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The Modes of Activation of Aldehyde Molecules in DecompositionReactions

By C. N. Hinshelwood, F.R.S., C. J. M. Fletcher, F. H. Verhoek, andC. A. Winkler

(Received April 18, 1934)

In reactions depending on the decomposition of a single substance the general relation between the rate, as represented by the reciprocal of the time of half change, and the initial pressure is given by a curve which at first rises from the origin and then bends round to become parallel to the pressure axis. The point at which the curve becomes horizontal, and the height of the limiting ordinate depend upon specific constants. Thus, if a molecule suffered a number of alternative decompositions, the total rate of change would be repre­sented by a curve which showed a corresponding number of changes of slope in particular regions of pressure, giving it a segmented appearance.

The curves for nitrous oxide* and for acetaldehydef show such an appearance, and the decompositions appear to be composite. Yet the products of reaction are the same over the whole pressure range. J The same chemical reaction thus occurs by mechanisms which differ in some physical way. This can be interpreted by assuming different types of activated molecule which have widely different decomposition probabilities.

The segmented curve can be roughly analysed into a series of curves each rising at first and then reaching a limiting height, as shown in fig. 6 of the paper of Fletcher and Hinshelwood, each curve corresponding to a different type of activated molecule. The slope of the rising, or “ second order,” part of each of these fundamental curves is proportional to the rate at which activated molecules are produced, and the limiting height to the probability of the decomposition of the particular type of activated molecule. The reaction corresponding to a small probability of decomposition will be observable experimentally only when the time between collisions is relatively long, that

* Musgrave and Hinshelwood, ‘ Proc. Roy. Soc.,’ A, vol. 135, p. 23 (1932); Hunter, ibid., vol. 144 , p. 386 (1934).

t Fletcher and Hinshelwood, ibid., vol. 141, p. 41 (1933).% At 400 mm acetaldehyde gives carbon monoxide and methane quantitatively. The

average of four analyses made on the products from acetaldehyde at 15-18 mm gave :— Gas: contraction on combustion: C 02 formed — 1 *00 : 1-22 : 0*98. Theoretical for CH4 + CO = 1 00 : 1-25 : 1 00 .

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328 C. N. Hinshelwood and others

is at low pressures, and then only if the rate of activation for that particular mode is greater than that of the other modes of activation. If a long time between collisions is required for reaction, a comparatively low pressure will be sufficient to maintain the equilibrium concentration of activated molecules, so that the curve for the reaction with a small decomposition probability bends over more quickly than that for the reaction with the higher probability. This condition is illustrated by the curves 1 and 2 of the figure above referred to. The curve which rises steeply and bends at low pressures thus means a high rate of activation and a small probability of decomposition.

It was suggested that the different activated states in acetaldehyde corre­spond to a location of the energy in different parts of the molecule. The essential process in the decomposition of an aldehyde is the migration of a hydrogen atom and the breaking of a C—C bond. Energy of activation com­municated by collision to any part of the molecule other than one of the two- bonds in question will have to be redistributed before decomposition can occur. Hence the widely varying probabilities of chemical reaction according to the location of the energy.

The problem now arises of attempting to correlate particular parts of the curve with the activation of specific degrees of freedom in the molecules of particular compounds. The most direct method is to investigate changes in the form of the curve when the structure of the molecule is varied in a known way. For this purpose a comparison has been made of the behaviour of acetaldehyde with formaldehyde, propionic aldehyde and chloral. The reaction in each substance consists in the removal of carbon monoxide from the molecule. With formaldehyde the residue is hydrogen, with acetaldehyde, methane, with chloral, chloroform which decomposes further, and with pro­pionic aldehyde, a variety of products of which ethane is the principal one.

The actual experimental results are described in the following papers and in a previous paper.* In the present paper the general conclusions are sum­marized and discussed. A general comparison of the shape of the curves for the four substances is shown in fig. 1. Except for chloral, which decomposes at a very much lower temperature, all the curves represent decomposition at 556° C.f

* ‘ Proc. Roy. Soc.,’ A, vol. 141, p. 41 (1933).t The temperatures were compared in terms of the same thermocouple (platinum-

platinum-rhodium). The restandardization lowers by 4° the temperatures given by Fletcher and Hinshelwood ; 560° of their paper should read 556°. The original temperat ure scale of Hinshelwood and Hutchison corresponds to a value between the old and the corrected value, but very close to the latter (‘ Proc. Roy. Soc.,5 A, vol. I l l , p. 380 (1926)).

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Decomposition Reactions 329

The most striking contrast revealed by the curves is the relative increase in the magnitude of the lower segments with increase in the degree of sub­stitution of the aldehyde. With formaldehyde, as far as has been determined, the curve consists of a line passing through the origin and showing no changes of slope. This suggests that in formaldehyde there is a single mode of activa­tion, as might be expected from its simple structure. Acetaldehyde shows additional modes of activation, which are still more pronounced with propionic aldehyde and with chloral. From all the evidence, we shall conclude that those corresponding to the lower segments represent the location of the activation

Initial p r e s s u r e (mm) F ig . 1

energy in parts of the molecule remote from the aldehyde group. The residual slope of the lines at high pressures, on the other hand, represents the simplest and most direct mode of activation for the decomposition of an aldehyde.

Although at 556° the rates of reaction at the lower pressures increase from formaldehyde to propionic aldehyde, at higher pressures the curves cross, so that formaldehyde decomposes most readily while with propionic aldehyde the line becomes almost horizontal. The part played by the simple mode of activation thus becomes less in absolute magnitude with increase in length of the hydrocarbon chain. This is in accord with what might be expected, from electronic theories of organic reactions, from the effect of the alkyl group in strengthening the attachment of the aldehyde hydrogen. On the other hand,

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330 C. N. Hinshelwood and others

the more complex modes of activation, as represented by the lower segments, are much in evidence, and become more so as the molecules are more heavily substituted. This can be understood in terms of our hypothesis, because there are many more ways in which the energy can be given to the molecule, e.g., it can reside in the more remote parts of the alkyl group, and have to find its way to the sensitive part of the molecule before decomposition can occur. As is well known from the general theory of unimolecular reactions, such a state of affairs gives rise to a high rate of reaction, because more degrees of freedom can contribute. For example, even though the energy may reside in the methyl group, the three degrees of freedom of the three C—H bonds give an enormously increased chance that the requisite energy is present, and this more than compensates for the smaller probability that it ultimately finds its way to the right place.

As far as concerns the general thesis that decomposition reactions may be kinetically composite, nothing need be assumed about the energies of activation at different pressures. What has just been said applies specifically to the identification of that mode of activation which happens to predominate at low pressures in the particular example of the aldehydes. This special interpretation is supported by a consideration of the experimental values of the energies of activation. These are given in Table I, which should be con­sidered in conjunction with fig. 1.

Table IPressure (mm Hg).... 25 30 ICO 200 350 450HCHO............................. — — — 44,000 — —CH3CHO....................... 55,000 — — 50,400 — 47,700C2H5CHO ..................... — 63,500 61,200 59,500 56,000 —CCI3CHO ..................... — — 49,200 49,200 — —

With formaldehyde the energy of activation corresponds to the simplest mode of activation, the number of molecules reacting being given by Z . e-E/IlT; in other words, practically every collision between two molecules possessing enough energy to activate one internal degree of freedom (two square terms) can lead to reaction (Fletcher, p. 357). With acetaldehyde the high pressure part of the reaction can be expressed in the same way,* but the energy of activation corresponding to the lower segments of the curve is greater.f To account for the rate of reaction at lower pressures, increasing numbers of degrees of freedom must be brought in.

* Hinshelwood and Hutchison, loc. cit. t Fletcher and Hinshelwood, loc. cit.

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Decomposition Reactions 331

The same increase in E with decreasing pressure is found with propionic aldehyde, but here the contribution of the “ high pressure ” part of the re­action is so small that, even at the highest pressures measured, E is still too great to allow the rate to be accounted for by activation in two square terms, p. 356. If the contribution from the “ low pressure ” mechanisms could be eliminated, it is possible that there might be a residual reaction with a rate given by the simple formula.

With chloral, the decomposition takes place at a comparable rate about 100° lower. This is what might be expected from the loosening of the aldehydic hydrogen by the chlorine atoms. The energy needed in the vital bond must be less, and if the reaction were of the “ second order ” type, as with formaldehyde, we should expect the E value to be correspondingly small. But, as the curves show, the reaction is predominantly of the type which we are associating with the more complex modes of activation. Accordingly we find, as with acetalde­hyde and propionic aldehyde at lower pressures, that the E value is much greater than would correspond to the simple mode of activation. Comparison of chloral with acetaldehyde raises the question why the simple mode itself should not be more in evidence. The explanation may well be that the chlorine atoms, whether by mere size or mass, or by some more specific effect, cause the energy of collision to enter the CC13 group preferentially.

So far no distinction has been made between the separate segments into which we consider the low pressure part of the curves for acetaldehyde and propionic aldehyde, p. 345, to be divided.

Although there is no question of any discontinuity in the curves, and it must be emphasized that no such discontinuity has been suggested, neverthe­less the changes of slope can hardly be represented except by functions with more than one set of independent constants.

Below 100 mm, with propionic aldehyde and acetaldehyde, there are two distinct regions of changing slope, namely at about 40 mm, and at 1 to 10 mm. The reaction which gives rise to the lowest bend is the one with the smallest transformation probability, as discussed above. In the special example of the aldehydes this reaction is the one with the highest energy of activation. It might therefore be assumed that the energy is located in the region of the molecule most remote from the aldehyde group, namely, the C—H links of the alkyl group. The probability of energy getting from here to the vital link is small, but, since the corresponding degrees of freedom are numerous, a high activation rate is possible. Thus we have a rapid rise with increasing pressure, and the early attainment of the small limiting value. The prominence of the

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332 C. N. Hinshelwood and others

lowest segment with propionic aldehyde, as compared with acetaldehyde, can be explained by the greater number of alkyl C—H links.

In this connection, we may consider the analysis of the propionic aldehyde reaction products. At the higher pressures the principal products are carbon monoxide and ethane. As the pressure falls ethylene and hydrogen make their appearance in increasing proportion, and they can be proved not to come from any secondary decomposition of the ethane. The increase corresponds more or less exactly to that in the proportion of the reaction corresponding to the lowest segment, p. 355. Without going into details of the process by which an aldehyde molecule at the moment of decomposition yields ethylene and hydro­gen rather than ethane, it is reasonable to suppose that this process has invol­ved the activation of one or more atoms of the alkyl group itself.

Having associated the lowest part of the curve with the activation of the alkyl group and the highest part with the direct activation of the vital part of the molecule, we might now suppose that any intermediate segments that are distinguishable arise either from modes of activation involving vibrations of specific linkages such as the C = 0 bond, or from particular deformation vibrations, or from particular combinations of various types of vibration. From the curves, two extra types of activation apparently have to be provided for, though there may be others not experimentally distinguishable.

One of these could be provided for by the vibrations of the carbonyl-group. Before speculating as to which other might be concerned, we should have to know what constitutes activation in the “ vital bond ” referred to above. This has been left unspecified because there seems no easy way of deciding whether the “ vital bond ” is that joining the aldehyde hydrogen to the carbon from which it must migrate in the reaction, or the bond joining the aldehyde carbon to the adjacent carbon from which it must be severed. Whichever it is, one of the others would be available to provide an extra mode of activation, and this would be expected to give the second highest transformation prob­ability, that is, to provide the segment next to the highest.

But the increase in the number of degrees of freedom which must be assumed in order to account for the reaction rate at low pressures shows that, in principle, combinations of different vibrations occur. Thus it may not be justifiable to identify too rigidly a given mode of activation with one single specific link. Even for the simplest mode of activation, the energy may not be definitely localizable either in the C—H or in the C—C link, but may reside in a deformation oscillation in which the angle between these two bonds changes.

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Decomposition Reactions 3 33

In the decomposition of propionic aldehyde, methane is formed in a primary reaction to some extent. I t is possible that the corresponding mode of activa­tion involves a bending oscillation, whereby the hydrogen of the aldehyde is brought within reach of the methyl group.

In conclusion, it may be pointed out that the view adopted here of a number of distinct transformation probabilities for various types of activated state is not inconsistent with those theories* which make any one probability a function of the total energy. What have been called transformation probabilities may quite well be energy functions rather than constants. But the point is that, even so, a number of such functions with considerably different mean values are necessary. If the course of the curves discussed above were expressed by one function only, it can be shown that the observed energy of activation would fall at low pressures. Actually it rises.

We are indebted to the Royal Society and to Imperial Chemical Industries, Ltd., for grants by which apparatus for these various investigations has been obtained.

Summary

The curves representing the variation with pressure of the time of half decomposition of acetaldehyde and of nitrous oxide indicate that the reactions are kinetically composite. A number of different modes of activation of the molecules are assumed. By a comparison of the behaviour of differently substituted aldehydes—formaldehyde, acetaldehyde, propionic aldehyde and trichloracetaldehyde—an attempt is made to characterize some of these various modes of activation.

* Rice and Ramsperger, 4 J. Amer. Chem. Soc.,’ vol. 49, p. 1617 (1927); Kassel, 4 J. Phys. Chem.,’ vol. 32, p. 225 (1928).

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