2804
FRAN~OIS DOTJEAND GEORGES GTJIOCHON
The Mechanism of Pyrolysis of Some Normal and Branched C, to C, Alkanes. Composition of Their Pyrolysis Products by Frangois Doue and Georges Guiochon Laboratoire Professeur L. Jacque, Ecole Polytechnique, Paris, France
(Received September 80, 1968)
The pyrolysis products of twelve Cato C9 alkanes have been analyzed by gas chromatography. The compositions of the products are tentatively predicted on the basis of a classical radical mechanism, taking into account radical isomerizations by 1-4 and 1-5 intramolecular transfers of hydrogen atoms. In the experimental conditions used in this work (temperature, 440-500’; pressure, 3 to 20 bars; reactor of clean sealed Pyrex tubes) the reaction is homogeneous, and hydrogen is formed only in small amounts. The experimental results are shown to be in agreement with theory if it is assumed that 1-5 hydrogen atom transfers are very fast and that the rates of 1-4 and intermolecular transfers are often similar, their relative value depending much on steric hindrance.
Introduction Recent work on the pyrolysis of light hydrocarbons, mainly n-butanel3+*and i ~ o b u t a n e , ~has !~ shown that these reactions are best described in terms of a free-radical chain mechanism.'^* The decomposition mechanism of light alkanes is now rather well established, and it seems worthwhile to extend to the pyrolysis of heavier alkanes the same concepts. In a previous workg we have shown that many quantitative results concerning the pyrolysis of ethane and n-butane could be used to predict the kinetics of heavy n-alkane decomposition and the composition of their pyrolysis products. Assuming that all primary radicals are effective chain carriers and that all possible primary and secondary heavy radicals of any given carbon number are always in thermodynamic equilibrium by fast 1-4, 1-5, and more distant hydrogen atoms transfers, we obtained the following expression for the pyrolysis rate
where p is the partial pressure of the CnH2%+2alkane, B is the ratio of the rate constants for the decomposition and the saturation of primary radicals, respectively, and AB is the rate constant of the pyrolysis under low pressure, the reaction being approximately first order in these condition^.^ The term 1 fp was introduced to take account of the fact that CnHZn+lprimary radicals can make up again the initial C,H2,+2 molecules by hydrogen atom transfers. In fact, f is small, and v Ap ( p B ) when p is not very high. This kinetic law was already observed by other a u t h o r ~ , ~ and - l ~ eq 1was shown to express rather well the experimental results for the pyrolysis of n-hexadecane in the rather wide range of the following experimental conditions : pressure, 0.5 to 150 bars; temperature, 380 to 520” ; degree of py-
+
+
The Journal of Physical Chemistry
rolysis, 0.2 to 5%. In this range of experimental conditions the reaction was shown to be homogeneous : the use of different methods of conditioning the reaction vessels did not change the initial reaction rate. The mechanism we have developedg has to be modified somewhat when applied to shorter n-alkanes or to branched chain alkanes. The rates of the different stages of the reaction are strongly dependent on the nature of the reacting radicals and, for instance, are very sensitive to the substitution of methyl groups on carbon atoms adjacent to the radical carbon. We show in this work, however, that some slight modifications allow predictions of the composition of the pyrolysis products of branched alkanes in excellent agreement with the experimental data.
Experimental Section The reaction was performed in small Pyrex tubes (0.5-ml inner volume) washed with nitric acid and rinsed in pure water, then dried and sealed under vacuum after introduction of the hydro~arbon.~The products were analyzed by programmed-temperature gas chromatography on a column packed with deactivated aluminaS9 Hydrogen was analyzed in the pyrolysis products using a Gow-Mac microcatharometer between the outlet of (1) J. H. Purnell and C. P. Quinn, Can. J . Chem., 43, 721 (1965). (2) .M,H. Back and M . L. Lin, CIC Symposium on the Kinetics of Pyrolytic Reactions, Ottawa, 1964. (3) C. P. Quinn, Proc. Roy. Soc., A275, 190 (19633. (4) N. H. Sagert and K. J. Laidler, Can. J . Chem., 41, 848 (1963). (5) J. Fusy, R. Martin, M. Dzierzynski, and M. Niclause, Bull. Sac. Chirn. Fr., 3783 (1966). (6) C. T. Brooks, Trans. Faraday Soc., 62, 935 (1966). (7) N. N. Semenov, “Some Problems in Chemical Kinetics and Reactivity,” Princeton University Press, Princeton, N. J., 1958.
(8) S. W. Benson, “The Foundations of Chemical Kinetics,” MCGraw-Hill Book Co. Inc., New York, N. Y . , 1960. (9) F. Doue and G. Guiochon, J . C h i n . Phys., 395 (1968). (10) D. R. Blackmore and Sir C. Hinshelwood, Proc. Roy. SOC., A268, 36 (1962).
2805
MECHANISM OF PYROLYSIS OF SOMENORMAL AND BRANCHED C6TO C9ALKANES rnI 40
n-Octane
3.M4thyl-Pentanc
-
rn x r(
30.
20
IO
?a.
-
10
s.
-
Ct
Figure 1. Variations of the number m of molecules of n-alkanes (0)and 1-alkenes ( X ) formed by pyrolysis of 100 molecules of n-octane vs. the number of carbon atoms of each product: theoretical curves and experimental points. Pyrolysis time 120 sec, temperature 442O, pressure, 8 bars.
C?'
Ct
Cs
C{
nQ
S;
2Uly
Ut
PC;
Figure 3. Pyrolysis products of 3-methylpentane. Same experimental conditions and symbols as for Figure 2.
3-M;thyl
~
Hexane
A aM6lhyl- Penlane
Ct
c,
cz
Ci
cs
e j
;bq
iwQ-
snq-
IC;
#e;
Cz
Cf
C3
cf
q
y
ne,
cs
anic,,.
IC,.
RC'.
ut
ur
Figure 2. Number m of molecules of the different products formed by the pyrolysis of 100 molecules of 2-methylpentane: experimental points. Solid line, case a; dashed line, case b. Experimental conditions: see Figure 1.
the column and the flame ionization detector; then argon is the carrier gas. This design does not seriously affect the efficiency of the column. The following alkanes have been pyrolyzed : n-hexane, n-octane, n-hexadecane, 2,2- and 2,3-dimethylbutane7 trimethylbutane (Flucka, Switzerland, purity > 95%) ; 2-methylpentane, 3-methylpentane, 3-methylhexane, 2,3- and 2,4-dimethylpentane, 2,2,4- and 2,3,4-trimethylpentane, 2,2,5-trimethylhexane (Eastman-Kodak, Rochester, purity > 95y0). The experimental conditions are the following: temperature, 3-methylhexane, 500°, all other alkanes, 443' ; pressure range, 2-10 bars; degree of pyrolysis, 1-4%.
Theoretical The initial products of the homogeneous decomposition of any alkane can be easily predicted as long as C-H bonds do not break at a measurable rate, and this was proved to be true in the experimental conditions
Figure 4. Pyrolysis products of 3-methylhexane. Temperature, 507"; other conditions as for Figure 2 .
used in this work.9 This can be done by considering the possible decomposition scheme of all the radicals obtained by abstraction of one of the hydrogen atoms from the alkane molecule. This decomposition is known to proceed via the rupture of one of the /3 C-C bonds. The radicals formed in that way can generally decompose themselves further or react via hydrogen atom transfers to give lower alkanes. However, the rate of hydrogen atom abstraction by secondary and tertiary radicals is small, and perhaps the rate of /3 C-H bond rupture is no longer negligible if there are no available /3 C-C bonds, especially for the small radicals where no isomerization is possible. Thus we may expect to find hydrogen in the pyrolysis products when isopropyl and tertiobutyl radicals are present. The composition of the pyrolysis products may be calculated as follows. 1. Kinetics Basis for the Calculation. The total number of molecules of pyrolysis products given by the decomposition of 100 alkane molecules in the range of Volume 78, Number 9
September 1069
2806
FRANCJOIS DOUEAND GEORGES GUIOCHON where E is the activation energy of the reaction
3u
m
R
+ R'H +RH + R'
and AH is the difference between the energies of R' and
R, respectively. We have extended eq 2 t o all the pri-
Figure 5. Pyrolysis products of 2,2-dimethylbutane. conditions and symbols as for Figure 2 .
Same
Im
2-3 DimGthyl-Butane rn
x
mary radicals. Therefore we have only to determine the difference in the energy of the various radicals present in the reaction vessel. (b) Some of the possible CnHZn+lradicals are undergoing very fast isomerization. To calculate the composition of the pyrolysis products we need the equilibrium constants, which can be derived from the differences in the free-radical energies, and the decomposition rates of these radicals. (c) Some radicals may undergo isomerization but the rate of this reaction is relatively low. I n this case it will be very difficult to predict quantitatively the concentrations of the different pyrolysis products. In this work we shall consider only the two cases (a) and (b) because in the third case calculation does not seem possible at the present time. If the experimental results are intermediary between the theoretical results of cases (a) and (b), this mill mean that some slow isomerizations are taking place between the corresponding radicals. In fact, many previous paper^'^^^^^^ have shown that isomerizations by 1-5 and more distant hydrogen-atom transfers are probably much faster than radical decomposition by ,f3 C-C bond ruptures. There is also pome evidence for 1-4 transfer^.^,'^ These transfers are not supposed to be so fast, since the nearer
'7
Trimithyl-butane
Figure 6. Pyrolysis products of 2,3-dimethylbutane. conditions and symbols as for Figure 2 .
Same 66.
experimental conditions given above was always less than 220. Thus, ninety per cent of the pyrolysis products a t least proceed directly from the first step of the reaction, Le., the decomposition of the initial radicals CnHzn+l. These radicals can behave in three different ways. (a) None of the different possible CnHZn+lradicals can undergo isomerization by internal hydrogen transfer. Then the Rice-Kossiakoff theory allows calculation of the ratios of the amounts of the different CnH2n+lradicals formed by removing successively one hydrogen atom from each of the different carbon atoms of the alkane molecule. I n this calculation we have used the relationship given by Boddy and Steacie" for hydrogen atom abstraction by methyl radicals
The Journal of Physical Chemistry
40
!
IC
Figure 7. Pyrolysis products of trimethylbutane. Same conditions and symbols as for Figure 2.
(11) P. J. Boddy and E. W. It. Steaoie, Can. J . Chem., 38, 1576 (1960). (12) H. H. Voge and G. M. Good, Ind. Eng. Chem., 73, 593 (1949). (13) A. S. Gordon and J. R. MoNesby, J . Chem. P h ~ s . ,31, 853 (1959). (14) A. S. Gordon and J. R. McNesby, ibid., 33, 1882 (1960).
2807
MECHANISM OF PYROLYSIS OF SOMENORMAL AND BRANCHED Ce TO C9 ALKANES
1
Dirnithyl-Pentane
:2
ml
x
ea
1
I
%
I
cz
c,
I
,
cam
c,
I
I
c.,
,
Ch
,
6 '
I
snt4-
tic4
'
nrq
ut
Figure 8. Pyrolysis products of 2,3-dimethylpentane. Same conditions and symbols as for Figure 2.
2.4 Dirn6fhyl- PsnlanQ x Y
C(
Cz
Ca'
Ca
Cam
isC4
i=Ci
Ha
Figure 9. Pyrolysis products of 2,4-dimethylpentane. conditions and symbols as for Figure 2.
Same
the linear position of the C . .Ha . . C bridge in the activated complex, the higher the rate of H-atom transfer. 2. Determination of the Enthalpy Digerences between Isomeric Free Radicals. (a) Values from 3.5 to 4.2 kcal/mol are found in the literature for the enthalpy difference AE between secondary and primary radicals. AE can be obtained from the composition of the pyrolysis products of n-hexane, using eq 1 and assuming the equilibrium K
cH2-CH2-CH2-CH2-CHz-CHa CH,-cH-CH2-CH,-C€12-CH3 with K = 2/3 exp(AE/RTj. Entropy changes between the two radicals are supposed to be very small and are not taken in account here. At a temperature of 443" the experimental results give a value of 8.5 for the equilibrium constant. Thus AE = 3.6 kcal/mol. This value has been used to calculate the composition of the
Figure 10. Pyrolysis products of 2,2,4-trimethylpentane. Same conditions and symbols as for Figure 2.
pyrolysis products of n-octane (Figure 1, full line). The experimental results are in very close agreement with the theory. (b) I n the Rice-Kossiakoff theory, the difference of enthalpy between tertiary and secondary radicals is supposed to be the same as the difference between secondary and primary radicals. This assumption will be shown to be valid for our experimental conditions. However, all secondary (or tertiary) radicals of a given carbon number do have not the same stabilities. For instance, methyl groups substituted on the carbon atom neighboring the radical carbon (cr-CH3) increase the stability of the radical. We have assumed that the increase of stability is proportional to the number of these methyl groups and does not depend on the nature of the radical. The value of the increase of stability AE' brought by one cr-CH3 has been determined from the composition of the pyrolysis products of 2-methylpentane, because in this reaction the two following radicals are in equilibrium
c-c-c-c-c I
c-c-c-c-ct I
A
B
C
C
From the ratio of the amounts of the products proceeding from the decomposition of (A) and (B), AE' can be calculated. The value resulting from experimental data is A E' = 1.6 kcal/mol. 3. Calculation of the Composition of the Pyrolysis Products. The values of the parameters reported above allow a quantitative estimation of the composition of the pyrolysis products. However, the amount of some paraffins cannot be calculated because the rate of hydrogen abstraction by secondary radicals is not known. Volume 73, Number 0 September 1063
FRANSOIS DOUEAND GEORGES GUIOCHON
2808
1
4a
c-c-c-c-c I
2-34 Trlm;lhyl-Pentane
C 4b
wl
X
Figure 11. Pyrolysis products of 2,3,4-trimethylpentane. Same conditions and symbols as for Figure 2.
2-24 TrimCthyl-Hexane
The 1-4b transfer seems faster than the 1-4a one since the amounts of 2-butene and 2-pentene found in the pyrolysis products are in agreement with case a, and the amount of l-butene is better accounted for by case b. For the other alkanes studied the composition of the pyrolysis products is intermediate between those predicted by cases a and b, respectively, and i t depends on the partial pressure of the alkane: the higher the pressure, the closer the agreement between the experimental results and the predictions of case a. It is probable that for these compounds the rate of 1-4 isomerization is of the same order of magnitude as the rate of hydrogen abstraction by the secondary radicals. Some other qualitative conclusions can be drawn from these experimental results (Figures 2 to 11). 1. Effect of the Presence of Methyl Groups on the Rate of 1-4 Isomerization. I n each case where one of the carbons 2 or 3 bears one or two methyl groups, the rate of 1-4 isomerization is increased. This effect results from an increase of the probability for the 1 and 4 carbon atoms being in a position adequate for the hydrogen transfer. For instance, the isomerization rate is greater with 2,2-dimethylbutane than with n-hexane. On the
C 1 1 2 3 4
1 2 3 4
c-c-c-c > c-c-c-c-c-c
'J u
C
Figure 12. Pyrolysis products of 2,2,5-trimethylhexanea Same conditions and symbols as for Figure 2.
Thus, two compositions have been calculated, corresponding to the two possibilities discussed above Case a: 1-5 isomerization is supposed to be very fast but there is no 1-4 isomerization Case b: 1-5 and 1-4 isomerizations are very fast
Experimental Results and Discussion As can be seen in Figures 2 to 11, which show the experimental results (points) and the theoretical predictions (full and dotted lines), the agreement between the experimental results and one of the two theories is strongly dependent on the nature of the pyrolyzed alkane. Case a is valid for the pyrolysis of 2-methylpentane, 2,3-dimethylbutane, and 3-methylhexane. Case b is valid for the pyrolysis of 2,2-dimethylbutane. I n the pyrolysis of 3-methylpentane there are two possible transfers, 1-4a and 1-4b. The Journal of Physical Chemistry
contrary, the substitution of methyl groups on one of the 1,4 or 5 carbon atoms lowers the rate of 1-4 hydrogen transfer, probably because of steric hindrance.
c1
I \ VC c-c-c-c-c
c-c-c-c I \
I
C
C
2. Formation of Hydrogen. Very small amounts of hydrogen were found in our experiments (Figures 1-6, 9-11). This corresponds to at most 6 molecules of hydrogen per 100 decomposed molecules of alkane at 500" and to only 3 at 450". The experimental results sup-
+
2809
RADIOLYSIS OF HYDROCARBON NzO SYSTEMS port the earlier suggestion that hydrogen formation is somewhat increased when tertiobutyl radicals appear in the pyrolysis (Figures 9,10,12). These results show that some simple modifications to the conventional Rice-Herzfeld-Kossiakoff mechanism
allow it to account for the experimental data from the pyrolysis of the normal and branched alkanes, including the composition of the pyrolysis products, as long as the pyrolysis yield is small enough to prevent any secondary effects of the pyrolysis products themselves.
Effect of Polar Molecules on Reaction of Negative Ions in Radiolysis of Hydrocarbon
+ Nitrous Oxide Systems in the Gas Phase
by J. L. Redpath and M. Simicl Laboratory of Radiation Chemistry, School of Chemistry, The University, Newcastle u p o n T y n e , England, and Pioneering Research Laboratory, U . S.A r m y Natick Laboratories, Natick, Massachusetts 01760 (Received October $4, 1968)
In the radiolysis of gaseous hydrocarbons containing small amounts of NzO (2 mol %) the radiation-produced electrons react with NzO leading eventually to G(N2) > G,. In the presence of polar molecules with low electron affinity (HzO, “3, CHICl) a limiting value of G(N2) = G, could be reached depending on the concentration of the polar compound and the temperature of the system. I t is suggested that the clustering of 0 - (or NzO-) in addition to proton transfer inhibits the formation of the excess nitrogen yield (AG(N2) = G(N2) - Ge). At temperatures above 80” G(N2) = 2G,.
Introduction
organic component, tend to cast some doubt on these
Interaction between ionic species produced in the gas phase and molecules with large dipole moments was recognized not long after the discovery of ionization phenomena. It was successfully applied in Wilson cloud chambers where either alcohol or water molecules formed large visible clusters around the ions produced by the passage of ionizing particles. Ion mobility measurements produced many discordant results due to the presence of small amounts of impurities. The low values of mobility of various positive and negative ions were later attributed to cluster formation.2 The initial step, ie., the formation of clusters with small numbers of molecules, has been neglected for a long time and only recently, with the development of high-pressure mass spectroscopy, has this process attracted due attention. Quantitative data such as A H , AG, and A S have been obtained for each particular stepJ3in several systems for positive ions and only very recently for some negative ones.4 The role of clusters in radiation chemistry was discussed by Lind6 as early as 1919, but it was not accepted because of the lack of conclusive evidence. It was proposed, more recently, that the decomposition of neutralized H,O + and NH4f ions depends on the size of their clusters.6 Unfortunately, other complicating factors, mainly chain thermo-radio1yt)ic decomposition of the
conclusion^.^ In this work we present some results indicating the formation of clustered 0- or NzO- ions and discuss a few specific reactions which discriminate between the clustered and bare negative ions.
Experimental Section All irradiations were carried out in annular Pyrex vessels (-340 ml) fitted with break-seals. Prior to use vessels were baked in air at -500” for a day and then Torr. Depumped at least for 0.5 hr down to sired amounts of gases were introduced by condensing from gas sample vessels filled at required PVT. Pure (1) Correspondence should be sent to M. Simic, U. 8. Army Natick Laboratories, Natick, Mass. 01760. (2) L. B. Loeb, “Basic Processes of Gaseous Electronics,” University of California Press, Berkeley and Los Angeles, Calif., 1961. (3) P. Kebarle, S. K. Searles, A. Zolla, J. Scarborough, and M. Arshadi, J. Amer. Chem Soc., 89, 6393 (1967). (4) (a) P. Kebarle, M.Arshadi, and J. Scarborough, J . Chem. Phys., 49, 817 (1968); (b) P. Kebarle, S. R. Searles, A. Zolla, J. Soarborough, and M . Arshadi, 25th International Mass Spectrometry Conference, Berlin, 1967. (5) S. C. Lind, J. Amer. Chem. SOC.,41, 531 (1919). (6) A. R. Anderson, B. Knight, and J. A. Winter, Nature, 209, 199 (1966); A. R. Anderson and J. A. Winter, “The Chemistry of Ionization and Excitation,” Taylor and Francis, Ltd., 1967, p 197. (7) G. R. A. Johnson and M. Simic, Nature, 212, 1570 (1966).
Volume 78,Number 9
September 1969
