velocity of propagation should decrease with increasing temperature and time. This is found to be true. O n the other hand, some of the results with cords preheated a t low temperatures or short times a t the higher temperatures are not consistent with this interpretation. First, the propagation velocity increased markedly with both time and temperature before its later decrease. This probably sho\vs the presence of two competing factors-the first causing an increase in velocity and the second a decrease. The latter factor is undoubtedly dilution by reaction products. The first may be caused by either or both of two things. I t may be a surface area effect, heating possibly causing greater porosity and, therefor’:, a greater exposed surface. I t may also be caused by a catalytic effect, the first small particles of product being the catalyst. This investigation supplies some support for both of the above possible causes of the initial velocity increase. No quantitative porosity imeasurements were attempted. However, microscopic examination of crystals heated in the evacuated hot stage. as \vel1 as those decomposing in the light beam, sho\ved considerable roughening of the surface. Moreover, these examinations shsowed tiny specks of reaction product. For the most part, these specks were associated with the highest velocity and their gro\vth with the velocity decrease. Xevertheless, their initiation a t earlier stages has been reported by Cook et al. (2) as a reijult of their electron microscopic study. Conceivably, the reaction product plays both roles, serving as a catalyst initially but as a diluent in larger amounts. Second, the ignition energy may be expected to follow the increased sensitivity of the material in the initial stages-that is, as the velocity increases, the ignition energy should decrease. The exact opposite was found-that is, any change in ignition energy from the “as received” value was always an increase. There are two possible explanations for this anomaly.
I t is entirely possible that the method of measuring the ignition energy was not sensitive enough to show a small decrease if it existed. As pointed out, the values reported were subject to errors due to electrode placement. This error appeared to be more serious a t the lower values of energy (0 to 5 joules). Agreement between individual cords in this energy region was no better than +lOyo. Any energy decrease over the “as received” value would necessarily fall in this area of poor reliability. Therefore, the decrease could simply have been missed. O n the other hand, the failure to find a n ignition energy decrease corresponding to the velocity increase may be a n indirect argument for the ”porosity effect.” Ignition energy is probably only slightly dependent on particle size or surface area, but should increase with degree of decomposition as a result of dilution of the explosive with reaction products and resulting decrease in concentration of the explosive itself. This is essentially what was found. In general, the results of this study appear to be consistent with basic observations made previously with crystals of pure lead azide.
literature Cited
(1) Bowden, F. P., Yaffe, A . D., “Fast Reactions in Solids,” Butterworths Scientific Publications, London, 1958. ( 2 ) Cook, M. A , , Head, N. L., Keyes, R. T., Thornley, G. M., Pitt, C. H., Institute of Metals and Explosives Research, University of Utah Salt Lake City, “Sensitivity of Lead Azide,” Int. Conf., London, 1963. ( 3 ) McXuslan, J. H. L., Ph.D. dissertation, Cambridge, 1957. (4) +McLaren,A. C., Research (London) 10, 409 (1957). ( 5 ) iVilliains, H. T., Ph.D. dissertation, Cambridge, 1951. RECEIVED for review August 3, 1966 ACCEPTED January 16, 1967
FREE RADICAL COMBINATION REACTIONS INVOLVING T H E METHYL RADICAL A T 1000”T O 1200”C. E R I C J . Y . SCOTT Mobil Oil Gorp., Princeton, Y. J .
u THE COURSE of work on the gas-phase reactions of hydrolcarbons with oxygen a t about 1000” C., it was observed that many products could be accounted for by a reaction of the type : R.1
+ R.2 = R1R2
(1)
where R . ’ and R . 2 are similar or dissimilar free radicals. Free radical combination reactions determine the main products in liquid-phase reactions catalyzed by peroxide at relatively low temperature. They also occur in gas-phase radical reactions as (chain-ending steps. However, in this instance the product R1R2 corresponds to a minor part of the entire reaction, the major products being determined by the chain-propagation reactions. In general, the reaction between two free radicals involves a negligible activation energy, so the reaction should occur readily. However, the
over-all rate is often relatively slow in gas-phase reactions, inasmuch as Reaction 1 involves trvo radicals which are present in small concentrations. Therefore, molecules often compete more favorably than radicals for other radicals, although radical-molecule reactions involve higher activation energies. In some gas-phase pyrolysis reactions the formation of major products may be interpreted as occurring by a free radical combination reaction. For example, ethane is formed from methane (Kevorkian, 1962, p. 395); 1.3-butadiene from ethylene (Kevorkian, 1962, p. 406) ; bibenzyl from toluene (Steacie, 1954). Also under some conditions all the p-methylbenzyl radicals formed from p-xylene yield p , p ’-dimethylbibenzyl (Errede and English, 1965). I n such instances, if chains occur, termination steps must predominate and the chains are short. However, high yield of dimer is not unVOL. 6
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MARCH 1967
67
The main products of the decomposition of mesityl oxide, diisobutylene, and a mixture of acetophenone and toluene are explained by free radical combination reactions. Prominent in these reactions is the methyl radical. Methyl radicals, formed by the pyrolysis of acetone or propane, are shown to combine with other free radicals a t 1000" C. For example, with vinyl radicals, propylene is formed; with allyl radicals, 1 butene; with methallyl radicals, 2-methyl- 1 -butene; and with benzyl radicals, ethylbenzene. There is no evidence that free radical addition to double bonds is significant a t 1000" C.
mica. Two concentric tubes (of Vycor and of unglazed porcelain) supported the mica and the reactor in position. For safety the induction coil was enclosed in a wire cage. The cage activated a limit switch which prevented the converter from operating if the cage were removed. Temperature was controlled to within 1 5 ' C. by an American Products Simplytrol regulator. Reactor temperature was measured by a thermocouple (TC1, Figure 1) positioned close to the reactor wall. No attempt was made to measure the gas temperature directly. Gases were metered and passed through concentric tubes heated to about 200' C. Immediately before mixing swirl was imparted to one of the streams by passing through a threaded portion of the outer tube. Liquid was injected at a prefixed rate by a positive displacement microsyringe pump supplied by Pressure Products Industries (Webb et al., 1962). T o prevent discontinuous injection in spurts it was necessary to operate the pump under a backing pressure of about 200 p.s.i.g. Flow in the liquid feed line was therefore restricted by partially closing a micrometer valve. Exit gases were cooled, metered, and vented. Exhaust gases could be thermally quenched by injecting helium gas a t 150 p.s.i.g. through a narrow stainless steel capillary (gage 33), positioned 1 inch from the reactor. Gas samples were taken by syringe for gas chromatographic analysis. The following columns were used: silica gel (for nitrogen and ethylene), 25% di-n-propyl sulfone on Chromosorb (for Ca to C j hydrocarbons), and 5% Carbowax 400 on Haloport F and 30% silicone rubber on Diatoport-S (for aromatics). Areas of gas chromatograph peaks were assumed proportional to weight
equivocal evidence that methyl, vinyl, and benzyl radicals combine to form ethane, 1,3-butadiene, and bibenzyl, respectively. Reaction 1 will probably occur when radical concentrations are high. High temperatures might be expected to produce high radical concentrations. For this reason we explored the extent to which free radical combination reactions are important a t 1000' C. and favorable structural prerequisites for such reactions. Experimental
Chemicals. The nitrogen was Airco (Seaford grade). Ethylene, propane, propylene, and isobutylene were Matheson (c.P. grade). The mesityl oxide and acetophenone were obtained from Matheson, Coleman and Bell, toluene and acetone from the Baker Chemical Co., and diisobutylene from the Eastman Kodak Co. The diisobutylene contained 85% 2,4,4-trimethyl-l -pentene and 15% 2,4,4-trimethyl-2-pentene. Apparatus and Procedure. The apparatus was designed to provide temperatures up to 1200' C. and contact times of about 0.001 second. T h e conventional flow system operates a t a few pounds' pressure (Figure 1). The reactor was a section of Inconel-600 tubing (2-mm. i.d.; 3.2-mm. 0.d.). Inconel-600 comprises 77% nickel, 15% chromium, 4% iron, and lesser amounts of copper, manganese, silicon, and carbon. A quartz liner (1-mm. i.d.) was inserted into the reactor. A closely fitting cylindrical bar of Inconel-600 (2 to 6 inches long and 0.75 inch in o.d.), surrounding the tubing, provided the heat sink. Residence time could be varied by using bars of different lengths. The bar was inductively heated by a coil which was activated by a 6-kw. Ajax Northrup spark-gap converter. The water-cooled induction coil (l'/?-inch i.d., 6 inches long) was insulated from the reactor by a cylindrical sheet of
QUENCH
r
IC,
ANALYSIS AND METERING SYSTEM
,
-
,, -.-,
i\,
%.
The reactor was first equilibrated for 2 to 3 minutes a t about 200' C. below minimum reaction temperature. Samples lvere taken for gas chromatographic analysis. After the temperature was raised by 50' or 100' C., samples were again taken. The procedure was repeated until the maximum ob-
P
1,
I I I'
TEMPERATURE' INDUCTION COIL REGULATOR TO CONVERTER
M'XER-PREHEATER
I
Ill
I
I
w Simplified diagram of reactor system P.
R.
F. LP. 60
l&EC
Pressure g a g e Pressure regulator Flowmeter Liquid pump
PRODUCT RESEARCH A N D DEVELOPMENT
I
18888
t Figure 1 .
I
served temperature was reached. Subsequently, the temperature was reduced by increments of 50' or 100' C. until the original temperature was attained. I n this way, any changes in reactor behavior could be averaged. The nitrogen content was used to evaluate any volume change which occurred during reaction. Mass spectrometric analyses were used to check results when gas chromatographic analyses were ambiguous. With aromatics, the liquid was condensed a t -198' C. and analyzed. Results varied slightly from reactor to reactor; however, for a series of runs corresponding to a table or figure the same reactor was used. Results and Discussion
Preliminary Evidence for Free Radical Combination. In a number of reactions we found that many products could be explained by Reactions 1 and 2 :
R.' R.1
+ R . 2 = R1R2
+ RZH
= RIH
Free radical combination
+ R.z
Hydrogen abstraction
(1) (2)
T h e product RIH indicates but does not prove that the radical R . 1 exists free in the system. Similarly, the product R1R2 suggests that free radicals R . ' and R . 2combine to form a stable product. At 925' C . , diisobutylene (85% 2,4,4-trimethyl-l-pentene; 15% 2,4,4-trimethyl-2-pentene) decomposes to give (among other products) 32% 2,5-dimethyl-l,5-hexadienecorresponding to the combination of two methallyl radicals [CHzC(CH3) :CH2] and 8% 2-methyl-l-butene, formed probably by the combination of :methallyl and methyl radicals. Also, 1,5-hexadiene (diallyl) is an important product when propylene is pyrolyzed or oxidized. This product probably arises from combination of two allyl radicals. The pyrolysis of a n equimolecular mixture of toluene and acetophenone yields benzene, ethylbenzene, bibenzyl, biphenyl, diphenylmethane, benzophenone, and benzylacetophenone. With the exception of benzene, all these products can be explained in terms of combination of free radicals. T h e products formed by pyrolysis of mesityl oxide (4methyl-3-pentene-2-one) are shown in Figure 2. Residence time is constant, so that conversion increases with increasing temperature. Isobutylene is formed initially; 1,3-butadiene, the most stable product, predominates at high conversions. At intermediate conditions, isoprene, 2-methyl-2-butene, and an unidentified product are important. A reasonable explanation of the reaction is that a dimethyl vinyl radical is formed by Reactions 3 and 4. This radical then abstracts a hydrogen atom to form isobutylene. (CH3)2C:CHCOCH3
-+
(CH3)zC:CHCO
+ CH3
+ CO (CH3)2C:CH2 + R *
(CH3)zC:CHCO + (CH3)zC:CH (CH3)gC:CH f-RH
+
(3) (4) (5)
T h e dimethyl vinyl radical may also combine with a methyl radical to form 2-methyl-2-butene, which could dehydrogenate or demethanate to form isoprene and 1,3-butadiene, respectively : -2 H
(CH3)gC:CH f C H I - + (CHI)&:CHCHi + CHz:C(CHa)CH:CH2 - CH, Isoprene CH2:CHCH:CHz 1,3-Butadiene
L
(6)
(7)
Reactions of Methyl Radicals with Olefins. In most of the above examples, methyl radicals appear to play an im-
go0
700
900
800
1000
1100
1200
1300
REACTOR TEMPERATURE ( T )
Figure 2.
Products from the pyrolysis of mesityl oxide X
1,3-Butadiene
0 Unidentified product 0 2-Methyl-2-butene
0
Isoprene
portant part in the formation of products. Consequently, reactions of methyl radicals with the relatively stable radicals formed from olefins were investigated. Methyl radicals were formed by the pyrolysis of either acetone or propane. Methyl radicals are known to play a part in both decomposition reactions (Steacie, 1954, pp. 151, 219). If free radical combination were important, the ultimate yields of products formed by the combination of a methyl radical with a free radical derived from the olefin should increase in the presence of propane or acetone. ACETONE RUNS. The results for the reactions of acetone with propylene, ethylene, and isobutylene are given in Tables I and I1 and Figure 3. Comparison is made with the pyrolysis of olefin in the absence of acetone. By varying residence time and temperature, enough runs were done so that results could be compared at either constant temperature or constant conversion. Comparison of runs 1 and 3 shows that the ultimate yield of 1-butene formed by the pyrolysis of propylene increases in the presence of acetone. 1-Butene is most probably formed by the combination of an allyl radical and a methyl radical:
+ CH3CH:CHz CH3 + dH2CH: CH2
CH3
+ CH2CH:CHz
-+
f CHd
(8)
CH3CH2CH: CH:!
(9)
1-Butene might also be produced by addition of a methyl radical to the double bond in propylene, followed by hydrogen abstraction or elimination : CH3
+ CHSCH: CHz
-fi
-+
CH3CHCHzCH3 C H I : CHCH2CH3
There are a number of objections to this mechanism. butenes could be formed by similar reactions: VOL. 6
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MARCH 1967
69
Table I. 1-Butene Formed from the Pyrolysis of Propylene-Acetone Mixtures' R u n No. 3 1 2 Nitrogen, mole % 64 21 38 Propylene, mole % 36 36 21 Acetone, mole yo .. 43 42 Reactor temp., ' C. 1140 1180 1220 1100 1140 1180 1100 1140 1180 Residence time? msec. 0.7 0.6 0.6 0.5 0.5 0.5 0.4 0.4 0.4 Conversion, ye 3 9 15 9 25 36 5 10 14 Ultimate yieldC
