N.,and Tilitschejew, M. D., Ibid., 6 2 3 , 658 (1939). (8) Tongberg, C. 0.. and Fenske, hl. R., ISD. ESG. CHEM.,24, 814 (1932). RECEIVED
.\.ISY 14, 1938.
