REACTIONS OF HYDROCARBONS I N THE GLOW DISCHARGE* BY ERNEST G . LINDER AND ARDITH P. DAVIS
Although the reactions in electrical discharge of a large number of hydrocarbons have already been studied by others, the experimental conditions of these various investigations have differed widely, and hence it is not possible to obtain much information from the literature concerning the relative behavior of this class of compounds under similar experimental circumstances. I t was the purpose of the research described in this article to study such relative behavior, and to find if there exist any simple relations between molecular structure and the nature of the reactions. Fifty-seven different hydrocarbons were investigated. No attempt was made to work out completely the chemical reactions of each substance in the discharge, such an undertaking being a tremendous task, since the determination of the reactions of even a single hydrocarbon is a long and difficult problem which has been solved satisfactorily in only a few cases. Instead, a standard method of procedure was developed and applied to each substance, and, although this did not give complete information regarding the reactions, it gave sufficient to reveal a number of interesting relations. Of the various types of electrical discharges in gases available for this work, the glow discharge (between metal electrodes, with D. C. current, a t a few millimeters gas pressure) was selected, because it was believed to be better understood and more controllable than any of the others. . 4 considerable amount of work, preliminary to the present research, was done by one of us, on the theory of chemical reactions in such a discharge, in order that the data might be better interpreted. Some of this has already been published elsewhere,*P2but it is now given in more complete form below.
Theory of Chemical Action in the Glow Discharge Aside from the difficulty of working out the chemical reactions occurring in gaseous electrical discharges, the principal obstacle to a satisfactory theory has been the lack of a means (either experimental or theoretical) of determining the rate of formation of activated molecules. Even a t present this remains a serious obstacle, but recent work has gone far towards its removal, so that it is now possible to form a picture of the principal phenomena, which is quite satisfactory in all its essential details. The present state of knowledge in regard to the fundamental elementary processes occurring in gas discharges has been well summarized by Compton and L a n g m ~ i rwhile , ~ theories dealing with the glow discharge itself, which are of particular significance in connection with the rate of formation of active molecules, have been developed by H. A. Wilson,4 Gunthersch~lze,~ Compton and MorseJGMorse,’ and Linder.*P The glow discharge may be divided into the following parts, starting from the cathode: ( I ) cathode glow, ( 2 ) Crookes dark space, (3) negative glow,
3650
ERNEST G. LINDER AND ARDITH P. DAVIS
(4) Faraday dark space, ( 5 ) positive column, and (6) anode glow. The cathode glow usually appears as a layer of velvety light covering the surface of the cathode. I t is a region of high positive ion density, and the luminosity is probably due to excitation of molecules by positive ion impact. The Crookes dark space is the region across which most of the potential drop of the discharge takes place, and where a large fraction of the energy dissipation occurs. It will be discussed more fully below. The negative glow is usually the most luminous region of t,he discharge. The positive ion and electron densities are greatest here, as is very likely also the density of excited molecules. The Faraday dark space resembles the Crookes dark space, but the energy dissipation is much less. The positive column is a region of almost uniform ion density and energy, unless it is striated, in which case the ion density and energy vary with the striations. However, in no cases are the electron energies comparable with those in the Crookes dark space, nor the positive ion and electron densities nearly as large as those of the negative glow, being only about one hundredth of the latter. The anode glow usually appears as a thin layer of light covering the anode surface. Here the electron energy is a little higher than in the positive column, and the light emission is likely due to electron impacts against molecules. I n discussing chemical action, only three of the above regions need be considered, namely, the Crookes dark space, the negative glow, and the positive column. The energy dissipation in the other regions is so small that the chemical action occurring in them is probably not large. Even in the positive column the reaction is usually negligible, since the energy dissipation per unit volume is small. The reaction in this region would become important only in long discharge tubes, where the positive column would have a large volume, since when the distance between the electrodes is increased, the positive column increases in length, the other regions of the discharge remaining practically unchanged in almost every way. Brewer and his co-workers: and Linder,* have shown experimentally that most of the chemical action occurs in the Crookes dark space and negative glow, in tubes of the usual dimensions, such as that used in the present work. Therefore, only these two regions will be considered in the following discussion. The principal phenomena of the dark space'" and negative glow are r e p resented in Fig. I . C is the cathode surface, the dark space extends from C to E, E is the dividing line between the dark space and the negative glow. The negative glow occupies the region from E to F. The lines E and F are not sharply defined in the actual discharge, but are made sharp in the drawing for the sake of simplicity and clarity. We shall consider the entire potential drop as existing between C and E, its value at any point being given by the parabolic curve V, in accordance with hston's result." In the negative glow, i.e., the region EF, the potential is constant, as represented by the horizontal line, and the region is therefore field-free. Suppose that an electron is generated at the point a on the cathode surface. Under the act,ion of the electrical field, it will travel away from c', gaining kinetic energy from the field as it advances. Such an electron will make
REACTIONS OF HYDROCARBONS I N THE GLOW DISCHARGE
3651
impacts with molecules. These impacts may be divided into three classes: ( I j Elastic impacts, i.e., those which involve no (or at most only small) energy changes of either the electron or the molecule concerned, and whose only effect is a possible change of the direction of motion of the electron and molecule, such as is represented at the points marked e. ( 2 ) Ionizing impacts, such as those represented by the points i. These always result in the production of a new electron, and a positive ion. (3) Exciting impacts, i.e., those which raise the molecule to a higher quantum state. Dissociating impacts are not included in this list, since dissociation is now regarded as the result of ionization or excitation.
FIG.I Representation of Ionization Phenomena in Crookes Dark Space and Negative Glow
As a consequence of ionizing impacts, each electron that leaves the cathode (primary electron) generates a large number of new electrons (secondary electrons), before it reaches E. This is shown by the branching line in Fig. I . Each fork (such as i) represents an ionizing impact and the consequent production of a new electron, which, in turn, advances and itself produces other new electrons. Thus the number of electrons increases exponentially with distance from the cathode. Townsend'l has given an equation to represent this increase, -pCD __
a=pCe x , where a is the number of ionizing collisions per centimeter of advance, p is the gas pressure, X is the field strength, and C and D are semi-empirical constants, which have been determined by Townsend for a number of gases. A slight modification of the equation to adopt it to such non-uniform fields as are found in the dark space, has been given by Compton and Morse.6
3652
ERNEST 0.LINDER ILVD ARDITH P. DAVIS
The elastic and exciting collisions, of course, play no part in this exponential increase of ionization, except that they rob the electron of energy which might otherwise be used in producing ions. It has been found that each primary electron is responsible for the formation of roughly 50 to 100 secondary electrons before reaching E.*t6 If n represents the total number of electrons produced by no primary electrons, the ratio n/no increases as shown by the curve in Fig. I . Obviously most of the electrons are generated close to E, and therefore fall through only a small potential drop before reaching E. The average electron energy is therefore low (in a discharge in water vapor it is about 0.15VI, expressed in volts, where V1 is the total cathode potential drop.). After the electrons reach E, they continue advancing into the negative glow (in Fig. I most' of the paths are not, drawn beyond E for the sake of clarity). Here they continue to make impacts until their energy supply is exhausted. Of course, they can gain no additional energy, since the negative glow is field-free. Path N represents that of an electron generated quite far back in the dark space and travelling all the way to E without making impacts, and consequently having high energy. Such electrons make more than the average number of collisions in the negative glow before their energy supply is exhausted. On the other hand, path Y represents the opposite extreme, Le., that of an electron of such low energy, that it makes no ionizing impacts, only elastic ones. L represents an average path. The number of activating impacts in the negative glow probably decreases asymptotically with increasing distance from E, somewhat as does the luminosity in that regi0n.1~ No attempt has been made to have more than rough quantitative accuracy in the figure, it being desired to present only a qualitative idea at present. We shall now consider for a moment the various kinds of active molecules formed by electron impacts in the dark space and negative glow. These may be divided into two classes, corresponding to the second and third kinds of impacts mentioned above, ( I ) positive ions, and ( 2 ) excited molecules. We shall consider first the relation of the discharge current to these. Each positive ion formed in the dark space moves towards the cathode under the action of the electric field. A typical path is illustrated by the dotted line AB in Fig. I , the ion being generated at A by an electron impact. The impacting electron and the new secondary electron produced, move on toward E, while the positive ion formed, moves back toward C. This ion, being much larger than an electron, especially in the case of large hydrocarbon molecules, such as are being considered here, makes new impacts. However, due to its small mean free path, the ion does not gain enough energy between collision to produce many ionized molecules. Hence the path of the positive ion is generally not branched, as is that of the electron. It is clear t,hat the number of positive ions reaching C per second is equal to the number of ionizing impacts made in the dark space per second. (This assumes no recombinations of ions and no ionization by positive ion impact, but these are known to be few in number in the dark space.) Since each
REACTIONS OF HYDROCARBOSS I N THE GLOW DISCHARGE
3653
electron that leaves the cathode produces many positive ions, the number of positive ions reaching C must be much greater than the number of electrons leaving (usually j o to IOO times greater), therefore the current at C is predominantly a positive ion current. The discharge current is a measure of the positive ion current to C (disregarding the small number of electrons leaving C). At E however, the current is mostly an electron current. The density of positive ions here is about equal to that of the electrons, but the latter carry most of the current by virtue of their greater mobility. Hence the discharge current is a measure also of the rate at which electrons cross E. The discharge current is therefore a measure of three rates: ( I ) the rate at which ionizing impacts are made in the dark space, ( 2 ) the rate a t which positive ions reach C, and (3) the rate at which electrons cross E. There is obviously no necessary relation between the discharge current and the rate of formation of positive ions in the negative glow, or the rate of formation of excited molecules in any region of the discharge. The determination of the mechanisms of the reactions of the active molecules formed in the discharge, is a distinct problem for each individual compound, and indeed, for each type of activated molecule for each compound. However, it is possible to make some general remarks which apply to all. Under the usual conditions of the glow discharge, such as those described below, the concentration of positive ions in the discharge is much smaller than that of normal molecules,2 hence most of the collisions of the positive ions will be with normal molecules. The density of excited molecules has not been well determined, but there is reason to believe that in most cases it is comparable with that of the positive ions.14 Consequently most of the collisions of the activated molecules will be with normal molecules, and the number of collisions of active molecules with other active molecules will be negligible. Therefore it would be expected that among the important primary reactions would be those taking place by the interaction of an activated molecule and a normal one. I n fact, many reactions reported in the literature have been explained by such interaction,ljI lfi,17, for example, the formation in the discharge, of dihydrodiphenyl from benzene as reported by Mignonac and de Snint-Aunay,15 CfiHs f Cs&’ -+ C12Hiz. (1)
Of course, an activated molecule may react with more than one normal molecule. This appears to occur for example, in the case of unsaturated hydrocarbons. But such a combination of three or more molecules does not occur simultaneously, except in rare cases, and strict,ly should therefore be regarded as a primary reaction (involving two molecules) followed by one or more secondary reactions, as the case may be. Another type of primary reaction which should be important in many cases is the activation of a single molecule by electron impact followed by its dissociation. Many reactions of this type have been observed,lg for example,
3654
ERNEST G. LINDER AND ARDITH P. DAVIS
as reported by Barton and Bartlett.*O Of course, in most cases the final products do not represent the results solely of primary reactions, but also of a number of secondary reactions, since in many cases the products of the primary reactions are in an active state, e.g., OH+ in reaction (2) above, and act as the initiators of further reactions almost immediately. I t usually requires special methods to isolate the primary products, as for example, the positive ray mass spectograph, or, a discharge tube immersed in liquid air so that the primary products are frozen out before they can react further. Activated molecules formed in the discharge may be divided into three classes: ( I ) positive ions formed in the dark space, ( 2 ) positive ions formed
FIG.2 Typical Behavior of Ions and Excited Molecules
in the negative glow, and (3) excited molecules. The typical behavior of these three types is illustrated in Fig. 2. Positive ions formed in the dark space, such as A Fig. 2 , travel toward the cathode, making collisions with neutral molecules on the way. Reactions may take place a t any of these collisions, or perhaps clusters may form, the ion picking up neutral molecules as it advances, as proposed by LindSZ1 However, it is doubtful that the final neutral reaction products are actually formed until the ion reaches the cathode and is neutralized. Positive ions formed in the negative glow are not acted upon by any field, and hence move in a random manner, such as is illustrated by the ion B in Fig. 2. Actually there is a small field causing them to drift toward the wall, where some are neutralized. Others are neutralized in the gas phase. The reactions they initiate are probably the same as those initiated by ions in the dark space, but these ions or clusters likely travel farther before neutralization. Furthermore, the positive ions formed in the dark space may participate in surface reactions on the cathode surface, since they are practically all neutralized there, whereas large numbers of the ions formed in the negative glow are neutralized in the gas phase, and hence can play no part in such reactions.
REACTI0.W O F HYDROCARBONS IX THE GLOW DISCHARGE
3655
Excited molecules, whether formed in the dark space or negative glow, are not acted upon by electric fields, since they are uncharged, hence they move in a random manner, as shown by molecule G in the figure. Such an excited molecule may either react, or may return to its normal state. The return to the normal state may be accomplished by the transfer of the excitation energy to another molecule in a collision, or by the emission of a light quantum. The light quantum niay pass out of the discharge, or may be absorbed by another molecule, producing another excited molecule. I n fact such a quantum may be passed on from one molecule to another until it is finally lost from the discharge or participates in a reaction. Nothing much more of a general nature can be said regarding these various types of activated molecules, since their detailed behavior is probably different for each different compound, and each type of activation. The rate of reaction in the glow discharge has been found by a number of workers1> z2, 23 to be almost directly proportional to the discharge current. Linder2 has shown however, that a probably more correct relation is 2 l 99
dq/dt = kWI, (3) where k is a constant determined by the kind of hydrocarbon, W is the average energy absorbed from the field by the electrons generated in the dark space, and I is the discharge current. W may not vary much as I is changed, so that frequently there is almost direct proportionality between dq’dt and I, in fact, in the normal discharge, W remains constant. I n the abnormal discharge, however, an increase in I causes an increase in W, so that the rate of reaction per unit current increases as the current increases. This is due to the fact that the electrons have greater energy and hence produce more activated m0lecules.~4 Over a gas pressure range of a t least 0.5 to I O mm, the rate and nature of the reaction is almost independent of the pressure for any given compound.ls 23 Increasing the pressure reduces the length of the mean free path of the electrons, but the width of the dark space also decreases, the cathode potential drop remaining constant. The result is that each electron makes the same number of collisions as before, and the reaction is unaffected. The theory presented in this section provides a fairly satisfactory, although still incomplete, theoretical basis for the interpretation of the chemical action in the glow discharge. It will be applied to the experimental data below. g3 22p
Discharge Apparatus At the beginning of the investigation many attempts were made to build a satisfactory apparatus using a hot tungsten filament as a source of electrons, and accelerating them by applying a potential between this and a neighboring grid. Such an apparatus would have the great advantage that the number and energy of the bombarding electrons could be vaned independently of each other. However, an insuperable difficulty was encountered in trying to eliminate or correct for the thermal decomposition caused by the hot filament. The principal difficulty is that although the amount of thermal decomposition
REACTIONS OF HYDROCARBONS I N THE GLOW DISCHARGE
3657
All runs were made at a pressure such that the width of the Crookes dark space was from about I to 3 mm. corresponding to pressures of from about 0.5 to I O mm. of mercury. A more exact fixation of the pressure was unnecessary in view of the small effect due to pressure changes. I n cases where the vapor pressure of the compound at room temperature was sufficiently high, the pressure was regulated by immersing E in a cooling bath of a suitable temperature, but in cases where it was below 0.5 mm, that part of the apparatus surrounded by the dashed line in Fig. 4, was enclosed in an oven and raised to R temperature sufficient to give the desired pressure.
,/
To McLeod qauqe ,andToepler p m 9
-- - -
r-----
:Y I I
TO
oily'
P P
I
I
FIG.4
Schematic sketch of discharge apparatus
I n some instances temperatures around 15o'C were necessary. I n these cases the ground joints a, could not be sealed with any of the ordinary waxes, but it was found that suitable seals could be made by melting sugar into the joints and wrapping cooling coils around them. The joints could then be kept hot enough to avoid too great a lowering of the vapor pressure in D, and yet sufficiently cool so that they remained vacuum tight. The discharge tube D, was of about z liters capacity. The electrodes e, were aluminum disks about 2 . 5 cm. in diameter. They were spaced 8.5 cm. apart. The leads to them were of heavy tungsten wire, glass covered. The larger ground joints enabled the electrodes to be removed easily for cleaning. in Fig. 3, was not used in any of The glass spiral shown between E and the work reported here. The trap TI was made with a bulb as shown, so as to prevent clogging by solid substances condensing at the surface line of the cooling solution. Trap TPwas used only in work with high vapor pressure compounds, whose vapors were not completely stopped by the first trap, and which interfered with the correct functioning of the McLeod gauge if allowed to enter S. I t was cooled with the same refrigerant as was used on trap TI.
3658
ERSEST G. LINDER AND ARDITH P. DAVIS
In nearly all cases the refrigerant on the traps was a carbon dioxide-ether slush. In a few cases of high molecular weight compounds, ice water was used, but in all instances, the gas samples for analysis mere passed through a coiled glass tube N, immersed in carbon dioxide and ether, after being raised to atmospheric pressure by the Toepler pump. Thus, in every case, the gas sample analyzed, contained only those gases not condensible at - 77OC, at atmospheric pressure. The current for the discharge was supplied by a transformer, and rectified by a single kenotron. Its pulsating nature was unimportant, as was shown by tests on a discharge in water vapor, the results of which agreed with those reported by Linder* for a smooth direct current. The voltages varied over a range of about joo to 1000 volts. The principal differences between the method used here and those usually employed in the study of reactions in electrical discharge, is that the gas passes through the discharge only once, and at a low pressure and high rate of flow. Thus, more advanced and complicated secondary reactionsprobably do not occur to as great an extent as they do FIG.5 Typical curves showing rate of pres- when the sample of gas is allowed to remain sure increase with time. I. Retene, at in the apparatus, or is circulated reo and I O ma. 11. Styrene, at 0, 5 and peatedly during the time of the discharge. 2 ma.
Procedure After inserting the hydrocarbon into the bulb E, and sealing the joints a, the system was always tested for air leaks or evolution of gases absorbed in the hydrocarbon. Only the solid hydrocarbons were found to give off appreciable quantities of absorbed gas, and melting them while in the bulb E a t low pressure, was found sufficient to cause these gases to be liberated a t once. The discharge was started only after the McLeod gauge showed no, or only a small, rate of pressure increase in S, and measurements of this rate of increase, if any, were made at the beginning so that it could be corrected for; for example, see the parts of the curves marked o in Fig. 5 . Upon starting the discharge, the temperature of E, or of the oven, was adjusted so that the width of the dark space was from one to three millimeters. Measurements were then made of the rate of pressure increase in S for discharge currents of 2 , j and I O ma., except in the case n-decane, dodecane and n-tetradecane, which were tested over a range of only o to 2 . j ma.' Typical curves are shown in Fig. j, in which the pressure in S is plotted against time for various currents. From such curves as these the time rate of pressure increase dp/dt, was determined by measuring the slope.
REACTIONS O F HYDROCARBONS IN THE GLOW DISCHARGE
3659
TABLE I Rates of Gas Production (cc/sec/ma Compound
dq/dt
n-pentane n-hexane n-heptane n-octane n-decane
60
dodecane n-tetradecane n-docosane benzene toluene
99 IO1 IOj
113 I33 I45
I55 25
52
o-xylene m-xylene p-xylene mesitylene durene
35
hexamethylbenzene hexaethylbenzene n-but ylbenzene sec-but ylbenzene tert-but ylbenzene
79
n-propylbenzene iso-propylbenzene ethylbenzene naphthalene dihydronaphthalene tetrahydronaphthalene dekahydronaphthalene cyclohexane methylcyclohexane 1-methylcyclohexene
47
63 71 62
83 81 80
66 71 58
64 39 i 2
71 '03
x
IO^) Compound
dq/dt
cyclohexene p-diphenylbenzene m-diphenylbenzene o-diphenylbenzene diphenyl
105
di-iso-but ylene dipentene pinene di-iso-amyl limonene
139
p-diethylbenzene m-diethylbenzene p-cymene p-menthane styrene 2 .2.4-trimethylpentane 1-heptene dibenzyl 2 .z.3-trimethylbutene methylnaphthalene
octylene triphenylmethane anthracene stilbene acenaphthene retene phenanthrene
51 41 52
43 114 IO1 I
18
I12
69 71
66 71 45 97
1'5 49
97 62 IOj
42
35 57 50
56 37
86 93 I Oj
When sufficient data had been obtained t o determine dp dt at 2 , j and ma., a long run at I O ma. was made for the purpose of obtaining sufficient gas for analysis. This gas was pumped from S by the Toepler pump I,, and then passed through the coil N, immersed in carbon dioxide and ether, into the pipette M. The gas analysis is described in a later section. The solid products of the reactions formed principally on the cathode, but also in smaller amounts on the anode and walls of the discharge tube. IO
3660
ERNEST G . LINDER AND ARDITH P. DAVIS
They were removed at the conclusion of the run, by scraping the various parts, then washed and weighed. Xo attempt has yet been made to investigate liquid products, but their formation was observed in many cases. Molecular Structure and the Rate of Gas Evolution Fairly definite and consistent relationships have been found between the structure of the molecule and the rate at which gas is evolved in the discharge, when the current is held constant. The rate at which gas is produced dqidt for all compounds investigated, is given in Table I. These rates seem to follow two empirical rules, which may be stated as follows: (I) In a series of similar compounds of increasing molecular weight (e.g., the normal paraffins) the rate of gas evolution increases with the molecular weight. ( 2 ) In a series of similar compounds of the same molecular weight (e.g., 0-, m-, and p-xylene) the rate of gas evolution increases with decreasing centralization. 30 20
IO
**
I
1
c
10 t
I
4 8 MYBER
IO
it
14
OF c m o n
Ib
18 t o
12
&TOMS
FIG.6 Rates of gas production for the normal paraffins
c c
c c
i
FIG.7 Rates of gas production for the mono-substituted benzene series:-benzene, toluene, ethylbenzene, n-propylbenzene, and n-butylbenzene.
There is more evidence for the first of these rules than for the second, many isomeric compounds not, being available. Also there are some exceptions to both rules. Some of the more striking results, showing the increase in rate of gas production with molecular size, are graphed in Figs. 6 to I O , which are, for the most part, self-explanatory. The diagrammatic representations of molecular structure, given along the horizontal axes, are somewhat conventionalized. The data represented by crosses in Fig, 6 will be discussed later. The somewhat large jump between the rate for n-pentane and n-hexane is likely due t o the different method of determining dq/'dt for n-pentane, necessitated by its high vapor pressure. Its vapor could not be completely stopped by the traps TI and TS, and its interference with the action of the McLeod
REACTIONS OF HYDROCARBONS I N THE GLOW DISCHARGE
3661
gauge made the determination of dq/dt unreliable by the use of that instrument; hence its rate was determined by pumping out the gas sample from S and measuring its volume at atmospheric pressure in a burette. The points for n-hexane, n-heptane, n-octane all seem to lie a little too high; this may well be due t o the presence of small amounts of their vapors in the gauge. This seems true especially in view of the fact that before the addition of trap TSt o the apparatus the points for n-hexane and n-heptane lay still much higher' than is shown in Fig. 6.
Rates of gas roduction for the methyl-substit uteb) benzene series:benzene, toluene, p x lene, mesitylene, durene, and hexamet&lbenzene.
FIG.9 Rates of gas production for benzene, diphenyl, and p-diphenylbenzene.
Durene seems to be an exception, as is evident from Fig. 8, and also tetrahydronaphthalene, as it may be seen from Fig. I O . In the latter caee, however, the departure is probably no greater than the experimental error. Typical series showing the effects of varying the centralization of the molecule are given in Fig. 1 1 . The 0-, m-, and p-diphenylbenzenes do not appear to obey the centralization rule. Other comparisons are listed in Table 11. In addition to size and centralization, saturation also seems to play #apart, for example, the rates for n-hexane, cyclohexane and benzene are o.oogg, 0.0086 and 0.002 j cc 'sec,'ma, respectively, differences which seem too large to be accounted for solely by the small changes in molecular weight. Styrene and ethylbenzene offer another example. However, the data on this point are meager, and in some cases the presence of a double bond seems to have no effect, as in the case of n-octane and octylene. Sometimes even the opposite effect is observed, as for n-heptane and 1-heptene, but the measurements on these compounds are not very reliable due to their high vapor pressure and the consequent probable presence of their vapors in the McLeod gauge, as has been mentioned before. Parallelism with Knock Rating. There is a striking parallelism between the rate of gas production and the knock rating as given by Lovell, Campbell
ERSEST G . LIXDER AKD AARDITH P. DAYIS
3662
FIG I I
FIG I O Rates of gas production for naphthalene, dihydronaphthalene, tetrah dronaphthalene and dekahydronapit halene . 1-
Series showing effect of centralization on rate of gas production. I nbutylbenzene, sec-butylbenzene, and tert-butylbenzene 11 p-xylene, mxylene, and o-xylene
TABLE I1 Some Comparisons between Structure and Rate of Gas Production Compound
d q / d t (cciseclma X r o 4 )
benzene hexamethylbenzene hexaethylbenaene
25
naphthalene diphenyl
39 43
acenaphthene -stilbene
50
anthracene phenanthrene triphenylmethane p-diphenylbenzene retene
79 83
57
35 37 42
5' 56
REACTIONS OF HTDROCARBOXS IN THE GLOW DISCHARGE
3663
and Boyd.25 These investigators have measured the knock ratings of twentyseven paraffin hydrocarbons. Definite relations have been found between molecular structure and the tendency to knock. The statement is made t,hat “in a homologous series the tendency to knock increases with the increasing length of the carbon chain, and in an isomeric series the tendency to knock usually decreases as the number of side chains is increased.,’ This is substantially a statement of the two rules enunciated above, governing the rate of gas production in the glow discharge. Discussion of Relations between Structure and Rate of Gas Eaolution. A satisfactory explanation of these results probably requires not only a knowledge of the general features of the glow discharge, such as have been described above, but also information on the fundamental elementary processes of ionization and excitation for the individual hydrocarbons. So far as the authors are aware, no work has ever been done on the critical potentials, probabilities of ionization and excitation, or modes of dissociation by electron impact, of any of the substances dealt with in this article, and until some such work has been carried out, a complete explanation is probably out of the question. However, in the light of the theory presented above, some idea of the significance of the comparisons in behavior can be obtained. In the first place, it is essential to understand what can be used as a basis of comparison. The most satisfactory would probably be the bombardment of each hydrocarbon by an equal number of equal-energy electrons. This is what was attempted in the apparatus employing the hot filament’ mentioned above. In the glow discharge this condition cannot be realized, since the electrons have a wide distribution of energies. Furthermore, this energy distribution is not’ known, nor even the average energy. This latter quantity can be calculatedJ2but the necessary data are not available in the present instances. Therefore, only the number of bombarding electrons is known (the discharge current is a measure of that, as explained above), the average energy (Win equation (3)) being undetermined. Hence the observed variations in the rates of gas production given in this article, may be due to changes in k or R,or both, in equation (3). It is quite certain that IT’ does change from one hydrocarbon to another, increasing in general with increasing molecular weight. Giintherschu1zez6 gives an empirical equation for the cathode potential drop,
v
= (0.245 M+4)v1o(,
(4)
where M is the molecular weight, ‘I; the ionization potential, and (Y a factor depending on the effectiveness of the collisions (“Stossverlust und Wirksamkeit der Stosse”). Although this equation has been shown to hold only for some mono- and diatomic gases, there is no reason to believe it invalid for gases having larger molecules. As a mat’ter of fact, the total voltage across the discharge tube in the experiments reported here was observed t o increase with molecular size in general, but due to the great unsteadiness of the voltage, and its dependence upon pressure, these measurements are not
3664
ERNEST G. LINDER AND ARDITH P. DAVIS
recorded here. (The fluctuations in the total voltage were in all probability due to changing conditions in the positive column and on the electrode surfaces, and not to changes in the cathode potential drop, which is the part of the total voltage drop concerned in the chemical action. Otherwise it is difficult to see how such straight lines, representing the relation between dp/dt and time, as those of Fig 5 , could have been obtained) The changes in the cathode potential drop can be attributed directly to molecular properties, as is suggested by equation (4). The quantum theory lends further support to this, for as the molecular size increases, the number of possible quantum states also increases, and hence the probability that an electron impact will produce an ion rather than an excited molecule, decreases In other pords, the more complex the molecule, the more the opportunities for an electron in an impact to expend its eneigy in other ways than ionization Hence, to produce ions at a given constant rate in the dark space, i e , to have a constant discharge current for each of a series of compounds, requires the expenditure of more energy in the cabey of the more complex molecules, Since in those cases a smaller percent of the total energy goes to produce ions, and a larger percent ton ard producing excited molecules The rate of reaction depends on the rate of formation of both ions and excited molecules, i e , dq dt = dx bM,
+
\There S is the rate of formation of ions, M the rate of formation of excited molecules, and a and b are constants The rate of reaction per ion can therefore be written, dq _ _ dt_ - a
N
+ bJI
S.
(5)
Hence it should increase as the ratio of excited molecules to ions JI S increases, as is probably the case as n e pass from less complex to more complex molecules In the case of the data given here on the effect of molecular size, for example in Figs 6 to I O , the rate of reaction is expressed in terms of gas dq dt evolution in cc sec ma This should be proportional to the quantity _1: In equation ( j ) Of course, the total number of positibe ions formed per second X is not measured, but only those formed in the dark space Hoaever, we shall make the plausible assumption that the ratio of the number formed in the dark space to those formed elsewhere is constant in any series of qimilar compounds, e g , the normal paraffins I t follows then that the relations between molecular size and rate of gas evolution may be attributed to variations of a, b and 31 S However, since a and b probably do not vary greatly in a series of similar compounds, most of the change 1s likely due L O changes in Yl K Hence n e conclude that the tncrease ~ i l hnioleculor ~ 7 t e in rate of gas eiolution per un7t ciorent is probably d u e fo the zncreasing ability of the lnrger molecules to absorb energy i n other w a y s t h a n by 7omzation, i e , to their larger number of degrees of freedom 27
REACTIONS OF HYDROCARBONS IN THE GLOW DISCHAFIGE
3665
Solid Products The rates of formation of the solid products are given in Table 111. The accuracy of these measurements was not high, because only small quantities were produced (0.1to 0 . 5 gm.), and although most of it formed on the cathode, some was deposited in a thin skin over the inside surface of the discharge tube and could not be entirely removed for weighing. The solid was wax-like in consistency, sometimes brittle. It' was dark brown or black in color, and had the appearance of having a high carbon content, although a few preliminary com- I( bustion analyses have not borne this out. The solid weighed was that portion which I remained after washings in carbon tetrachloride, benzene and ether, except in a few 'I cases where the amount, of solid formed was so small that it had to be determined by weighing the electrodes before and after the run. In these cases the solid was not washed at all. The rate of formation of this solidforeach 2 hydrocarbon is plotted in Fig. 12, against :,IO the hydrogen to carbon ratio of the original ; hydrocarbon from which it was formed. 2 8 Although the points are scattered, the graph shows a distinct increase in the rate of 0 formation with decrease in the hydrogen to carbon ratio. This is in agreement with , the results of Lind,**who found that larger ion clusters form in the case of unsaturates than in the case of saturates. ~
Gas Analyses Ob 1.0 I4 1.8 2I HYDIOCFN TO CARBON RATIO The complete data on the gas analyses FIG.12 are given in Table IS'. These gases were Data On the formation Of produced in the discharge at a current of solids from hydrocarbons I O ma, and a pressure from 0.5 to I O mm, and passed, before analysis, through a coiled glass tube immersed in a dry ice-ether mixture. The analysis apparatus was similar to the one designed by Shepherd'g a t the Bureau of Standards, and embodied the following special features: I) The balance point of the manometer was obtained by electrical contact. This method was suggested by Greggo and its accuracy determined by Weaver and Ledig.31 I t has been found very satisfactory. A burette illuminator was made by placing a small electric light bulb 2) behind a sheet of tracing cloth. A reflector, made of wood and painted on the inside with aluminum paint, was placed behind the light. This enabled the burette to be read accurately.
3666
ERNEST G . LINDER AND ARDITH P. DAVIS
TABLE I11 Data on Formation of Solids
H, C, hydrogen to carbon ratio; W, rate of solid formation (gm sec ma X 109 LV
Compound
H C
toluene o-xylene pxylene mesitylene n-butylbenzene
I
16
4.39
1
24
I .69
.3;
1
24
I
1
32
2.43
I 40
2.91
sec-butyl benzene
I
40
2.
13
t ert-but ylbenzene
1
2
73
I
40 21
1
32
2.90
I
31
1 .53
I
;-'
ethylbenzene n-propylbenzene iso-propylbenzene hexamethylbenzene cyclohexane n-heptane n-pentane di-iso-but ylene
2 00
'
2.82
2.76 0.51
35
2
28
0
2
40
I ,2 1
19
2 00
I ,
met hylc yclohexanp dipentene pinene di-iso-amyl limonene
2 00
0.19
60
1.66
n-docosane retene acenaphthene diphenyl
2
I
I bo
1.88
2
0.62 i .62
2 0
I a0 IO
I 00 0 0
83 83
I .
5.1
.j .08
6-39 ;. 65
Compound
H C
p-diethylbenzene m-die thy1benzene p-cymene p-met hane styrene
I
2 . 2 .4-trimethylpentane 1-heptene 2 . 2 .3-trimethylbutene dibenzyl dekahydronaphthalene
dihydronaphthalene cyclohexene 1-methylcyclohexene octylme naphthalene triphenylmethane hexaet h ylbenzene p-diphenylbenzene m-diphenylbenzene o-diphenylbenzene anthracene phenanthrene stilbene
I
80 80
1
If. I .
77
2.94
40
2.09
2 00
0.90
I 00
1.32
21
I .32 0.77 0.58
2
2 00 2 00
80
5.04 I .j 2
I 00
j.40
I 00
I
I
67
I
7'
1.06 .36 0.80 7.51 I
2
00
0
80
0
84
1
67
o
j8
11.50
0 0
78 78
1 8 .I O 11.60
0
72
6.91 1.29
0 72
9.70 8.82
o 86
i 24 '
3) A stopcock with two bores. one a two-millimeter, and the other a constricted bore, was placed in the mercury line at the bottom of the burette. This permitted easy regulation of the mercury level. 4) Capillary T-stopcocks, with large barrels, were used in the distributor. The bore of the capillary tubing was from 1 . j t o 2 . 0 millimeters. Capillary stopcocks with small barrels were tried and found t o be unreliable, because of leaks. 5 ) The water jacket was made large enough to accommodate the burette, compensator and manometer. 6) Dennis-Friedrichs pipettes were used for absorptions.
REACTIONS OF HYDROCARBONS IN THE GLOW DIBCHABGE
3667
TABLEIV Summary of Gas Analyses Hydrogen
Acetylenes
Ethylenes
Paraffans
n-pentane n-hept ane n-octane n-docosane benzene
6 0 , zyo 46.0 48.9 55.7 46.0
1o.0yo
17.i%
9.7 13.8
26.9 16.6
I2,Iyo 17.5
11.2
25.7
40.5
4.4
toluene o-:cy lene m-xylene p-xylene mesitylene
54.8 60.2
29.0 16.0
3.0
52.2
25.1
.6 18.9
6.8 6.3 7.4
15.8
i 3 .O 55.1
hexamethylbenzene hexaethylbenzene n-but ylbenzene sec-but ylbenzene tert-but ylbenzene
54.7 37.8 56.4 50.6 45.5
11.4 20.6 9.6 16.8 18.8
13.I 17.8 I8 . o 16.6 11.9
20.9 23.8 16.I 16.2 23.8
n-propylbenzene iso-propylbenzene ethylbenzene naphthalene dihydronaphthalene
43 .o 51.8 50.7 42.8 54.8
12.8 14.4 19.6 32.2 23.4
27.0
17.2
12.1
21.6 '7.3 1.7
Compound
11
7.8
12.4 23.3 19.7
tetrahydronaphthalene dekahydronaphthalene cyclohexane methylcyclohexane 1-methylcyclohexene
57 .o
17 .o
20
52.5
12.8
46 . o 47 . o 40.8
.6 12.6
30.0 32.1 26.6 31.1
cyclohexene diphenyl di-iso-but ylene dipentene pinene
48.7 43.2 57.3 48.4 53.7
16.5 37.4
di-iso-amyl limonene p-diethylbenzene m-diethylbenzene p-cymene
56.6 58.4 36.0 49.8 46.2
13.2 12
I1
.o
.o
30.0 17.3 13.8
20.8
7.4 9.2 13.7 16 . o 9.1 18,7
2.1
6 .o 4.7 8.7 13.8 15.5
4.8 2.1
'7.9 13.1
11.5
27
16.2
18.2
I2
'5.1 14.9 11.4 1 4 .I
15.5
I8 .o
12.9 8.7
25.7
27.0
16.9 16.6
2 j . 2
I2
.o
.o
.o
19.2
3668
ERNEST G. LINDER AND ARDITH P. DAVIS
TABLE IV (Continued) Summary of Gas Analyses Compound
Hydrogen
p-menthane styrene 2 . 2 +trimethylpentane oct ylene triphenylmethane
5% 45 7 53 2 38 i jo I
1-heptene retene 2 . z .3-trimethylbutene anthracene stilbene
36 3 39 8 37 9 44 9 46 8
52
Acetylenes I
Ethylenes 28
1%
Paraffins 18 .o%
18 8 3 5
30 7
11
0
36.4
27
3
21
8
2.1
I
25
4
12.3
21
I
I j 0
24.1 2 7 .3
7 9
36 2 41 8
acenaphthene phenanthrene ;)
4%
2 j
26
I
9
18 2 6 8
4.7 16.2 10.9 0.8
0.; I
.h
73 3
1.4
28 0
4 3
The combustion pipette was constructed as described by Shepherd.
It has given service that is entirely satisfactory. 8 ) A copper oxide tube was used for combustion of hydrogen. I t was heated by means of a small electric furnace made from a porcelain porous cup. Connection with the copper oxide tube was made by means of two stopcocks placed in the distributor with the outlets pointing upward. Shepherd describes the method for exact analysis and also for what he calls technical analysis. The laborious procedure necessary for exact analysis was not followed because gas samples collected under conditions as near identical as possible were found to vary more than the error would be in technical analysis. The error in our analyses was not more than three or four tenths of a percent of the whole sample on each constituent. Ethylenes and acetylenes were absorbed together in fuming sulphuric acid. Oxygen was absorbed in alkaline pyrogallol. Hydrogen was determined by combustion over copper oxide heated to about 2 ; o O C . After these determinations had been carried out the gas that remained consisted of paraffins and nitrogen. The volume of paraffins was determined by combustion in the hot platinum wire pipette. Acetylene was determined in a separate sample of gas.3* The acetylene was allowed to react with ammoniacal cuprous chloride. The copper acetylide was filtered off, washed with dilute ammonia and dissolved in hot dilute hydrochloric acid. Bromine water was then added to oxidize the copper to the cupric condition. The solution was then evaporated to a thick syrup or to dryness. The copper was then determined by the potassium iodide titration method as described in Scott’s Standard Methods of Analysis. Dennis and Nichols33 say that acetylene is “best determined by leading it through an ammoniacal cuprous chloride solution, a reddish brown precipitate being thrown down. . . . The acetylene
REACTIONS OF HYDROCARBONS IN THE GLOW DISCHARGE
3669
may be determined by taking advantage of the fact that the moist precipitate contains carbon and copper in the atomic proportions of I : I . ” As is evident from Table IV, the composition of the gas evolved from the various hydrocarbons has been found not to vary over a wide range. Furthermore, no evident relations have been found between gas composition and the molecular structure of the original hydrocarbon. The presence of large amounts of unsaturates in most of the gases is of interest in view of the fact that Lind and GlockleP4 report their absence in gases collected from discharges in hydrocarbons in a Siemens ozonizer. However, these latter runs were of a duration of eight and a half hours, and a flow method was not used. I n a run of eighty minutes duration they found small amounts of ethylene and acetylene. Hence, they conclude that unsaturates are formed in the early stages of the reaction, but later used up. The work reported here supports this view, since probably not many secondary reactions occur, and if unsaturates are formed in the first few reactions they should be present in the gas collected, as was found to be the case.
Additivity of Rate of Gas Evolution There is some evidence that the rate of gas evolution in the discharge is an additive property of the molecule. For example, the points indicated by crosses in Fig. 6, were obtained by subtracting the rate for benzene from those for toluene, ethylbenzene, n-propylbenzene, and n-butylbenzene. The data thus obtained lie on the normal paraffin curve close to the positions that apparently would be occupied by methane, ethane, propane and butane. Also in the case of Figs. 7 and 8, it is apparent that the rates for pxylene and ethylbenzene are nearly the same, as are also those for n-propylbenzene and mesitylene, suggesting that the carbon groups have the same effect regardless of their arrangement around the benzene ring, providing that they are not too close together. The comparison does not hold good for the higher members of these series, possibly because centralization or crowding, begins to play a part. Such additivity as is suggested here, would be in harmony with the theory proposed above to explain the increase in rate of gas evolution with molecular size, if the number of possible quantum states of a molecule were the sum of the states of its separate parts. Sources of Compounds and Accuracy of Data As was stated in the introduction, the purpose of this research was not to study in detail the reactions of individual hydrocarbons, but to make a survey of the entire class of compounds with the idea of finding any outstanding relations which might exist between molecular structure and the nature of the react,ions. For this reason, and also because of the prohibitive amount of work and time that would otherwise be required, no great pains were taken in order that each datum be of high accuracy. Consequently, the greatest value of the work lies not in individual figures, but in comparison between numbers of data, each in itself not necessarily highly accurate.
3670
ERNEST G. LIWDER AWD ARDITH P. DAVIS
The hydrocarbons used were obtained from the Eastman Kodak Company, with exceptions as follows: acenaphthene, diphenyl, cyclohexane, anthracene, and triphenylmethane were manufactured by the Kahlbaum Chemische Fabrik, Berlin; 1-heptene, and 2.2.3-trimethylbutene were supplied by Dr. T. A. Boyd of the General hlotors Laboratory, Detroit; methylnaphthalene was supplied by Dr. S. P. Miller of The Barrett Company, Xew York, and ethylbenzene and n-docosane were prepared by the Department of Organic Chemistry of Cornel1 University. We wish to take this opportunity to thank all those who helped us in obtaining some of these compounds. In determining the rates of gas evolution, one run each was made a t current values of 2, 5 , and I O ma, (with the exception of the three paraffins mentioned above) and the value of dq dt at each of these currents obtained. An average dqldt was then determined by plotting these and drawing the best straight line through the three points. Most of these data are reproducible to within about I O percent. The accuracy of the weighings of the solid product is less, owing to the aforementioned difficulty of collecting it, and also to variations in the washing procedure necessitated by differences in the solubilities of the original hydrocarbons which had to be removed. Only one gas analysis was made of the gases evolved from each hydrocarbon, except when accidents or doubtful results necessitated repetitions. However, considerable work was done in developing the analysis apparatus and procedure, so that the results are probably quite reliable. The method should be accurate to within two or three tenths of a percent. The formation of solid products on the electrodes, principally the cathode, leads to some ambiguity in the interpretation of the data on gas evolution, since the solid is subjected to continuous bombardment by positive ions and electrons. Thus, complicated and advanced secondary reactions resulting in gas emission, may take place on the electrode surfaces. The gas emission would then not be entirely due to primary or early secondary reactions. However, there is some reason t o believe that these solid deposits are a minor factor, at least in so far as the effects on rate of gas production is concerned. In a previous paperLit was reported t h a t runs of a duration of 80 minutes showed no departure from the linear relation between pressure and time, as shown in Fig. 5 , in spiteof the fact that at the beginning of the run the electrodes were clean, whereas at the end they were coated with a heavy layer of solid. Furthermore, McLennan, Perrin, and I r e t ~ nhave ~ ~ bombarded with cathode rays a similar solid product formed from acetylene, and found no evidence of gas evolution. Runs with an electrodeless discharge are being contemplated, which should clear up this point. summary A theory of chemical action in the glow discharge is presented. An apparatus for subjecting hydrocarbons in the gaseous phase to the glow discharge, and collecting the reaction products, is described. Data on fifty-seven hydrocarbons are given.
REACTIOSS OF HYDROCARBOSS IN THE GLOW DISCHARGE
3671
I t has been found that in a series of similar compounds of increasing molecular weight (e.g., the normal paraffins) the rate of gas evolution per unit current increases with the molecular weight; whereas in. a series of the same molecular weight, (e.g., 0-, m-, and p-xylene), it increases with decreasing centralization. The increase in the rate of gas evolution with molecular size is attributed to the increasing ability of the larger molecules to absorb energy in other ways than by ionization, Le., to their larger number of degrees of freedom. The amount of insoluble solid formed has been found to increase generally with decrease in the hydrogen to carbon ratio of the original hydrocarbon. Analyses are given of the gases evolved in the glow discharge for fortyseven hydrocarbons. Acknowledgment In conclusion the authors wish to express their appreciation to those of the Departments of Physics and Chemistry of Cornell University who were helpful in this work, and to Professor Vladimir Karapetoff, who is in general charge of the above-mentioned research program at Cornel1 on the fundamentals of the disintegration of organic electrical insulation. They wish especially to thank the President of the Detroit Edison Company, Mr. Alex DOXY, and the Chief of Research, Mr. C. F. Hirshfeld, for the financial support which made the above-described work possible, and for permission to publish the results. (’omell Cniuersity, Ithaca. S . 1‘.
References and Footnotes * The work described in this article is a part of an investi ation of the fundamentals of the (disintegration of organic dielectrics being carried out a t 8ornell University. I t is supported by a fund provided by the Detroit Edison Company. I Ernest G. Linder: Phys. Rev., 1 2 ) 36, 13;j ( 1 9 3 ~ ) . * Ernest’G. Linder: Phys. Rev., 38, 6 j 9 (1931). K . T . Compton and I. 1.angmuir: Reviews of Mod. Phys., 2 r z ) , 123 (1930);3 I Z ) , 191 (1931). H.A . Wilson: Phys. Rev., 1 2 ) 8 , z z i :1928). 5.4. Guntherschulze: 2 . Physik, 33, 810 (192jj. K.T. Compton and P. M. Morse: Phys. Rev., ( 2 ) 30, 3 0 j (1928). P. 11.Morse: Phys. Rev., ( 2 ) 31, 1003 (1928i. 8 Ernest G. Linder: Phys. Rev., (forthcoming paper). 4. K. Rrever and J. FV. Westhaver: J. Phys. Chem., 34, I j3, 2343 i1930); A Brewer and P. D. Kueck: 35, 1281, 1293 i1931i. The term “dark space” is used in this article to mean the Crookes dark space. ;.e., the region het1r-een the cathode and the edge of the field-free region in the negative glow, but not the visible edge of. the glow, as is sometimes meant. F. W. Aston: Proc. Roy. SOC.,84, j26 (1911I . lZ J. S. Townsend: “The Theory of Ionization of Gases by Collision,” Chap. I. l 3 A. Guntherschulze: 2. Physik, 33, 810 \192j). l 4 Compton and Langmuir: Reviews of Mod. Phys., 2 ( z ) , 133 (1930). lS C. Mignonac, and R . V. de Saint-Aunay: Bull. SOC.Chim., 47, j23, 522 (1930); Compt. rend., 189, 106; 188, 959 (1929). 16R. H. Pease: J. Am. Chem. Soc., 52, 1158 (1930). S.C. Lind: “The Chemical Effects of Alpha Particles and Electrons,” Chap. 1 2 . Brewer and Kueck: J. Phys. Chem., 35, 1298 (1931).
36i2
ERNEST G. LINDER AXD ARDITH P. D.4VIB
I 8 H . R. Stewart and A. R . Olson: J. Am. Chem. Soc., 53, 1236 (1931). Also many papers on positive ray analysis by H. D. Smyth, Hogness, Barton, etc., especially in the Physical Review. Barton and Bartlett: Phys. Rev., (2)31, 822 (1928). ?L S. C. Lind: “The Chemical Effects of Alpha Particles and Electrons.” z2P. d. Kirby: Proc. Roy. Soc., 85 A , 151 (1911); Phil. Mag., 13, 289 (1907). 23 A. Guntherschulze: Z. Physik, 21, j o (1924). 24 I n calculating the total amount of ionization in their discharges, Brewer and his associates have assumed that W is a constant, independent of the current, and equal to the entire normal cathode potential drop. Both of these assumptions appear unjustified, especially the latter, since by far the greater number of the electrons fall through only a small part of the total cathode potential drop (about I j percent of it). See above discussion and also references ( 2 ) and (6). 25W. G. Lovell, J. M . Campbell, and T. A . Boyd: I n d . Eng. Chem., 23, 26 (1931). 26A. Guntherschulze: Z. Physik, 20, I 53 (1923). 27 For a further discussion of the relation between molecular activation and the chemical yield, see Ernest G. Linder: Phys. Rev. (forthcoming paper). 28 S. C. Lind: “The Chemical Effects of Alpha Particles and Electrons,” p. I q j . M. Shepard: Bur. Standards J. Res., 6, I Z I (1930). aoGregg:Ind. Eng. Chem., 9, 528 (1917). 31 Weaver and Ledig: J. Am. Chem. Soc., 42, 1177 (1920). 32 I n this acetylene analysis the assumption was made that acetylene itself was the only member of the series present. This is quite plausible since the gases were passed through a coil immersed in a carhon dioxide ether slush, the temperature of which was -77°C. whereas the boiling point of the second member of the acetylene series CaH, is -27. j°C: 33 Dennis and Nichols: “Gas Analysis,” 265 (1929’). This method was not used however, in the cases of toluene, mesitylene, n-octane, benzene, m-xylene and o-xylene, the acetylenes for these substances being determined by absorption in ammoniacal silver nitrate solution, and ethylenes by absorption in bromine water. 34 Lind and Glockler: J. Am. Chem. Soc., 52, 4450 (1930). 35 J. C. McLennan, M. W. Perrin and H. J. C. Ireton: Proc. Roy. SOC.,125A,246 (1929).
