48 R. K. Rao and S. G. Krishnamurty thus classified. The largest term Ap 4SH gives the ionization potential of Br hi as 35-7 V. R eferences Bhattacharyya 1929 Nature, Lond., 123, 150. Bloch, L. and E. 1927 Ann. Phys., Paris, 7, 205. Deb 1930 Proc. Roy. Soc. A, 127, 197. Krishnamurty and Rao, K. R. 1935 Proc. Roy. Soc. A, 149, 56. — 1937 Proc. Roy. Soc. A, 158, 562. Lacroute 1935 Ann. Phys., Paris, 3, 5. Martin 1935 Phys. Rev. 48, 938. Rao, K. R. 1929 Proc. Roy. Soc. A, 125, 238. — 1932 Proc. Phys. Soc. 44, 594. — 1936 Nature, Lond., 138, 168. Rao, K. R. and Badami, J. S. 1931 Proc. Roy. Soc. A, 131, 154. Saha and Mazumdar 1928 Indian J . Phys. 3, 67. Vaudet 1927 C.R. Acad. Sci., Paris, 185, 1270. The Combustion of Aromatic and Alicyclic Hydrocarbons I—The Slow Combustion of Benzene, Toluene, Ethylbenzene, n-Propylbenzene, n-Butylbenzene, o-Xylene, m-Xylene, p-Xylene and Mesitylene B y J. H. B urgoyne, P h .D. (Communicated by A. C. G. Egerton, —Received 1 I ntroduction Recent work upon the kinetics of the oxidation of aliphatic hydrocarbons has led to the recognition of certain characteristic features that find a ready interpretation in terms of the chain theory of chemical reaction. Thus, for example, both paraffins and olefines exhibit well-defined in - duction periods, pressure limits of inflammability and a marked sensitivity to the influence of surface, that point directly to the intervention of reac- tion chains; and although the precise nature of the chain mechanisms is somewhat uncertain a great deal of information is available as to their length, branching characteristics, mutual interactions and stability. on September 6, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from
Transcript
48 R. K. Rao and S. G. K rishnam urty
thus classified. The largest term Ap 4SH gives the ionization potential of Br hi as 35-7 V.
References
B hattacharyya 1929 Nature, Lond., 123, 150.Bloch, L. and E. 1927 A nn. Phys., Paris, 7, 205.Deb 1930 Proc. Roy. Soc. A, 127, 197.K rishnam urty and Rao, K. R . 1935 Proc. Roy. Soc. A , 149, 56.
— 1937 Proc. Roy. Soc. A, 158, 562.Lacroute 1935 A nn. Phys., Paris, 3, 5.M artin 1935 Phys. Rev. 48, 938.Rao, K. R. 1929 Proc. Roy. Soc. A, 125, 238.— 1932 Proc. Phys. Soc. 44 , 594.— 1936 Nature, Lond., 138, 168.
Rao, K. R. and Badami, J . S. 1931 Proc. Roy. Soc. A, 131, 154. Saha and Mazumdar 1928 Indian J . Phys. 3 , 67.Vaudet 1927 C.R. Acad. Sci., Paris, 185, 1270.
The Combustion of Arom atic and Alicyclic Hydrocarbons
I—The Slow Combustion of Benzene, Toluene, Ethylbenzene, n-Propylbenzene, n-Butylbenzene, o-Xylene, m-Xylene, p-X ylene and Mesitylene
By J. H. Burgoyne, Ph .D.
(Communicated by A. C. G. Egerton, —Received 18 February 1937)
Introduction
Recent work upon the kinetics of the oxidation of aliphatic hydrocarbons has led to the recognition of certain characteristic features that find a ready interpretation in terms of the chain theory of chemical reaction. Thus, for example, both paraffins and olefines exhibit well-defined induction periods, pressure limits of inflammability and a marked sensitivity to the influence of surface, that point directly to the intervention of reaction chains; and although the precise nature of the chain mechanisms is somewhat uncertain a great deal of information is available as to their length, branching characteristics, mutual interactions and stability.
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Corresponding data for alicyclic and aromatic compounds are, however, very scanty and only in one instance has a comprehensive systematic kinetic study been made. Fort and Hinshelwood (1930) and Amiel (1933 a, b, 1936) have investigated the slow combustion of benzene and find that whilst it shows a general resemblance to ethylene there are certain respects in which significant differences occur. Fort and Hinshelwood concluded that benzene is oxidized by a chain mechanism, the chains initiated predominantly in the gaseous phase being of short continuation.
Some two years ago, the author, in collaboration with D. M. Newitt, made a comparative study of the slow oxidations of benzene, toluene and ethylbenzene at high pressures (Newitt and Burgoyne 1936) with a view to determining the relative reactivity of the side chain and nucleus and identifying the steps leading to the rupture of the latter. The results showed that whilst in the case of benzene the formation of a single series of hydroxy-intermediates preceded the breakdown of the ring, with toluene and ethylbenzene both nuclear and side-chain oxidations occurred simultaneously. Furthermore, the phenyl group in ethylbenzene appeared to expose the a-carbon atom of the side-chain to oxygen attack.
In continuation of this work, benzene and a series of its derivatives have now been examined at normal pressures and the present paper contains the results of comparative experiments with benzene, toluene, ethylbenzene, w-propylbenzene, w-butylbenzene, o-, m-, and p-xylenes and mesitylene. The results confirm the view, previously expressed, that chain mechanisms are involved in the combustion of aromatic hydrocarbons, but that the chain characteristics in the case of benzene are rather sharply distinguished from those of its alkyl derivatives.
Apparatus and E xperimental Procedure
The method employed has been to follow the course of the slow combustion of the various hydrocarbons in a closed silica vessel manometrically.
The apparatus used is illustrated in fig. 1. The reaction mixture is made up with the aid of the manometer C in the previously evacuated 2 1. bulb B from the liquid hydrocarbon contained in A, and pure oxygen from a 10 1. gas-holder in which it is stored over a 50 % aqueous solution of glycerine. The bulbs A and B and their capillary connexions are immersed in a thermostatically controlled bath at a temperature sufficiently high to maintain the requisite pressure of hydrocarbon vapour in the prepared mixture. The desired quantity of this mixture is then transferred through electrically heated capillaries to the evacuated silica vessel D (capacity
Vol. CLXI—A. E
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= 550c.c.) which is maintained at a suitable reaction temperature. The combustion is followed by the change in pressure as indicated by the manometer E ; and at its conclusion or at some predetermined intermediate stage a sample of the contents of the vessel is withdrawn into the gas burette F for subsequent analysis.
J . H. Burgoyne
Oxygen
Nitrogen
F ig . 1
The oxygen used was prepared by heating recrystallized potassium permanganate; before passing into the bulb it traversed two tubes packed with freshly distilled phosphorous pentoxide. The pure hydrocarbons as purchased were dried over anhydrous sodium sulphate and fractionally distilled, the end fractions being rejected.
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Correlation of Pressure Change with Course of the R eaction
Aromatic and Alicyclic Hydrocarbons 51
It was necessary first to ascertain experimentally the relation between pressure change and oxygen consumption for certain typical mixtures of each of the hydrocarbons. A series of experiments was carried out with a given mixture at a fixed initial pressure and temperature, and the reaction arrested at various intermediate stages by removing the silica vessel from
Time in min.F ig . 2— 1 Pressure increase i
2 Oxygen consumed I 100 mm. C6H 5. C2H5 + 100 mm. 0 2 + 200 mm. N2 a t3 C 02 formed j 438° C.4 CO formed J5 Pressure increase 50 mm. C6H 6 + 50 mm. 0 2 a t 561° C.6 Pressure increase 50 mm. 1 : 3 : 5 C6H 3(CH3)3 + 50 mm. 0 2 a t 485° C.
the furnace and rapidly chilling. The cold products were then analysed and a comparison made between the oxygen consumption, the amount of oxides of carbon formed and the rise of pressure. As an example, the results for an ethylbenzene-oxygen-nitrogen mixture reacting at 438° C. are shown by means of curves in fig. 2. The composition of the initial mixture and the products at the end of the reaction are given in Table I.
The curves in fig. 2 show that the amount of oxygen consumed is closely related to the pressure increase throughout the greater part of the reaction;
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moreover, after a preliminary acceleration the rate of pressure increase remains substantially constant until the reaction is about two-thirds complete. Similar results are given by the other hydrocarbons, exemplified in fig. 2 by typical curves for benzene and mesitylene, and it has therefore been assumed in all cases that the time taken for the pressure increment to increase from 20 to 60 % of its final value (t20_60) is inversely proportional tothe reaction rate.
T a b l e I
Initial gases mm. Products mm.c6h 5. c 2h 5 100-0 co2 14-8o2 100-0 CO 55-3N2 200-0 H 2 + aliphatic 1-5
400-0 hydrocarbons Residual ethylbenzene,
interm ediate products and steam 181-6
n 2 200-0453-2
In the initial stages of the reaction the ratio oxygen consumed/pressure increase is much greater than at later stages and it is probable that during this period the initial oxidation products are formed with little or no pressure change (Fort and Hinshelwood 1930). The two oxides of carbon are found in the products at all stages, but as the ratio hydrocarbon/oxygen increases there is, as might be expected, a corresponding increase in the ratio C0/C02.
Having established, in the above manner, a basis of comparison for the reaction rates of related hydrocarbons, the influence of temperature, concentration, dilution and surface factors upon the oxidations were next determined.
The Products of Combustion
The results of a detailed examination of the products of combustion of benzene, toluene and ethylbenzene at high pressures have been described in a previous communication (Newitt and Burgoyne 1936). In the present series of experiments a complete determination of all the intermediate products was not feasible and usually the gaseous products only were analysed. The predominating reactions were those resulting in the forma-
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tion of the two oxides of carbon and steam according to the general equations:
^ t t + 6 ^ - 2 n + 6 "b1 5 + 3 ^ ^
2 ' 2 ~
1n Ti i
9 + 2 n n^ n -h 6x l 2 n + 6 1 2 ° 2 ~
(n + 6) C02 + + 3) H20,
(n + 6) CO + (w + 3) H20.
( 1 )
( 2 )
The percentage pressure increase at constant volume associated with these reactions for the nine aromatic hydrocarbons under consideration is given in Table II.
T a b l e II% pressure increase due to form ation of
H ydrocarbon C 0 2 + H 20 by eq. (1) CO + H aO by eq. (2)
By experiment it is found that the products in all cases correspond with the occurrence of both reactions in proportions dependent upon temperature, concentration, dilution and surface factors. I t is possible, therefore, to compare, as in Table III, the behaviour of the various hydrocarbons provided the reaction temperatures are selected to give equal rates of oxidation and the initial partial pressures of the reactants are maintained constant.
Table III—Combustion of E quimolecular Hydrocarbon-oxygen Mixtures at 100 mm. Initial Pressure
PercentageReaction Final pressure R atio 0 2 appearing
tem perature increment CO/COa in as oxidesHydrocarbon ° A mm. products of carbon
The ratio C0/C02 is substantially higher for ethyl-, propyl- and butyl- benzenes than in the other series, and the percentage of oxygen accounted for as oxides of carbon is much less. This latter fact emphasizes the conclusion that, whereas with most of the hydrocarbons considered the total reaction is adequately expressed by the equations (1) and (2), in the case of those derivatives having a long side-chain, appreciable amounts of oxygenated intermediates (aldehydes, ketones, etc.) survive in the products.
In comparing the figures of Tables II and III it must be borne in mind that during the combustion of an equimolecular hydrocarbon-oxygen mixture under the stated conditions, all the oxygen is used up, but only about 20 % of the hydrocarbon is oxidized.
The Influence of Temperature
In order to compare the influence of temperature upon the rates of oxidation of the various hydrocarbons, equimolecular hydrocarbon- oxygen mixtures were employed, the concentrations of reactants being such that at 800° A their partial pressures were 50 mm. (i.e. 0*001 g.mol./l.
F ig . 3
approximately). No difficulty was experienced in obtaining reproducible pressure-time curves, the rate of reaction being far less sensitive to surface effects than is the case with the paraffin hydrocarbons. The results are shown graphically in fig. 3, in which the logarithm of the times taken for the reactions to proceed from 20 to 60 % completion are plotted against the
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reciprocals of the absolute temperatures. A linear relationship holds, in general, within the limits of experimental error, and since 0-6o is inversely proportional to the velocity constant k, the equation to the lines, namely
l ° g lO ^20-60 =
may be written in the form of the Arrhenius equation:
Elogeh = KT + C’
where E, the energy of activation, is numerically equal to 2*30 Abeing the gradients of the lines in fig. 3. The values of E, obtained thus, were corrected for the variation of molecular velocity with temperature and the amended results are summarized in Table IV.
Table IV—Energies of Activation for the Oxidation of Aromatic Hydrocarbons
HydrocarbonBenzene Toluene Ethylbenzene n-Propylbenzene w-Butylbenzene o-Xylene m -Xylene p -Xylene Mesitylene
A ctivation energy E kcal./g.mol.
68-049-041-539-034038-039040*036-0
Further confirmation of these values has been obtained from experiments in which the concentrations of the reactants differed widely from those in the series under consideration, the results in all cases showing satisfactory agreement.
The case of w-butylbenzene alone calls for special comment. I t was found that above about 400° C. the combustion of this hydrocarbon obeyed Arrhenius’s equation fairly well, giving a value of 34 kcal., but below 400° C. the temperature coefficient of the reaction appeared to decrease, ultimately becoming negative. By increasing the initial pressure of the equimolecular mixture in the temperature range 300-400° C., the occurrence of cool flames was revealed upwards of about 120 mm. Further experiments are in hand with the object of examining in detail this phenomenon, which has not, up to the present, been observed in connexion with any of the other hydrocarbons under consideration.
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If in fig. 3 a line is drawn parallel to the axis of 1 /T, the points of intersection with the “ Arrhenius lines” give the temperatures at which the different hydrocarbons are oxidised at equal rates. In fig. 4 a series of these values is plotted against the corresponding values of E, and smooth connecting curves have been drawn to emphasize the structural relationships of the following three groups of hydrocarbons:
(1) Benzene and derivatives with a single side-chain of varying length.(2) Isomeric xylenes.(3) Hydrocarbons with symmetrical molecules.
Benzene
Toluene^
Ethylbenzene
Mesitylene-Butylbenzene
Tem perature of equal ra te of combustion °A
F ig. 4
The conclusions to be drawn regarding the relative ease of oxidation of benzene and its homologues, which are illustrated clearly in fig. 4, may be summarized as follows:
(1) The introduction of an unbranched, saturated side-chain into the benzene nucleus facilitates combustion to a degree dependent on the length of the chain.
(2) The activating effect of adding more than one side-chain is progressive, subject to the modification that
(3) The symmetrical disposition of a given number of equal groups round the nucleus constitutes the least reactive configuration.
Assuming the activation energies relate to primary hydrocarbon- oxygen encounters, the calculated reaction velocities are for benzene several million times less and for toluene about one thousand times less than
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the experimental values; for the remaining hydrocarbons the differences between the calculated and experimental figures are not so marked although the former are always lower.
The Influence of Concentration Factors
For each of the nine hydrocarbons three series of comparative experiments have been made under varying conditions of concentration. Starting with an equimolecular mixture at 100 mm. total pressure and at a suitable reaction temperature, the effect of the following factors upon the velocity of combustion and the analysis of the products were studied:
(1) Variation of the initial pressure between 50 and 500 mm.(2) Progressive increase of partial pressure of oxygen to 350 mm. or
more.(3) Progressive increase of partial pressure of hydrocarbon to 300 mm. I t was found that, with the exception of benzene, the rate of reaction was
directly proportional to a power of the total or partial pressure undergoing variation, i.e.
1t----- = K . p n,
<20—60
where p is the pressure and n and K are constants.The index “n ” expresses the dependence of the reaction velocity upon
the various concentration factors and provides a useful basis for comparing the behaviour of the different hydrocarbons.
In Table V are set out the values of n for the various hydrocarbons, relative to the total pressure of the equimolecular mixture, the initial partial pressure of oxygen and the initial partial pressure of hydrocarbon, respectively.
Table V—Values of Indices expressing the Dependence of theR eaction Velocity upon the Total Pressure and Partial Pressures
of R eactantsP artial pressure of
Values of n relative to ... Total pressure Oxygen HydrocarbonBenzene (1-9) (0-2) (1-7)Toluene 2-0 10 0-8Ethylbenzene 2-0 0-3 2-0n-Propylbenzene 2-1 - 0 1 2-2n-Butylbenzene 21 -0-4 —o-Xylene 1-0 10 0m-Xylene 1-7 0-8 0-6p -Xylene 1-6 0-9 0-2Mesitylene 1-6 11 0-2
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is benzene. It was found that the rate of reaction of 50 mm. C6H6 + 50 mm. 02 was not much increased when the partial pressure of oxygen was raised to 150 mm., in accordance with the value n = 0-2 given in the above table. For higher concentrations of oxygen however, a much increased dependence of the reaction velocity upon this factor was noticed. For the theoretical mixture (50 mm. C6H6 + 375 mm. 0 2) n was 1*5 relative to the partial pressure of oxygen and 2-5 relative to the total pressure. Experiments with mixtures containing 25 mm. of benzene and from twenty to thirty times this amount of oxygen indicated that in the limiting case the reaction rate is proportional to the square of the oxygen concentration. In this connexion it may be recalled that the energy of activation is the same for the equimolecular and theoretical mixtures.
Referring to Table V, attention may be drawn to the following features:(1) For benzene and those derivatives containing a single side-chain, the
rate of reaction depends approximately upon the square of the total pressure.
(2) Ethyl-, propyl- and butylbenzenes fall into a series in which the oxygen concentration has a small and progressively diminishing influence; with butylbenzene, indeed, oxygen definitely retards the rate of reaction; the effect of the hydrocarbon concentration is substantially the inverse of the oxygen effect.
(3) Toluene is exceptional in that hydrocarbon and oxygen exert approximately equal effects.
(4) The hydrocarbons containing more than one methyl group attached to the nucleus show a greater degree of complexity, an important feature being the marked influence of oxygen as compared with hydrocarbon concentration.
The composition of the gaseous products is not materially influenced by an increase of total pressure, although there is a definite small increase in the CO/C02 ratio. When, however, the relative proportions of hydrocarbon and oxygen are varied, considerable changes occur. In Table VI is given the composition of the products from a series of experiments with benzene in which the hydrocarbon and oxygen concentrations respectively were the only varying factors; and these results may be taken as representative of the remaining hydrocarbons examined.
It will be observed that when the ratio oxygen/benzene is less than about
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4, the oxygen is completely consumed and the observed pressure increase indicates that a certain amount of hydrocarbon survives in the products; when the ratio is 6 or more, the whole of the benzene is oxidized. Those derivatives having a single side-chain behave in a similar manner to benzene, but when more than one methyl group is present, the hydrocarbon is capable of reacting with upwards of four times its own volume of oxygen; to this rule o-xylene is an exception, being similar to the single side-chain derivatives in this respect.
The formation of carbon monoxide is favoured by a defect of oxygen, that of the dioxide by an excess, whilst ethylene, hydrogen and aliphatic hydrocarbons are found in considerable quantities in the products if the ratio hydrocarbon/oxygen is high. Acetylene is practically always produced, but only assumes measurable proportions when ignition occurs.
The Influence of Surface Factors
(a) The Condition of the Surface. I t is well known that the rates of oxidation of certain combustibles are very sensitive to changes in the surface of the reaction vessel and extreme precautions are often necessary in order to maintain the surface in such a uniform state as to give reproducible results. This phenomenon, however, is not so marked with aromatic as with paraffin hydrocarbons and one or two preliminary experiments in the apparatus usually sufficed to stabilize its surface. Of the nine hydrocarbons examined benzene and its mono-alkyl derivatives are the least sensitive and mesitylene the most sensitive in this respect.
(b) The Surface/ Volume Ratio. When a combustion proceeds by a chain mechanism in which the chains are broken on the walls, an increase in the surface/volume ratio of the reaction vessel generally causes a retardation of the reaction rate. A series of experiments was carried out to determine whether in the case of the aromatic hydrocarbons any such influence is operative. For this purpose, comparative oxidations were made with the vessel (i) empty and (ii) packed with 20 silica rods, whereby its surface volume ratio was increased 3-4 times. The results are recorded in Table VII.
In all cases, the rate of reaction in the packed vessel was appreciably slower, but whereas in the case of benzene the decrease was about 50 %, for the remaining hydrocarbons it did not exceed 30 %; the retarding effect, moreover, showed a progressive diminution on ascending the homologous series.
The only appreciable effect upon the gaseous products of increasing the
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surface/volume ratio is to diminish the ratio C0/C02, there being a corresponding decrease in the pressure rise. Thus for ethylbenzene (cf. Table VII) the following figures were obtained:
Total pressure increase
R atio CO/COa mm.U npacked bulb {S/V —0-87) 2-72 8T8Packed bulb (S /V — 2-9) 2-15 76-6
The Influence of Dilution
To obtain further information as to the chain characteristics of the reactions a considerable number of observations were made upon the influence of progressive dilutions with nitrogen on the reaction velocity of certain hydrocarbon-oxygen mixtures. The general effect was the same in all cases but varied slightly in degree. Thus, the addition of small amounts of diluent either had no effect on the reaction rate or caused a minor retardation. An increase in the partial pressure of the diluent caused a considerable acceleration which persisted over the greater part of the range of concentration it was feasible to employ in the present apparatus. At the highest partial pressures of diluent the reaction rate showed signs of reaching a maximum, but further experiments are required to establish this observation.
Three series of experiments were carried out with each hydrocarbon, in which (1) the same hydrocarbon-oxygen mixture was progressively diluted with nitrogen at various reaction temperatures; (2) a given mixture was progressively diluted with nitrogen at a constant reaction temperature, the vessel being (a) empty, and ( b)packed with 20 silica rods; (3) the effect of using argon or helium as diluent in place of nitrogen was observed.
The results obtained for benzene will be considered briefly. In Table VIII are given the relative rates of reaction of a 50 mm. C6H6-(- 200 mm. 02
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mixture, diluted with various amounts of nitrogen and reacting at various temperatures in (a) an unpacked vessel and ( ) a vessel packed so as to increase the surface/volume ratio 3-4 times.
Table VIII—Influence of Diluent Nitrogen on the Rate of Reaction of 50 mm. C6H6 + 200 mm. 0 2
Ratio 2̂0-60 undiluted m ixture 2̂0 —60 diluted m ixture
a t
mm. of N2 535° C. 555° C. 555° C. 565° C. 565° C.added (unpacked) (unpacked) (packed) (unpacked) (packed)
It will be seen that whilst temperature has no marked influence on the accelerating action of the diluent an increase in the surface/volume ratio of the vessel tends to suppress it.
In Table IX, the effect of replacing nitrogen by helium is illustrated; it is seen that the latter has considerably the less accelerating influence. When argon is the diluent, the relative reaction rates do not differ appreciably from the figures obtained for nitrogen.
Table IX—Comparison of Effects of N itrogen and Helium in the Dilution of a 100 mm. C6H6+ 100 mm. 0 2 Mixture at 550° C.
t-. , • 2̂0—60 undiluted mixture .R a t io --------- —----- -----;-------- in presence of<20-60 diluted m ixture
Similar results were obtained with all the other hydrocarbons examined, although the observed effects were less pronounced than in the case of benzene; comparative figures for toluene, ethylbenzene, m-xylene and mesitylene are given in Table X and need no further comment.
Discussion
The effects of surface/volume ratio and of diluents upon the velocity of reaction suggest that a chain mechanism is operative and also afford
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Table X —The Influence of D iluent N itrogen on the R ate of Aromatic Combustion
. £2o-60 undilu ted m ixture K atio -------- ——---- —----- ---------
M ixture and experim entalconditions 50 100 200 300 400
50 mm. C6H 5CH3 | 0-98+ 150 mm. 0 2 a t 520° C.j
0-95 (1-01)*111
1-23 (1-06)1-27
50 mm. C6H 5C2H 5 ]+ 150 mm. 0 2 a t 464° C.J 114
(1-06)119
(1-09)1-44 1-48
50 mm. m — C6H 4(CH3)2 | + 150 mm. 0 2 a t 459° C.J
(1-06)115 1-43
50 mm. C6H 3(CH3)3 ]+ 150 mm. 0 2 a t 481° C.J 101 114 1-25
* The figures in brackets refer to experim ents in the packed bulb.
certain information as to the nature of the chains involved. Thus the retardation caused by an increase of the surface/volume ratio is explained on the assumption that chains are deactivated on the walls of the vessel. If in a cylindrical vessel they are deactivated solely by this means, their length may be shown to be proportional to the square of the diameter of the vessel, and hence inversely proportional to the square of the surface/ volume ratio R. This latter relationship also holds good for vessels of other regular shapes (Bursian and Sorokin 1931), and thus if the number of initial centres remains constant the rate of reaction o) is given by
where A is a constant. If the chains are simultaneously deactivated in the gas phase
oj = K 5
where a and b denote the proportions of the total change due to chains which are ultimately deactivated in the gas phase and on the walls respectively. For two values of the velocity Wj and corresponding with surface/volume ratios of R± and R2,
b _ (co^-oj^Ba B2m2 — R
If it be assumed that the walls play no part in initiating chains and that the above relationship holds in the case of a cylindrical vessel packed with
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rods, the values of 6/a and the percentage of the reactions due to chains breaking on the walls and in the gas phase may be calculated for five representative hydrocarbon-oxygen mixtures from the data in Table V II; the results are recorded in Table XI.
Table XI—The D eactivation of Reaction Chains on the WallsAND IN THE GAS PHASE FOR VARIOUS HYDROCARBON-OXYGEN MIXTURES
Comparing the figures for benzene, toluene and ethylbenzene, it will be seen that the ratio 6/a and hence the average chain lengths diminish with increasing molecular weight. This result is in harmony with the corresponding variations in the activation energies of the hydrocarbon and in the calculated and observed reaction velocities.
The addition of increasing proportions of diluents to the reacting medium accelerates the reaction as shown by the results in Table VIII. If from
Table XII—The D eactivation of Chains on the Walls and in the Gas Phase in the Combustion of 50 mm. Benzene + 200 mm. Oxygen :
Effect of Dilution with Nitrogen
Percentage of reaction due to chains which rupture
Diluent added w2/wi b/a On walls In gas phaseAt 555° C .:
Nil 0-55 0-78 43 57150 mm. 0-51 0-88 47 53350 mm. 0-40 1-46 59 41
A t 565° C .:Nil 0-51 0-88 47 53100 mm. 0-49 0-97 49 51250 mm. 0-45 116 54 46350 mm. 0-43 1-27 56 44
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these figures the corresponding values of the ratio b/a are calculated (see Table XII), the cause of the acceleration is seen to be due to an increase in the average chain length as though the diluent were impeding the diffusion of active centres to the walls.
I t is possible that the diluent may act also as a deactivating medium, for with excessive dilution the reaction velocity shows signs of passing through a maximum. With the present apparatus, however, it was not possible to proceed to such high dilutions as would satisfactorily establish this point.
In addition to the specific effects of surface/volume ratio and of diluents, further evidence in support of a chain mechanism is furnished by the close adherence of the pressure-time curves to theoretical chain relationships, although of course such agreement may be of merely empirical significance. It has been shown (Kowalsky 1933; Semenoff 1933) that the complete course of a chain reaction may be represented by the expression
c _ 100 5 - 1 + e- ^ ’
whence1 -
1100 - £
+ constant,
where £ is the percentage of the total reaction taking place during a time t from the start, 6 is the time measured from the point of half-change and <p is a parameter depending upon the properties of the initial reacting medium. It has been found that the above relationships are valid for £ = 10 — 90 % in the case of aromatic combustions. This fact is illustrated in fig. 5, where
logi 100 - £is plotted against t for a few hydrocarbon-oxygen mixtures
chosen at random.It is not proposed to discuss the actual mechanism by which chains are
propagated in the various hydrocarbon-oxygen media until further work, now in progress, has been completed. It may be pointed out, however, that no single mechanism is likely to be applicable both to benzene and its homologues since nuclear and side chain oxidations may proceed simultaneously and the point at which the benzene ring ultimately breaks may be influenced by the position and number of the side chains.
The author is indebted to Dr. D. M. Newitt for many valuable discussions and to the Department of Scientific and Industrial Research for a grant with the aid of which part of the above investigation has been carried out.
Vol. CLXI—A. F
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The kinetics of the slow combustion of benzene and eight alkyl derivatives have been studied and compared. I t has been shown that the rate of such combustions can be judged with a considerable degree of accuracy by manometric observations.
F ig. 5— 1, 50 mm. C6H 6 + 50m m . Oa a t 565° C.; 2, 50 mm. C8H6 + 50 mm. 0 2 544° C .; 3, 50 mm. C6H5. CH3 + 50 mm. 0 2 a t 503° C.; 4, 50 mm. C6H5C3H 7 + 50 mm. 0 2 a t 438-5° C .; 5, 50 mm. 1 : 3 : 5 C6H 3(CH3)3 + 50 mm. 0 2 a t 481° C.
The influence upon the various combustions of factors of temperature, concentration, dilution and surface have been investigated, and as a result it is now clear that the hydrocarbons considered may be variously classified. Generally speaking, benzene stands considerably apart from its derivatives, and exhibits evidence of chain mechanisms in a more pronounced manner than do its derivatives.
In the matter of reactivity, three classes have been established:(1) the derivatives with a single side chain,(2) the isomeric xylenes,(3) the hydrocarbons with symmetrical molecules,
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and it is suggested that relationships in the matter of the effectiveness of the primary encounter of hydrocarbon and oxygen molecules are to be sought along these lines.
From a kinetical point of view, having regard to the influence of concentration factors, three classes are again established but of a different constitution. They are:
(1) benzene,(2) derivatives with methyl groups attached to the nucleus (toluene, the
xylenes and mesitylene),(3) derivatives with a single side chain containing more than one
carbon atom.Conclusions as to reaction mechanism are evidently to be based on this
Bursian and Sorokin 1931 Z phys. Chem. B, 12, 247. F o rt and Hinshelwood 1930 Proc. Roy. Soc. A , 127, 218. Kowalsky 1933 Phys. Z . Sowjet. 4, 723.N ewitt and Burgoyne 1936 Proc. Roy. Soc. A, 153, 448. Semenoff 1933 Phys. Z . Sowjet. 4, 906.
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