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The combustion of aromatic and alicyclic rspa.ro ??The Slow Combustion of Benzene, Toluene,...

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  • 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.


    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

    IThe 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)


    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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  • Aromatic and Alicyclic Hydrocarbons 49

    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. CLXIA. E

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  • 50

    = 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



    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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  • 52 J. H. Burgoyne

    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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  • Aromatic and Alicyclic Hydrocarbons 53

    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)

    Benzene 5-9 64Toluene 10-0 69Ethylbenzene, Xylenes 13-0 73n-Propylbenzene, Mesitylene 15-4 76n-Butylbenzene 17-2 79

    By experiment it is found that the products in all cases correspond wit

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