2872
INDUSTRIAL AND ENGINEERING CHEMISTRY DISCUSSION OF RESULTS
The quaternary ammonium hydroxide types of anion exchange resins are effective catalysts for the cyanoethylation of most primary and secondary alcohols. The reaction is sufficiently vigorous that it is desirable to hold the reaction temperature below 15’ C. In some cases, such as with allyl alcohol, methyl cellosolve, furfuryl alcohol, and benzyl alcohol, TT hich are less reactive, temperatures of about 45” C. Rere found t o be more satisfactory. The conversions to alkoxypropionitriles are very good, comparable t o those obtained using conventional basic catalysts. Isolation of the product from the reaction mixture was somewhat simplified since there n a s no problem of neutralizing the base used as a catalyst or otherwise removing it from the The resin can be recovered by simple iiltration so that the I eaction product contained only the alkoxypropionitrile along with the unreacted slcohol and acrylonitrile. The product can be obtained by a simple distillation. I n order to prepare the dicyanoethylated product of ethj lene glycol it was neceesary to use a very large excess of acrylonitrile; otherwise considerable quantities of a lower boiling material are obtained which can react further Kith more acrylonitrile. JT’ith a ratio of 2 moles of acrylonitrile to 1 mole of ethylene glycol, only a 327, of the glycol was converted to the dicyanoethylated product, but if a 6 to 1 mole ratio n a s used an 85% conversion to the dicyanoethylated product was obtained. tert-Butyl alcohol can not be made t o react JT-ith acrylonitrile in the presence of a strongly basic anion exchange resin and in fact can be used as a solvent in which to carry out other cyanoethylation reactions. This has also been observed v,7hen conventional basic catalysts are used (83). There seems to be little difference in catalytic activity among the commercially available, strongly basic anion exchange resins. There was, however, some tendency for all of these resins to become inactive after a fern runs. This is possibly the result
Vol. 44, No. 12
of the formation of a film of polyacrylonitrile on the surface of the resin. LITERATURE CITED
American Cyanamid Co., Brit. Patent 544,421 (April 13, 1942). Bruson, H. A. (to The Resinous Products 6: Chemical Co.), U. S. Patent 2,280,790 (ilpril28, 1942). Ibid., 2,280,792 (April28, 1942). Ibid., 2,401,607 (June 4, 1946). Bruson, H. A , , aiid Riener, T. TT., J . Am. Chenz. Soc., 65, 23 (1943).
Conant, J. B., and Tuttle, Ory. Syntheses, 1, 45 (1921). Cope. A. C . , J . A m . Cheni. Soc., 59,2327 (1937). Cope, -4.C., and Hoffman, C. AI., I b i d . , 63, 3456 (1941). Cope, A. C., Hoffman, C. M.,TTyckoff, C., and Ilardenhergh, E., I b i d . , 63,3452 (1941).
Fischer, F. G., and Marshall, Ber., 64, 2825 (1931). Galat, A,. J . Am. Chem. Soc., 70, 3945 (1948). Haskell, V.C., and Hammett. L. P., Ibid., 71, 1284 (1939). Hoffer, IT., S\viss Patent 173,737 (April 1, 1935). Jenny, H. J., Colloid Sci., 1 , 2 (1946). Knoevenagel, E., B e r . , 29, 175 (1896). I b i d . , 31, 735 (1898).
Xuhn, R., Badstubner, TT., and Grundmann, C., Ibid.. 69, 98 (1936).
Kunin, R., aiid Myers, R. J . , “Ion Exchange Resins,” p. 138, Iiew York, John TTiley and Sons, Inc., 1950. Lapwort,h,A., and McKae, J. A . , J . Chenz. Soc., 121,2741 (1922). Levesque, C. L., and Craig, A. M., ISD. ENG.CHEM,,40, 96 (1948). MacGregor and Pugh, J . Chein. Soc., 1945,535. RIcRae, J. A , and Manske, R. H. J., Ibid., 1928, 484. “Organic Reactions,” Vol. V, Chap. 2 , Kew P o r k , John m’iley and Sons, Inc., 1949. Perkin, W. H., J . Chem. Soc., 61, 837 (1892). Pratt, E. F.! and Werble, E., J . Am. Chem. SOC.,7 2 , 4638 (1950). Sussman, S., IKD.ESG.CHSM..38, 1228 (1946). Thomas, G. G., and Davies, C. V., Nature, 159,373 (1927). Unlermohlen, W. P., J . Am. Chem. Soc., 67, 1505 (1945). Wurtz, A , , Cornpt. rend., 74, 1361 (1872). RECEIVED for review November 2, 1961. ACCEPTED August 15, 19.52. Presented in part before the Division of Organic Chemistry a t the 119th Meeting of the AVERICAN C H E m c h L SOCIETY,Cleveland, Ohio, 1951.
Thermal Cracking of Alkyl Phenols J
MECHANISM OF DEALKYLATION B. W. JONES AND M. B. NEUWORTH Research and Development Division, Pittsburgh Consolidation Coal Co., Library, P a .
I
T HAS been recognized that many of the compounds found in
high temperature tar represent thermally cracked products of homologs present in low temperature tar because of the difference in carbonization temperature. Specifically, it has been shown by studies in an experimental coal carbonization retort that the phenols from high temperature carbonization tar are made up of substantially more phenol and smaller quantities of xylenols than the corresponding fraction from low temperature tar ( 4 , 10). This Tas attributed to the thermal splitting of alkyl groups a t the higher temperature. A more quantitative correlation of these differences in composition is of interest in order to permit an understanding of some of the important reactions occurring during carbonization. The direct interpretation of the processes involved in the production of various alkyl phenols is extremely difficult owing to multiplicity of concurrent reactions. A considerable simplification can be effected by studying the controlled thermal crack-
ing of pure alkyl phenols under conditions which permit a determination of the relative rates of cracking and the quantitative product distribution. Thermal cracking studies on the individual isomeric cresols have been carried out by a number of earlier workers (6, 11, 12). The mechanisms TT-hich %-ereproposed as a result of their studies are incompatible TTith currently accepted theories of high temperature cracking chemistry, owing, in part, t o the severe thermal treatment resulting in a masking of the primary processes. To overcome these difficulties a n experimental apparatus was designed in which controlled temperatures and residence times as short as 0.05 second could be maintained. It ?vas felt t h a t under these conditions the primary processes would be more evident. I n addition, a precise feeding system for alkyl phenols was developed to permit accurate material balances. For convenience, the alkyl phenols %-erediluted with superheated stpam. This had the advantage of supplying an inert atmosphere and
December 1952
v
INDUSTRIAL AND ENGINEERING CHEMISTRY
d h i n a t i n g reactor plugging by simultaneous gasification of cirbon laydown. I n addition, steam did not dilute the noncondensable product gas and thus permitted more exact gas analyses. The use of infrared analytical techniques permitted the quantitative analysis of the relatively simple mixture of compounds resulting from the thermal cracking of individual alkyl phenols with a precision which would be quite difficult to attain by distillation or chemical methods for relatively small amounts of cracked product, particularly where analysis of close boiling isomers was involved. The application of the infrared analysis method described by Friedel et al. (6) to the quantitative determination of the low boiling phenols obtained from low temperature and high temperature tar samples permits a direct comparison of the relative percentages of the individual phenols in greater detail than was possible by techniques available to earlier workers. An attempt will be made to interpret the differences in composition in terms of the behavior of the individual alkyl phenols studied.
2873
during the material balance period was checked by weighing the water into the reservoir during the material balance period.
PRODUCT RECOVERY SYSTEM. The plug closure a t the bottom of the cracking unit contained an opening for the cracking vapors. A steel steam condenser connected directly to the plug provided rapid quenching of the cracked vapors. A water-jacketed receiver with bottom withdrawal stopcock was joined directly to the steam condenser by means of a glass-metal ball and socket joint. Condensation of the major portion of the steam and t a r acids occurred in this condenser. The remaining vapors from this condenser passed through a series of two or three dry-ice traps joined successively by ball and socket joints. The noncondensable gas from the final dry-ice trap was metered through a wet test meter. A glass tee in the line provided a convenient opening for taking gas samples.
EXPEKIMENTAL
*
CRACKISGREACTORS.Two cracking reactors were used, having volumes of 13 and 75 ml. in the cracking zone, respectively. The cracking apparatus is shown schematically in Figure 1. The larger cracking reactor consisted of a 1-inch stainless steel pipe, 27 inches long. The tar acid entry was surrounded by superheated steam entering a t about 480" C. which vaporized and diluted the tar acid vapors, no external heat being supplied a t this point. The resulting mixture was preheated in a 12-inch zone by means of an external electric heater. The preheat zone maximum temperature was adjusted manually so that no significant cracking occurred. A 1/4-inch thermowell extended down through the preheat zone to within 1/2-inch of the cracking zone. The temperature a t the bottom of the thermowell was recorded continuously. The cracking zone, 13 inches long, was packed with quartz chips surrounding a l/r-inch thermocouple well passing up through the center of this zone. An electric furnace surrounded the tube and contained three heaters, each manually controlled. Temperature settings were determined by three thermocouples placed in the thermowell so that they corresponded to the centers of the three heaters. The temperature variations recorded by these thermocouples on a Brown electronic temperature recorder did not exceed f 5" C. The top and bottom of the reactor were sealed by means of threaded plugs, closure being made by means of soft copper gaskets. The smaller reactor was identical in design except the cracking zone was reduced from 1to '/$inch inside diameter. STEAMFEEDING SYSTEM. Water for superheated steam was supplied t o the cracking unit by pumping distilled water with a small centrifugal pump. To maintain the low liquid feed rates required for cracking, a side stream was used for steam production, the bulk of the pump output being returned to the water reservoir. A schematic diagram of the steam feeder is included in Figure 1. The ratio of water going to the two streams was controlled by needle valves. The water feed for steam production was metered through a rotameter, the rate being manually controlled by means of a needle valve upstream of the rotameter. Additional adjustment was made by controlling the speed of the centrifugal pump. This arrangement results in uniform liquid water feed in the range of 1 to 10 ml. per minute. To achieve smooth vaporization of the water, it was introduced into the vaporization tube proper through a hypodermic needle extending about 2 inches into the heated area. The vaporization tube, a 1-inch stainless steel pipe, 32 inches long, was packed with stainless steel metal textile for improved heat transfer. The tube was heated by two electric heaters with separate manual control. The temperature of the superheated steam was measured in a thermocouple well a t the exit end of the superheater and recorded continuously. The amount of water actually fed
Figure 1.
Schematic Diagram of Cracking Apparatus
OPERATIONOF THE CRACKING UNIT. The unit was purged with steam prior to feeding phenols. The alkyl phenol feed was then started and all temperatures were adjusted. During this period all the cracking products were vented through the bottom of the water-jacketed receiver. When steady state conditions were reached the vapors were permitted to go through the recovery train and wet test meter. The alkyl phenol feed to the cracking unit during the material balance period was measured by the elapsed time during this period. The condensed steam and organic layer were withdrawn intermittently from the water-jacketed receiver. Usually three spot gas samples were taken during the material balance period. The total gas production was measured with the wet test meter. For each cracking experiment about 200 to 500 grams of alkyl phenol were fed t o the cracking unit. ALKYLPHENOL FEEDER (9). A feeder for alkyl phenols was used, based on a motor-driven syringe design. The details of this pump will be described elsewhere. Feed rates in the range of 0.8 to 13.0 ml. per minute were possible with a precision of *I%. MATERIALS
The pure compounds, except p-cresol, used in this study were purchased from Reilly Tar and Chemical Co. in a minimum purity of 95 to 98%. The p-cresol was a synthetic product obtained from Hercules Powder Co. These compounds were further purified by redistillation in vacuo on a 25 X 150 em. Cannon packed column, a heart cut being retained in each case. The
INDUSTRIAL AND ENGINEERING CHEMISTRY
2874
purified cuts were checked qualitatively for isomer impurities by infrared analysis. ANALYSIS OF CR.4CKED PRODUCTS
The n-ater and organic layers x-ere separated by decantation, The wat,er and contents of the dry-ice traps were extracted exhaustively with ethyl ether. The ether extract was then freed of ether on an efficient column. The solvent-free extract was combined n-ith the previously separated organic phase and vacuum distilled in a Claisen flask to determine nondistillable residue. The distillate from this separation was then distilled analytical1~under vacuum in a Cannon packed column, 1.9 X 100 cm. The distillatioii was carried well on to the distillation flat of t,he feed alliyl phenol, using the unconverted alkyl phenol as a barking liquid t o ensure the recovery of all Ion-er boiling liquid produrts. The result,ing liquid distillate TTXS analyzed by infrared absorpt.ion analysis for both phenol and aromatic hydrocarbon components. &4nalyseswithin + 1% absolute mere possible by this technique as shown by analyses on synthetic mixtures of known composition. In the case of the sj-lenols, the cracked liquid products were more complex and required recovering the distillate in several fractions to prevent interference in the infrared analysis.
I
8.5
9.0
+x
9.5 (OK)
Figure 2. Variation in Log Specific Reaction Rate Constants with Reciprocal of Absolute Temperature for Several Alkyl Phenols
The noncondensable gas was analyzed by a conventional Orsat analysis which provided the composition of the gas as ell as its carbon content. The carbon content of the residue, cracked liquid products, and gas permitted an accurate determination of the fraction converted by a carbon balance on the feed and cracked product. PREPARATION AND ANALYSIS O F L O W BOILIXGP H E N O L S O F TARORIGIK. A sample of coal tar distillate boiling up to 250" C. from a commercial by-product coke oven tar mas the source of low boiling phenols representing high temperature carbonization. The low boiling phenols of low temperature origin were extracted from the distillate of a representative sample of tar produced by the Disco process. The distillates xere extracted three times with a threefold excess of 10% sodium hydroxide. During contact with caustic
Vol. 44, No. 12
the solutions were kept in an inert atmosphere to suppress ouidntion. The combined caustic extracts were made up to 20% sodium hydroxide by the addition of solid sodium hydroxide. The resulting solution was then wvashpd exhaustively with benzene to remove hydrocarbons and tar bases. In the case of the high temperature tar distillate it was necessary to dilute the distillate with benzene to dissolve naphthalene and heat the distillate during extraction. The recovered benzene washings were x-ashed with 10% sodium hydroxide to recover phenolq lost by hydrolysis. The combined .odium hydroxide extracts were sprung with excess 30% sulfuric acid. The free phenol? vere separated from the aqueous layer, followed by exhaustive extraction of the aqueous layer a i t h bmzene. The organic la) er and the benzene extract were combined and dried by azeotropic distillation. The dried benzene extract was stripped free of benzene on a Cannon packed distillation column, 1.9 X 100 cm. This was followd by fractional distillation into 6 cuts TT-hich were analyzed by the method of Friedel et al. (5) on Baird double beam infrared spectrophotometer with rock salt optics. DISCUSSION OF RESULTS
A series of cracking experiments was made on the isomeric cresols varying the temperature and residence time. These dat,a are summarized in Table I. The marked thermal stability of m-eresol as compared with 0- or p-cresol is indicated from its significantly lower conversion at the same temperature and residence time. Nakai ( 1 2 ) and Kosaka ( 1 1 ) recognized the greater stability of m-cresol. The differences were not as marked as shown by the results in this investigation. This was probably due to the unavailability of sufficiently precise aiialytical methods or the effect of secondary processes resulting from the extremely high conversions. The product distribution clearly indicates that phenol is one of the primary cracking product,s of the converted cresol. In the case of 0- or p-cresol, molar yields from 35 to 54% were obtained. Any interpretation of this cracking process must be in accord with the observed recovery of phenol, benzene, and toluene in molar yields of over 80% of the cracked cresol in some cases. A kinetic analysis of the results of cracking o- and ni-cresol was made. A typical plot of specific reaction rates for cracking of the alkyl phenol against t,he reciprocal of the absolute temperature is shown in Figure 2, assuming the reaction is first order and homogeneous, Although sufficient data have not been accumulated to establish the homogeneity and first-order character of t,his reaction, the vork of Szivarc (13) on the pyrolysis of the isomeric xylenes provides some basis for this assumption. The activation energies calculated from this data are 75 and 69 kcal., respectively, for ?n- and o-cresol. It is of interest in this connection that Szwarc reported values of 77.5 and 74 kcal. for the cracking of m- and o-xylene, respectively. The effect of increasing the number of methyl groups on the reaction rate is shown in a similar plot of the cracking data on 2,4-xylenol in Figure 2 . A higher cracking rate than that of the cresols is observed. Preliminary results on 3,4-xylenol show a similar increase in cracking rate over the cresols. The aotivation energy calculated for 2,4-xylenol is 70 kcal., which is essentially identical to the value for o-cresol. The information obtained in this lyork, coupled with the kinet,ic studies of Szlyarc ( I S ) on the isomeric xylenes, permits one to postulate a cracking mechanism for methyl-substituted phenols. The rate-determining step appears t'o be free-radical rupture of a C-H bond in the methyl group to a hydroxpbenzyl radical and a hydrogen atom (step I ) . This hydrogen atom attacks a second molecule of cresol in one of three ways-Le., by complex formation followed by ejection of a met,hyI radical with the formation of phenol (step 2a); hydrocracking of the hydroxyl
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1952
2875
m-cresol-p-cresol mixture resulting from cracking 3,4-xylenol analyzed 75% met,a and 25% para. It p-Cresol appears on the basis of these results that if demethylation occurs via hydrogen atom attack, that 0.05 this attack is preferential in the ortho and para 12.7 position with respect to the hydroxyl group. Evidence for free-radical attack occurring selectively in the ortho and para position is found in free35.2 radical alkylation of substituted benzenes by benzoyl 16.5 peroxide (5). A further implication of results ob5.9 6.7 tained from cracking 3,4-xylenol is that t,he higher 32.3 thermal stability of meta-substituted phenols ~ i l l 50.0 persist in a mixture with ortho- and para-substituted 90.0 17.2 phenols. This was observed experimentally when a ... mixture of m- and p-cresol was cracked. p-Cresol cracked a t approximately twice the rate of the mcresol. Samples of low boiling phenols from low and high temperature tar were analyzed by the infrared technique of Friedel et al. (6) for the isomer distribution in the Ce t o C8 range. A knowledge of the detailed isomer distribution permits an interpretation of some of the reactions occurring during carbonization of coal in the light of the results obtained in this investigation of pure alkyl phenols. I n comparing the isomer distributions in Table 111, the alkyl phenols from high temperature tar, which have been subjected to temperatures as high as 1000' C., may be considered as thermally cracked products of the more primary low temperature tar alkyl phenols produced in range of 450' to 500" C.
TABLEI. SUMMARY OF DATA ON CRACKING OF ISOMERIC CRESOLS
.
Temp., C. Residence time, see. % converMo!e sion Products, moles/ 100 moles cracked Liquids Phenol Benzene Toluene Residue Gases CH4
co H2
CO2
CaH4
o-Cresol 788 816
766
871
816
n-Cresol 843
871
0,05
0.05 4.85
0.05 11.5
0.05 21.6
0.5 16.8
0.5 21.0
0.05 14.7
44.0
54.4 14.3 9.4 3.2 40.7 60.2 44.0 5.5
46.3 26.0
41.2' 23.0 23.0 2.2 28.2 47.5 41.0 6.0
21.5 29.5 30.5 0.9 34.4 71.8 45.3 3.7
...
10.9
2.1 42.9 66.0 50.8 7.3 .
I
.
I
.
...
.
23.9 25.4 14.9 4.6 21.0 94.8 127.0 21.6
...
21.7 40.3 13.0 2.6 25.2 86.0 119.0 23.3 2.2
28.5 32.8 14.8 1.5 27.7 101.8 122.2 21.7 6.9
group to form water and a tolyl radical (step 2b); or formation of molecular hydrogen and a hydroxybenzyl radical (step 2c). The ultimate fate of the methyl radical and the tolyl radical is t o form methane and toluene, respectively, by hydrogen capture. The primary product formed from the hydroxybenzyl radical is a dimer (step 3). T o account for the substantially higher yields of demethylated product that are actually obtained, secondary cracking of the dimer is proposed. Supporting evidence for this type of secondary cracking is found in the recent work of Horrex and Miles ( 7 ) . Significant yields of benzene were obtained from the pyrolysis of dibenzyl, the hydrocarbon analog of the dimers suggested in this investigation. This mechan& is in accord with t h e observed product distribution obtained from cracking the various alkyl phenols. This mechanism should explain the decreased stability of alkyl phenols containing an increased number of methyl groups and the decreased stability of ortho- and para-substituted phenols as compared with meta-substituted phenols. As the number of methyl groups increases the probability of C-H bond rupture increases, resulting in a higher cracking rate. The increased cracking rate observed in the case of ortho- and para-substituted phenols is due to a weakening of the C-H bond in the methyl group as a result of interaction of the methyl group and hydroxyl group. Similar interaction is not possible in the meta position. Since C-H bond rupture is considered the rate-determining step, any process which weakens this bond will be reflected in higher cracking rates.
*
TABLE11. SUMMARY OF DATA ON CRACKING 2,4-
AND
XYLENOLS
E X k i e % n e , sec. Weight % conversion Products, moles/100 moles cracked Liquids Phenol o-Cresol p-Cresol Benzene Toluene m-X ylene Residue Gases CH4
co
Hz COa
763 0.5 27.2
2,4-Xylenol 788 0.5 40.7
843 0.05 37.9
1.0 20.1
4.2 13.8
4.9 13.6
15.2 2.3 22.6 10.3
12.2 4.5 15.9 5.0
13.1 8.4 28.6 5.5
5.8
2.9
5.3
42.8 57.9 70.4 8.1
45.6 85.7 119.0 15.7
40.6 53.1 51.6 6.3
3,4-
3,4-Xylenol 80 1 0.05 19.9
1.8 31.0 (mcresol) 9 5 1 2 . 1 (oxylene) 8.8 42.6 52.2 36.5 2.8
A knowledge of the composition of the cresols resulting from cracking 2,4- and 3,4-xylenol provides further insight into .the cracking process. These data are shown in Table 11. The o-cresol-p-cresol ratio from 2,4-xylenol cracking varies from 1.1 to 1.3 depending on the cracking temperature. The
TABLE 111. COMPOSITION OF Low BOILINGPHENOLS Phenol o-Cresol m-Cresol p-Cresol 2 6-Xylenol 2'4-xylenol 2:5-~ylenol 2,3-Xylenol 3 5-Xylenol ;-Ethylphenol p-Ethylphenol 3,4-Xylenol
High Temperature Tar, Wt. % 29.0 14.4 24 5
ii.4
1.0
2.4 3.9 0.8 5.5 2.4 2.0 2.7
Low Temperature Tar,
Wt.
70
11.0 15.4 lA R
9.1
5.5 16.5 8.6 3.7 8.9 4.8 1.0 1.2
The increasing stability of the phenols with decreasing niolecular weight is indicated from the relative amounts of phenol and isomeric cresols. The phenol concentration in the CSto Cs phenol fraction of high temperature tar is almost three times the amount in a similar fraction of low temperature tar. The combined phenol and cresols in high temperature tar represent 79% of the low boiling phenols as compared with 50% in the case of low temperature tar. This is in accord nith the observed decrease in thermal stability with increasing molecular weight. The relative stability of m-cresol as compared with 0- or p-cresol is shown by the increase in the amount of m-cresol from 37% of the total cresols in low temperature tar to 49% in high temperature tar. A comparison of the analyses of the isomeric xylenols, in the two tars shows that 3,5-xylenol having both methyl groups in the meta position suffered the least destruction, increasing from 15% of the xylenol fraction in low temperature tar to 35% in high temperature tar. The samples of low boiling phenols contained approximately 2% of unknown CS phenols. The analyses of the remaining low boiling phenols were adjusted to 100%. Similar qualitative distributions have been reported ( 2 , 8, 10, 14) for phenols from low and high temperature tars. Bergtsson ( 1 ) examined three tars resulting from carbonization of three different rank coals in the BM-AGA retort a t 900' C. Using the infrared technique of Friedel (6),he showed that the mcresol fraction of the total cresols varied from 43 to 45%, indi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2876
cating the importance of the thermal treatment as compared with variations in the coal composition. ACKYOWLEDGMENT
The authors wish to express their appreciation to G. L. Barthauer and R. J. Friedrich of the analytical group for their development of the infrared analytical techniques applied to cracking of pure compounds, t u R. A. Friedel of the Office of Synthetic Liquid Fuels, Bruceton, Pa., for his assistance in the infrared analyses on phenols of tar origin, to Nrs. Irene Pigman and James Wagner for carrying out the experimental program, and to Everett Gorin for valuable discussions on the cracking mechanism. LITERATURE CITED
(1) Bergtsson, E., U.
P.Bur. Mines, R e p t . Invest. 4755 (December
1950).
Vol. 44, No. 12
( 2 ) Briatom, W.-4., J . Inst. Fuels, 20, 109 (1947). (3) Dietrich, W., H e h . Chim. Acta, 8, 149 (1925). (4) Fisher, C. H., U. 8.Bur. Mines, Bull. 412 (1938). (5) Friedel, R. A , et al., A n a l . Chem., 22, 418 (1950). (6) Hagemann, A., 2. angew. Chem., 42, 355, 503 (1929). ( 7 ) Horrex, C., and Miles, 8. E., Discussions Faraday Soc., 10, 187 (1961). 18) Jager, A., and Katwinkel, G., Brennstof-Chem., 31, 65 (1940). (9) Jones. B. W.. Jones. S. A.. and lieuworth. M. B.. IIUD. ENG CHEX,44,2233 (1952). (10) Kaplan, E. H., U. S. BLIP. Mines, Tech. P a p e r 690 (1946). (11) Kosaka, L., J . SOC.Chem. I n d . ( J a p a n ) , 30,108 (1931). (12) Nakai, R., B u l l . Chem. SOC.J a p a n , 5, 136 (1930). (13) Szwarc, M., J . Chem. Phua., 16, 128 (1948). (14) Wilson, P. J., and Wells, J. H., “Coal, Coke and Coal Chemicals,” p. 374, New York, McGraw-Hill Book Co., 1950. RECEIVED for review May 15, 1952.
ACCEPTED August 13, 1952.
Effect of Associated Salts on the Po~vrnerization of utadiene by Organoium Reagents J
AVERY A. MORTON, FRANK H. BOLTON’, FRANCES W. COLLINS, AND EDWARD F. CLUFF Department of Chemistry, ikfassachusetts Institute of Technology, Cambridge, Mass.
T
HE polymerization of dienes by sodium metal is probably
the oldest known method for producing synthetic rubber. A course for the reaction has been traced by Ziegler and coworkers through the 1,4-disodium-2-butene intermediate and thence as an organosodium reagent through a series of adduct compounds to a rubber which is primarily the result of 1,2- rather than the 1,4-chain growth required to make it relatively similar to Hevea, The same type of polymer is obtained if the first stage with sodium metal is omitted and any active organosodium reagent is substituted as the starting agent. This sodium process gives an unsatisfactory product when used with butadiene and isoprene, but in World War I the Germans applied it to 2,3-dimethylbutadiene to give a moderately suitable polymer. Subsequently the emulsion process was developed and yielded a better type of product in which around 80% of the butadiene was joined end to end, that is, 1,4-. The present synthetic rubber for general use belongs with the emulsion class. All things considered, the rubber made by the emulsion process is distinctly different from that by the sodium method, so much so that an examination of physical properties alone serves easily t o differentiate the t7vo materials, even if their sources are unknown. The purpose of this paper is to point out how the sodium process as practiced with an organosodium reagent as the starting point can be altered so as to produce differences .ivhich are even greater than the significant ones found beti\-een t,he soqium and emulsion types of polymerization. The change is brought about by association of the organosodium compound-a reagent insoluble in the reaction media-with other solid sodium salts of a less reactive type, even of such slight reactivity as found in sodium chloride. In proper kind and proportion, however, the effect of the additional salts can be astonishing, both as to the rate of polymerization and the structure of the polymer. The particular combination rrhich has given the most outstanding result is allylsodium associated with sodium isopropoxide and sodium chloride and is known as an alfin catalyst ( 12 ) . Allylso1
Present address, Research Laboratory, Dow Chemical Co., Midland,
Mich.
dium by itself is slowacting and yields a polymer having an intrinsic viscosity less than 1 and having around VOyo external double bonds, the product of 1,2-addition. By the presence of the two associated salts the same quantity of allylsodium achieves polymerization a t enormous speeds to polymers with intrinsic viscosities of 12 or more with 70 to 80% 1,4-structure. Emulsion polymerization can scarcely achieve intrinsic viscosities much above 3. The full scope of these changes, the exact composition of the catalyst, and the degree to which the behavior of an organosodium reagent is controlled by the associated salts were not a t first realized. Indeed, in the beginning, the name alfin was given on the assumption that only two components, the salt of an alcohol and an olefin, were necessary ( 1 7 ) . However, as this paper will show, sodium chloride is as essential as the other tnTo salts. Substitution of sodium chloride by many other related salts is possible. Each single salt of the three ‘component mixture has some influence, but all three are needed to obtain the effects achieved as an alfin catalyst. The cation required for these salts is equally specific, sodium being of prime importance. h limited substitution of sodium by potassium is possible and, in a few cases, may cause a faster rate of polymerization. The lithium ion cannot be used in general without impairment of activity, although in some cases its bad influence can be partly compensated by association with a large anion, such as the iodide in place of chloride. As the organosodium reagent becomes more active and effective, it also becomes more specific. For instance, an ordinary organosodium compound without high specificity will cause styrene to polymerize faster than butadiene, whereas the alfin catalyst xi11 cause butadiene to polymerize much faster than styrene ( 1 6 ) . A given quantity of catalyst \vi11 have a great effect upon butadiene, much less on isoprene, and scarcely any on 2,3-dimethylbutadiene ( 6 ) , the monomer for the old methvl rubber. This high specificity is an important feature of this development where, for the first time, a clear demonstration has been possible in a field where specificity is sorely needed, because it is apparent that if the synthesis of polpdienes of specific struc-
