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(9) Dept. Sci. Ind. Research Brit., Report, p. 69, 1929-30. (33) Mason, J., IVature, 129, KO.3246, 97 (1932). (10) Dubrov, Lavrovskii, Goldstein, Fish, and Mikhovskaya, (34) Mason, J., Smale, C. A., Thompson, R. N., and m e e l e r , T. S., Nejtyanoe Khozyaistvo, 22, 19 (1932). J. Chem. SOC.,1931, 3150. (11) Egloff, Levinson, and H e m a n n , J. Inst. Petroleum Tech., 18, (35) Mason, J., and Wheeler, T. S., J. Chem. Soc., 1931, 2282. 282 (1932). (36) McAfee, J. IND. ENG.CHEX.,7, 737 (1915). (11A) Egloff and Lowry, Ibid., 14, 558 (1928). (37) Sash, A. W., Stanley, H. M., and Bowen, 4. R., J. Inst. Pe (12) Egloff, Lowry, and Schaad, Zbid., 16, 133 (1930). troleum Tech., 16, 830 (1930). (13) Egloff and Moore, Met. & Chem. Eng., 15, 67 (1916). (38) Nef, Ann., 318, 14 (1901). (14) Egloff, G., and MorreU, J. C. (to Universal Oil Products CO.), (39) Naamlooae Vennootschap de Bataavsche Petroleum M a a b U. S. Patent 1,705,180 (1929). schappij, British Patent 344,470 (1928). (15) Ellis, “Hydrogenation of Organic Substances,” 3rd. ed., P. 528, (40) Naamloose Vennoot,schap de Bataavsche Petroleum Maatschap Van Nostrand, 1930. pij, and Peski, A. J. van, German Patent 548,982 (1928); (16) Fischer and Niggemann, Ges. Abhandl. Kenntnis Kohle, 1, British Patent 302,349 (1929): New Zealand Patent 61,931 231 (1917). (1929). (17) Friedel and Crafts, British Patent 4769 (1877). (41) Naamlooze Vennootschap Mijnbouw-en Cultuurmaatschappii (17A) Gault and Sigwalt, Ann. combustibles liquides, 2, 309, 543 Boeton, British Patent 333,553 (1930) ; French Patent (1927): Egloff and Lowry, J . Inst. Petroleum Tech., 14, 562 671,035 (1929). (1928). (42) Ibid., French Patent 671,116 (1929). (17B) Graetz, Ann. combustibles Ziquides, 2, 69 (1927); Egloff and (43) Otto, M.. Brennstof-Chem., 8, 321 (1927); Hofmann, F., and Lowry, J. Inst. Petroleum Tech., 14, 562 (1928). Otto, M.. British Patent 293,487 (1928); Hofmann, F., Otto, (18) Grignard and Stratford, Compt. rend., 178, 2149 (1924). M., and Stegemann, W., British Patent 313,067 (1929). (19) Henderson and Gangloff, J. Am. Chem. SOC.,38, 1382 (1916); (44) Pease, R. N., and Walz, G. F., J. Am. Chem. SOC., 53,3728 (1931). 39, 1420 (1917); Steele, J. Chem. SOC.,83, 1470 (1903); Gus(45) Peski, A. J. van, British Patent 315,890 (1928). tavin, Compt. rend., 136, 1065 (1903); 140, 940 (1905). (46) Pictet and Lerczynska, Bull. SOC. chim., 19, 326 (1916). (20) Heusler, Z. angew. Chem., 10, 280 (1896). (47) Rabek, T. I., Brennstof-Chem., 11, 189 (1930). (48) Schleede and Luckow, Ber., 55B, 3710 (1922). (21) Hofmann. .Fa.and Wulff, C., British Patent 307,802 (1929). (22) Hurd, C. D., “Pyrolysis of Carbon Compounds,” p. 125, Chemi(49) Schneider, M. I . T. Pub. Abstr. (Jan., 1928); Egloff, Lowry, and cal Catalog, 1929, Schaad, J. Znst. PetroZeum Tech., 16, 205 (1930). (23) I. G. Farbenindustrie, British Patents 265,601, 273,665 (1928); (50) Scholl, Seer, and Weltsenbock, Ber., 43, 2203 (1910): Scholl 295,990 (1929); Michel, R. (to I. G . Farbenindustrie), U. S. and Mansfield, Ibid., 43, 1737 (1910); Scholl and Seer, Ann., Patents 1,667,214 (1928), 1,741,472 (1929). 394, 111 (1912). (24) I. G. Farbenindustrie, British Patents 299,086 (1928), 309,199 (51) Soc. industrielle Hydrocarbures & derives (from International (1929), 320,846 (1929) ; French Patent 650,799 (1929). Industrial & Chemical Co.), British Patent 352,688 (1930). (25) I. G. Farbenindustrie, British Patents 303,761 (1929), 347,727 (52) Spilker, A. L. H., Zerbe, C., and Ges. fur Teerverwertung. (1930) ; French Patent 699,319 (1931), addition 36,080 British Patents 277,974,279,055 (1928) ; Chem. Age, 17, 496, (1930); Galle, E., Hofmann, G., and Born, H . (to I. G. Farben557 (1927). induatrie), Canadian Patent 285,916 (1928); u. s. Patent (53) Stanley, H. M., J. SOC.Chem. Ind., 49, 349 (1930). 1,854,146 (1932). (54) Stratford, Ann. combustibles liquides, 4, 83, 317 (1929). (26) I. G. Farbenindustrie, British Patent 326,322 (1930). (55) Sullivan, Voorhees, Neeley. and Shankland, IND. ENQ.CHEM., (27) Ipatiev and Routala, Ber., 46, 1748 (1913). 23, 604 (1931). (28) Joseph, Res. chim. ind., 36, 16 (1927). (56) Varga and Amasi, Brennstof-Chem., 12, 327 (1931). (29) Kling, A. J., and Florentin, J.-M.F.D., British Patent 253,507 (57) Weston, P. E., and Hass, H. B., J. Am. Chem. SOC.,54, 3337 (1927). (1932). (30) L a c b a n , Refiner Natural Gasoline Mfr.9 1% KO. 11- 72 (58) Wheeler, T. S., Binnie, D., and Imperial Chem. Ind., British (1931) ; (to Richfield Oil Co.), U.S. Patents 1,790,622,1,809,170 Patent 353,913 (1930). (1931): Richfield oil c0.9 French Patents 6959077-78 (1931) (59) Winter, H., and Free, G., Brennstof-Chem., 12, 451 (1931). (31) Mardick, J. R. (to Universal oil Products COJ, u. 8. Patent (60) Wood, Lowy, and Faragher, IND. ENO.CHEM., 16, 1116 (1924). 1,706,629 (1929). RECEIVED May 31, 1933. (32) Martin, E., and Fuchs, 0.. 2.Elektrochem., 27, 150 (1921). -4.7
Pyrolysis of Unsaturated Hydrocarbons CHARLES D. Hum, Northwestern University, Evanston, Ill. A survey is made of recent developments in the This duration is directly ProporXCEPT for prior work on pyrolysis of unsaturated hydrocarbons and a tional to the volume, v, of the ethylene and acetylene, tube and the temperature, T’, of p r a c t i c a l l y Our mechanism proposed which correlates the various the entering gas, but inversely knowledge concerningthe pyrolydata. The mechanism incorporates the .fact that . proportional to the rate of flow, sis of unsaturated h y d r o c a r unsaturated hydrocarbons pyrolyze characterisF , of gas through the tube and bons has been a c c u m u l a t e d within the past decade. In this tically into (1) simpler products, (2) isomers to the temperature, T“, of the which include branched chain hydrocarbons from heated gas. F is usually conperiod precise work has been sidered as the average of the carried out on a score of unsatustraight chain members, (3) dehydrogenation inflow and the outflow, Thus rates. T h e s t u d i e s h a v e inproducts*and (4) po1yrners* cluded members from all of the contact time = VT‘IFT”. It is logical to a s s u m e t h a t i m p o r t a n t families-namely, The importance of the contact time is mentioned and the influence of metal tubes discussed. the contact time a t a n y given olefins, acetylenes, allenes, and other dienes. Except for methr e a c t ion temperature, r a t h e r than the size of the tube or the ods to analyze hydrocarbon mixtures and methods to synthesize pure unsaturated hydrocar- rate of flow of gas through the tube, should be the controlling bons which were developed in this period, success could not factor in the pyrolysis. Experiments with both saturated and have been achieved. The present paper will endeavor to survey unsaturated hydrocarbons have demonstrated the correctness the important findings in this field and to correlate the evidence. of such an assumption. Confirmatory data with isobutylene, and n- and isobutane are presented in Table I. The reaction CONTACT TIME temperatures were 700’ C. and the reaction tubes Pyrex glass When a gas is passed into a hot reaction tube, it remains or quartz. These data were obtained in experiments with for a definite period of time (contact time) before escaping. F. D. Pilgrim and L. K. Eilers.
E
January, 1934
I N D U S T R I A L A N D E IL'G I N E E R I N G C H E hl I S T R Y
Contact times of similar magnitude were obtained with the Other hydrocarbons could have been listed and other expeributanes by maintaining a rate of flow seventy-five to eighty- mental conditions could have been included. The present five times smaller in the smaller tube. Not only were the selections, however, are not only representative but compaextents of decomposition comparable, but also analyses of the rable since they were all performed at 650" C. reaction products from the small and large tubes were found Some striking facts are revealed: to be practically identical. Isobutylene provided an analo(1) The five hydrocarbons may be grouped in three stability gous picture. Similar conclusions have been reached by zones. As judged by the long contact time required for a 26 Geniesse and Reuter (4) in their study of the pyrolysis of per cent decomposition, ethylene is easily the most stable. hydrocarbon oils. Although identical results may be antici- Propylene and isobutylene are next, with the former somewhat pated in small or large tubes wherein surface effects do not the more stable of the two, whereas the 1- and 2-butenes are to be the least stable, and of these the more susceptible appear, it is apparent that a given quantity of substance can seen compound is the 1-butene. be treated much more rapidly in large than in small tubes (2) The paraffin content of the gas is high. Methane prebecause of the faster rate of flow. If large quantities are in dominates in this saturated portion. (3) Propylene and ethylene are always formed from the consideration, as in industrial cracking operations, this is an higher olefins. important detail. (4) Acetylene is not an important reaction product except from ethylene. Hydrogen assumes less importance as a reaction TABLE I. FACTORS 1NFLVESCISG EXTENTO F DECOMPOSITION product in the higher olefins. --ISOBUTYLENE-TZ-BUTANEIEOBUTANE (5) Allene is not mentioned as a gaseous product. It is Tube vol., cc. 264 77 5.4 5.9 550 5.9 550 entirely absent in the gaseous products. In the case of isoRate of flow, cc./min.: butylene, for example, an equation would be balanced if allene 368 104 6.6 4.45 390 4.7 330 Entering Exit 407 108 7.2 8.6 7411 7.8 595 Tvere the other roduct besides methane, but no allene is formed. Final vol. from 1000 c c . Hence, a eimpg equation must be inadequate to portray the of substance 1106 1040 1075 1815 1870 1679 1800 effects. Contact time, sec. 12 13 14 15 13 17 21 Extent of decomposition, (6) Secondary effects must contribute to the nature of the % 48.4 48 38 73 75 77 70 products of the reaction since most of the reaction products are inert at the temperature of the experiment. Obviously, Several unsaturated hydrocarbons, such as ethylene, propyl- not these secondary effects become accentuated with higher perene, and isobutylene, are known to require a higher tempera- centage decompositions such as are caused by higher temperature (700" C.) of decomposition than the corresponding tures or longer contact times. saturated hydrocarbons (600 "). There are other unsaturated PRODUCTS O F REhRRANGEblENT AND DEHYDROGENATION hydrocarbons, however, such as allene, diallyl, and 4-methyl1-pentene where this is not the case since they decompose The question of determining whether or not simple olefins appreciably in the 400" to 500" C. range. Differences in might rearrange into isomers necessarily awaited a study of structure in the various unsaturated hydrocarbons must give the 1- and 2-butenes for its answer. As shown in Table 11, rise to these effects. there was found to be a substantial rearrangement of 1-butene into 2-butene during the pyrolysis of the former, and vice EXPERIMENTAL FINDINGS WITH USSATURATED versa. The ratio of lower boiling 2-butene (boiling point HYDROCARBONS 0.5" C.)l to higher boiling %butene (boiling point 2.5" C.) It is convenient to visualize the reaction products of pyrolp- was practically 3 to 2 whenever 2-butene was encountered sis experiments as grouped in three distillation ranges: as a reaction product. Analyses of these unsaturated hydrocarbons were by precise fractional distillation, details for (1) Products, especially gases, which are lower boiling than the original hydrocarbon. which will appear elsewhere. (2) Substances in the same distillation range as the original. I n the pyrolysis of 1-pentene and 2-pentene at 550" to 600" In addition t o recovered substance, this group especially includes C. it has been established (IS)also that isomeric unsaturated isomeric hydrocarbons and dehydrogenation products. hydrocarbons were important reaction products. In these (3) Higher boiling products, especially olymers and aromatic hydrocarbons. These three classes of progcts will be considered experiments the contact time varied between 8 and 19 seconds. in turn. One-seventh to two-fifths of the total reaction products conTo emphasize the greater compIexity encountered in the sisted of these isomeric pentylenes. One might guess that pyrolysis of unsaturated hydrocarbons as compared with 1-pentene would give rise to 2-pentene and vice versa. Actusaturated hydrocarbons, it should be remembered that ally this change was noticed, but as a matter of fact more isoparaffin hydrocarbons undergo only the first two of these propylethylene was formed than any other pentylene. Small three transformations. The higher boiling polymeric prod- amounts of trimethylethylene were identified also. Since ucts are not generated from paraffin hydrocarbons except it was known that the original pentenes were very pure, the rearrangement into isopropylethylene, involving as it does as secondary effects. the change of a CSchain into a Cc chain, must be a matter of T A B L E 11. MOLES OF' MAJORG.4sEoUs PRODUCTS FROM 100 importance in this type of reaction. MOLESOF DECOMPOBED HYDROCARBON This isomerization of a straight-chain into a branched chain ExTENT Isocompound was not observed with the butenes, for they gave CONOF MERIC TACT DESMALLER OLErise to no isobutylene. TIMECOMPN.CUH~U+, Hn OLEFINS CnHl FINB LIQUIDS Evidence for isomeric products has also been obtained with % bv wt. of olefin higher a - o l e h s (9). Being a-olefins, these compounds Sec. '% decampn. should possess lower boiling points (9) and lower index of reEthylene (90la 3 5 M 5 0 26 24 10 13 .. Some Propylene (14) 30 16 42 8.3 i i 3 .. 20-25 fraction values than the aorresponding j3-olefins. Therefore, Isobutylene (16) 27 23 47 15 20 3.5 40 the presence of considerable quantities of liquids with some1-Butene (19) 11 6i 44 3.7 33 (C3Hd .. 20 30 16 (CZHI) what higher boiling points and higher index of refraction 2-Butene (13) 13 44 28 5.5 19 (C3He) .. 25 30 4 (CZH4) values than the original materials provides strong evidence a To calculate the contact time it is assumed that the volume of Walker's for the presence of isomers. tube was 300 to 400 cc. A typical experiment with diallyl a t 500" C. and 13 seconds SIMPLER PRODUCTS OF THE PYROLYSIS contact time gave results as follows: From 22.5 grams of Some data from representative experiments with ethylene, 1 T h e lower boiling is the cis iaomer according to Boord, J . Am. Cham. propylene, and the three butylenes are given in Table 11. S a c , 6 4 , 756 (1932), and the higher boiling the trans isomer. I
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Vol. 26. No. 1
diallyl 16.1 grams of liquids were recovered. Of the latter, formed. In their unpolymerized state they are discernible 4.4 grams were diallyl (boiling point 58-60" C., naz 1.406), by their comparatively high boiling point and index of rebut there was a 60-65' fraction which weighed 3.7 grams fraction values: (n': 1.414) and a 65-75' fraction which weighed 2.7 grams BOILINQPOINT REFRACTIVEINDEXR E F E B ~ N C B (n': 1.440). The presence of 1,4hexadiene (6) (boiling c. 1 3-Pentadiene 42-44 1 4402 16') (r) point 64-66' C.), and 2,4-hexadiene (82) (boiling point 80-82', fihenylbutadiene 90 (15 mm.) 1:SlZS {16') (18) nz2 1.4463) is strongly indicated in these fractions. Hexatriene 77-78 1.4884 (13') (86) 4Phenyl-1-butene (boiling point 175-178' C., n2z 2-Propenylidenecyclohexane 159-161 ........ (27) 1.5090) underwent pyrolysis to give similar results. For exThus, in the sequence of related compounds-1-alkene to ample, in a run a t 550', 12 grams from an original 50 grams ' were recovered which boiled between 170' and 185" C. The 2-alkene to l13-alkadiene-there is an evident increase in the 7 grams of this which boiled a t 170-178" gave 1.5225 as an magnitude of boiling point and refractive index values which index of refraction. The 178-185' fraction had an nz: value is of service in detecting the presence of dehydrogenated prodof 1.5268. It is probable that considerable 4-phenyl-2- ucts as well as rearranged products. The case of 4phenyl-1-butene (I) will be used to summarize butene (66)(boiling point 176' C., n': 1.5101) or l-phenyl-lbutene (17) (boiling point 189", nl! 1.5414) was in this frac- the foregoing: tion. In a similar way, allylcyclohexane (boiling point 149" C., n'; 1.451) after pyrolysis a t 500-600' yielded a distillate in the 145-150' zone which was not pure allylcyclohexane, for its n': value was high (1.460). Likewise, p-allyltoluene / (boiling point 180" C., n': 1.5082) underwent rearrangement, CH3 B. p., C. 175 176 189 inasmuch as the 176-181 'fraction from an experiment a t 575' n D 1.5090 (20") 1.5101 (20") 1.5414 (16") possessed a value for nzi of 1.521; the 181-189' C. fraction I I1 I11 was 1.527. Eight grams of the former and 11 of the latter came from an original 40 grams of the allyltoluene (IO). Rearrangement of the same nature has been noticed in the pyrolysis of acetylenes. Methylacetylene changes easily at 550' C. into allene (or its polymers), and ethylacetylene changes into methylallene (19): \/ O
04;
CH-CECH
+CH-C=CHz
An analogous rearrangement of benzohydryl-tert-butylacetylene into a,a-diphenyl?-tertbutylallene (99) takes place together with some dimerization, on distillation: Me8CC=C-CHPh2 4Me8CCH=C=CPhZ Recent work (11) on the pyrolysis of 1-hexyne (butylacetylene) points to an identical conclusion. Such facts as these call for a reinterpretation of the older data of Guest (6) who heated 1-heptyne at 350' to 380' C. over soda lime or pumice and reported a pronounced rearrangement into isomeric "disubstituted acetylenes." The structure of the latter was not determined definitely. The analytical evidence for carbon and hydrogen and the absence of the -C=CH group is satisfactory for dialkylacetylenes, but is equally satisfactory for allenes. There is good reason to believe, therefore, that alkylacetylenes rearrange into allenes. The latter, of course, may continue this process of rearrangement and produce dialkylacetylenes: RCH~CH~CECH +RCH2CH=C=CH2 +RCHZCzCCHs etc.
DEHYDROGENATION PRODUCTS In the distillation range of the undecomposed hydrocarbon and ita isomers also are to be found the products of dehydrogenation. Proof for the presence of butadiene in the gases from 1- or 2-butene a t 600' to 650' C. is fairly easy because bromination yields a crystalline tetrabromide. Identification of naphthalene in the products from 4phenyl-1-butene is simplified because of its crystalline nature. Uusually, however, the problem is more difficult. For example, 1- or 2-pentene may yield pentadiene; 4-phenyl-1-butene may yield phenylbutadiene; or diallyl may yield hexatriene. In a compound like allylcyclohexane (CaH16) several dehydrogenation products are possible, such as CeHlr, CgH12, CgHlO, the first of which would be 2-propenylidenecyclohexane. These compounds probably polymerize extensively when
CH
($HI
90(15 mm.) 1.6128 (16")
IV
218
Solid V
The evidence for the presence of 11, 111, or IV rests on the formation of liquids possessing higher boiling points and higher refractive index values than I. Naphthalene (V) was present in considerable quantity. There was formed also much higher boiling, polymeric material, from which some stilbene (PhCH=CHPh) was isolated. Toluene was the important component of the lower boiling liquids, and propylene was the most important product of the gas. 4-Phenyl-1-butene and phenyl allyl ether possess a structural resemblance: Ph-CH2-CsH6 and Ph-O-CpHs. At 200' C. the ether is known to undergo a rearrangement into o-all ylphenol :
For an analogous reaction to occur with the phenylbutene, the product of rearrangement would be o-allyltoluene,
ncH3
Special experiments were designed to investigate this possibility. The phenylbutene was passed and recirculated through a furnace for 97 hours a t 400" C. without obtaining enough allyltoluene to identify positively. As a matter of fact, it may be recalled that n-allylaniline (Ph-NH-CaHd likewise failed to rearrange into o-allylaniline, but a t 700' it did give rise to quinoline (1) in much the same manner that 4-phenyl-1-butene yields naphthalene.
HIGHER BOILINGPRODUCTS The case for the polymerization of olefins by pyrolytic methods has been demonstrated repeatedly. For example,
January, 1934
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
gaseous olefins such as ethylene, propylene, or isobutylene give rise to large yields of liquid products when they are passed through a tube a t 700" C. for a limited hot contact time. At lower temperatures there are required longer contact times. Pressure also favors the effect because of the diminution in gaseous volume. I n this connection, Dunstan ( 2 ) has reported an 80 to 87 per cent conversion of ethylene or propylene into nonaromatic liquids by heating a t 380" to 400" C. for 2.5 to 3 hours under maximum pressures of 67 to 94 atmospheres. Higher olefins also polymerize, as witnessed by the formation of high-boiling liquids and tars during the pyrolysis of diallyl, allylcyclohexane, 4-phenyl-l-butene, or p-allyltoluene (9, 10).
Aromatic hydrocarbons predominate in the liquids from experiments with olefins at high temperatures, but a t lower temperatures they may be totally absent. Isoprene is illustrative. Between 90" and 250" it gradually polymerizes to rubber whereas a t 750" it rapidly gives rise to benzene, toluene, naphthalene, and other aromatic hydrocarbons. The experimental truth of the acetylene-to-benzene polymerization is far from the simple reaction, 3Cd3.2 -+-CaHO, credited to it by most authors of elementary organic textbooks. As a matter of fact, between 450" and 600", acetylene polymerizes to a complex of products of a wide range of volatility (21) containing not only benzene but also hydrocarbons with an aliphatic type of unsaturation.
ANALOQIES FROM OTHER SERIES
53
the gas evolved was propylene, not allene. This formation of toluidine without allene from allyltoluidine is comparable to the formation of propylene without allene from diallyl, or of methane without allene from butylene:
Recently, Wittig and Leo ($1) observed that a t distillation temperature 1,1,6,6-tetraphenyldiallyl (Ph&=CHCH2CH2CH=CPh2) gave about a 50 per cent yield of a,a-diphenylpropylene and a polymerized residue which they assumed to be polymerized diphenylallene. Evidently the diphenylallene should be eliminated from consideration. The reaction mechanism is strictly analogous to the case of diallyl. Obviously the simple equation type of mechanism is inadequate for the representation of these facts. This is in contrast to some other cases of pyrolysis where the simple equation mechanism fits the evidence much better. To illustrate: n-butane gives rise to approximately equivalent amounts of methane and propylene (or of ethane and ethylene) when the extent of decomposition is kept low. Must one conclude, therefore, that butene has one mechanism of pyrolysis and butane another? This seems objectionable. Hence, the conclusion is reached that the simple equation for butane is a summation effect caused by fortuitous circumstances.
bfECHANIShl
Various suggestions have been offered to explain the proAny satisfactory interpretation of the facts about hydro- duction of simpler substances from a hydrocarbon as it undercarbon decomposition must rest upon a broader basis than goes high-temperature pyrolysis. The most satisfactory one hydrocarbons alone. The pyrolysis of tetraalkyllead (do), is the suggestion that radicals are intermediates in this change. for example, has provided satisfactory evidence that alkyl Rice's mechanism (23) for the pyrolysis of saturated hydroradicals are the primary reaction products. Since it is un- carbons is based on this idea. Because the C-H bond is likely that pyrolytic scission of a C-Pb bond is materially stronger than the C-C bond, Rice postulated C-C scission different from that of a C-C bond (for both involve the as the primary effect. A radical thus formed is considered rupture of an electron pair), one is forced to conclude that to do one of two things: (1) It may appropriate a hydrogen radicals are intermediates also in the pyrolysis of other or- atom from a neighboring hydrocarbon molecule with which ganic substances. it collides, thereby starting a reaction chain; (2) it may deDirect evidence for this contention was found by Rice and compose prior to such a collision into an olefin and a simpler co-workers who pyrolyzed acetone, propane, etc., and con- radical which disappears by collision as before. If the deducted the products over a mercury-covered surface which composing radical is ethyl, isopropyl, or tert-butyl, the simpler caused organic mercury derivatives to be formed. These radical is hydrogen; otherwise, it is an alkyl radical. dialkylmercury vapors were converted by reaction with merThe fundamental correctness of this theory is not unchalcuric bromide into crystalline alkylmercuric bromides and lenged a t the present time because there are recent estimates readily identified. From the pyrolysis of acetone, methyl of the strength of the C-C bond which are considerably in radicals were confirmed since methylmercuric bromide was excess of the activation energy (namely, approximately isolated. Similarly, propane (CH,CH2CH3) was interpreted 65 Calories). This difficulty has been treated by Rice (24). to break momentarily into methyl and ethyl radicals for Radicals have also been proposed (16) as intermediates in both methyl- and ethylmercuric bromides were formed. the pyrolysis of unsaturated hydrocarbons. Presumably Ethers provide another useful analogy. Unsaturated the mechanism follows a similar course with unsaturated ethers, such as allyl phenyl ether, are particularly thermolabile. as with saturated hydrocarbons, but added complexities That heat ruptures the 0-C bond is evident inasmuch as the occur in the former. Besides C-C bonds, there are C=C resultant compound is allylphenol. Phenyl vinyl ether and C=C bonds to be considered. Furthermore, the strength (Ph-O-CH=CH2) and phenyl butenyl ether (Ph-0of the C-H and C-C bonds is influenced by their proximity CH2CH2CH=CH2) are much more stable. Therefore, one to the unsaturated location. The customary values (8) for must conclude that the allyl group is peculiarly instrumental the strength of carbon to carbon bonds are: C-C, 71 in weakening the 0-C bond in phenyl allyl ether (Ph-0Calories;2 C=C, 125; C=C, 166. On this basis, it may be CH&H=CHz). assumed safely that the C Z C bond in CLC"C=C is There is a similar weakness of the n-allyl bond. One greater than 71 Calories, whereas the C L C bond is less than needs only to reflux diallyltoluidine, for example, to cause an evolution of propylene. Toluidine and tarry material com- 71. The C-H bonds of methane and benzene, respectively, prise the residue. The moderate conditions under which this are given as 92 and 101 Calories. Hence it may be assumed breakdown occurs emphasize the labile nature of the allyl that the C A H bond in H H is greater than 92 calories, 81 a ! group. Furthermore, there is the inference that the carbonc-c=c allyl bond in unsaturated hydrocarbons may be also a position of weakness. 2 Kistiakowsky and Gershinowitz, J . Chem. Phys., 1, 438 (1933),glvo Another significant fact from the above experiment is that the C-C value a8 77 Caloriea
I N D U S TR I A L A N D E N G I NE E R I N G C H E M I STR Y
54
whereas the C L H bond is somewhat less than 92. It seems unlikely that the C Z C value is enough greater than 71 or that C L H is enough less than 92 so that the latter would be weaker than the former. Thus it may be assumed that in simple o l e h s the two electrons between carbon-to-carbon bonds are the first to break. An exception would arise in a case wherein the C L C bond is also a C Z C bond, as in
H
I
I
or in CBHs-C-C=C, etc. Two univalent radicals appear when the C-C single bond breaks. It might be imagined that the C = C or C=C bonds would be unaffected because of their greater energy content. True it is that complete rupture is excluded for this reason. However, complete rupture of a double bond would involve four electrons, not two, whereas rupture of a triple bond would involve six. It is reasonable to believe that heat would affect two of the four or six electrons in C=C or C=C as readily as it does the two in C-C. This concept may be visualized as follows: (1) Rupture of two electrons of the single bond R3C: CR3 +2R3C. (2) Rupture of two electrons of the double bond RzC: :CRz +IhC:CIh (3) Rupture of two electrons hf the triple bond RC: : :CR +RC: :CR
. .
With this picture, the C=C double bond would give rise to one bivalent radical, C-C, comparable to the two univalent
I I
radicals from the C-C single bond. Similarly, the C=C triple bond would give rise to one bivalent radical, C=C.
I
1
I n general, the fate of the bivalent radicals should be similar to that of the univalent radicals. Radicals formed from saturated and unsaturated hydrocarbons may be expected to appropriate hydrogen atoms by collision with unchanged molecules. This effect is favored the higher the temperature. No other effect occurs in the collision of a radical with a saturated molecule. However, with unsaturated molecules another possibility presents itself-namely, the addition of the radicals to the unsaturated bonds. The following equation is illustrative: R R RR R. + C::C + R:C;C. HH
I 2-1
comes in contact with unchanged ethylene molecules. It may appropriate hydrogen, a reaction leading to ethane and acetylene, or it may add to the double bond: CHAHg
I
or
I
CH-CHz
1
1
Reacfion 8.
-CH~-CHZ-CHZ-CHZ-CH~-CHZ-, (B)
+ CH-CH, 4CHsCHz + CHFCHI + 2 C H d H z +CHsCHs + ~CHFCH-
et c.
Reaction 1 calls for higher temperatures than reaction 2. The univalent radicals of (1) may add also to ethylene as in (2), but a t their high temperature of formation it is to be expected that the greater part of the C H s C H z and C H p C H radicals would change into C1H4 and C2H2,respectively, by detachment of a hydrogen atom. Some ethane would escape, but much of it would pyrolyze further into 2CH3-, thence into 2CHd as in reaction 1or into 2CHaCH&Hr, etc., as in reaction 2. The product (2B) may isomerize to cyclohexane. Dehydrogenation of (A) and (B) would be brought about by collision with other radicals. Thus, (A) would give rise to butadiene and (B) to cyclohexene or benaene. The lower the temperature, the greater should be the tendency for a long polymeric chain. The actual products of the ethylene pyrolysis are in k e e p ing with this conception. For example, Schneider and Frolich (&?), by their method of extrapolation to zero conversion, found the following to be the initial, stable products; hydrogen, butadiene, propylene and higher olefins, acetylene, ethane, methane. ACETYLENE.Acetylene should yield a bivalent radical in the manner of ethylene: Reaction S.
CH=CH
I
I
+ CH=CH
or CH=CH f 2CH=CH
I
I
+CHFCH I +CHFCH~
At high temperatures the CH=C-
H-
+ 2C.
+ CH=C+ 2CH3C-
radical would change into
Reaction 4.
CH=CH-CH=CH-CH=CH,
1
HH
The R&H-CHRis a new radical which may repeat the process. Since this reaction is impossible with the saturated hydrocarbons, it explains why they decompose exclusively in t,he direction of simpler products. The addition reaction, which gives rise to the polymeric materials, should be favored a t the lower temperatures and at higher pressures. It should be especially favored when a large quantity of olefin is present, which is the situation when unsaturated hydrocarbons are pyrolyzed, but which is not the case when saturated hydrocarbons are taken. Some specific illustrations will be cited. ETHYLENE.The bivalent radical from ethylene (listed below as CH CH2) may be expected to do two things as it
Reaction 1.
The addition reaction may be considered to have the following sequence:
H
C=C-C-C=C
Vol. 26, No. 1
(B)
I
etc.
Substances A and B are comparable to those from ethylene. The vinyl and ethynyl radicals from reaction 3may also participate in reaction 4. (B) would isomerize readily to benzene, but it is apparent that the liquids formed from acetylene should contain many more substances than benzene alone. Propylene should dissociate essentially in two PROPYLENE. ways: (1) into CHsCHZ=CH- radicals, (2) into the bivalent CH&H-CHz radical. In a dehydrogenation type
+
I
I
of collision with unused propylene, the methyl radical becomes methane, the vinyl radical becomes ethylene, and the propylene radical becomes either the propyl radical or propane. In the hot zone, propane breaks rapidly via methyl and ethyl radicals, The fate of the methyl, ethyl, and propyl radicals follows Rice's formulation. Dehydrogenation of CaHe by collision with a radical should give rise to an allyl radical, C H M H C H p . This radical must perpetuate itself by collision with propylene until an addition reaction occurs. The addition of radicals to propylene follows the plan suggested for ethylene. Thus: R-
CsHe
+ CsHs+R-C8H6- +R-CsH0-GHt.-,
etc
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1934
Here, R may represent the univalent methyl, vinyl, and allyl radicals and also the bivalent propylene radical. Dehydrogenation, ring closures, etc., occur as before. REARRANGEMENTS. The three butylenes follow the mechanism given for propylene. However, with 1-butene the lower decomposition temperature is noteworthy. It is associated with the more facile rupture of the C B C bond. This bond and the C B H bond are involved in the rearrangements which are noted. The pyrolytic rearrangement of 1-butene into 2-butene and vice versa is an instance of the "allylic" rearrangement ( l e ) . In such a three-carbon system there are many known examples to illustrate the rearrangement of >C=C-C < into >C-C=C
