Kinetics of gaseous reactions : the low temperature oxidation of aliphatic ethers

One of the most remarkable characteristics of the low-temperature, slow-oxidation of organic compounds is the influence of structure on the reaction rate. With the normal paraffin hydrocarbons, for example, the rate of oxidation, in strong contrast to the rate of pyrolysis, increases markedly with the length of the carbon chain (Cullis, C.F., Hinshelwood, C.N. and Mulcahy, M.F.H; Disc. Faraday Soc.: 1947, 2, 117). Structural sensitivity has recently been found to be quite general and any alteration in the molecule whether by branching or substitution of chloro, amino or carbonyl groups is reflected in the oxidation rate (Hinshelwood, C.N.; Disc. Faraday Soc., April 1951). The primary purpose of the present investigation is to determine the influence of R′O substitution (regarding an ether, R′O-R, as a hydrocarbon with an alkoxy substituent) upon oxidation. It has been suggested that sensitivity of oxidation reactions to the structure of the combustible substance may have, in part at least, a kinetic explanation (Hinshelwood, C.N.; Disc. Faraday Soc., April 1951). This hypothesis is reviewed in the thesis, and in its essentials it may be stated an follows. Low-temperature, slow oxidation takes place by a mechanism depending on the formation of a peroxide which slowly decomposes into radicals, thereby setting up branching chains. The peroxide forms by a aeries of reactions of the following type: RH + O2 → R− + HO2−, R− + O2 → RO2−, and RO2− + RH → ROOR″ (where R″ may be H). Decomposition of the peroxide molecule leads to RO− and R″O− which in general break down, for example, by the reaction R1CH2O− → R1− + HCHO. The smaller radical reacts further and my either continue the chain or terminate it. Solution of the rate equations for the individual reaction steps shows that the rate of oxidation depends on the rate of initial attack of oxygen on the organic substance and also on the relative ease of splitting of the peroxide bond. Both factors may be influenced by the structure of the organic reactant, but because of the peculiar kinetics of the branching chain reaction, the oxidation rate gives a magnified and sensitive measure of the influence of the structure of R on the -O-O- linkage, the rupture of which is responsible for branching. The results of previous investigations indicate that the peroxide bond is weakened by substituents in R which are electron attractive (i.e. exert an inductive effect represented by −I) and is strengthened by the introduction of methyl groups which have a +I effect. This is in agreement with the view (Walsh, A.D., J. Chem. Soc.: 1948, 398) that bonds between strongly electronegative elements should be strengthened by attachment of electron repelling groups since such groups facilitate the expansion of atomic orbitals and allow increased over-lap without the occurrence of nuclear repulsion. The second factor on which the rate of oxidation depends is the rate of the chain-initiation process. Before taking part in the initiation reaction the attacking oxygen must be changed, more or less completely, into the biradical form. In this change electrons must be drawn from the O=O bond, and the very initial stages of the redistribution may well be helped by the action of a positive centre at the seat of reaction which would tend to draw electrons towards it. If this were so −I substituents would favour attack by creating the necessary positive centre, while +I substituents, such as the methyl group, would weaken the attack. Thus it appears that if the influence of substituents can be assessed in terms of the ease of approach of the attacking agent and the initiation of the requisite re-organisation, the effects of substitution on both the chain-branching and chain-initiating steps are in the same direction and tend to reinforce one another. Apart from the influence of structure on the rate of oxidation, the most striking kinetic characteristics which this scheme summarises are: the autocatalytic nature of the reaction; the increase of the reaction rate with concentration of organic reactant; the independence, over a considerable range, of initial oxygen pressure and the reaction rate. In fact, an elaboration of the theory (Bardwell, J.; Thesis, Oxford, 1950) predicts that oxygen in excess will have an inhibitory affect on the rate. If the oxidation of ethers takes place according to a mechanism basically similar to that which has just been described, these kinetic features should be in evidence. The object of the present investigation is to determine the extent to which the reactions of ethers are comparable in mechanism to those of other compounds previously studied, and where the mechanisms are comparable, what is the influence of the alkoxy group. The experimental methods employed were similar to those used in previous work on the oxidation of paraffins, amines and ketones in this laboratory. Ether vapour and oxygen were admitted separately to a closed pyrex reaction vessel at constant temperature and the reaction wan followed by the change in pressure and by chemical analysis. In the kinetics of their slow oxidation dimethyl, methyl ethyl, methyl normal-propyl, diethyl, and di-normal-propyl ethers are generally similar to the other organic compounds which have been studied. During the combustion of ethers the pressure decreases at first and then increases, the rate passing through a maximum before the end of the reaction. The maximum rate of oxidation increases with the initial pressure of ether but is reduced by oxygen in excess. The length of time between the admission of the reactants to the reaction vessel and the establishment of the maximum rate (the induction period) is shortened by an increase in initial ether pressure but is lengthened by an increase in the initial pressure of oxygen in the range studied. In the low temperature region an increase in temperature favours oxidation. The maximum in the concentration of peroxides of the ROOR″ type during the slow reaction coincides with the maximum rate of pressure increase. The slow oxidation of ethers is insensitive to additions of inert gas, but the nature of the surface of the reaction vessel may influence the pressure change during the reaction, without greatly affecting the formation of peroxides. With standardization of experimental procedure reasonable reproducibility can be achieved. Previous results on the maximum rate of oxidation of normal-pentane and the corresponding induction period having been generally confirmed in the present apparatus, the relative rates of oxidation of the ethers and pentane are as follows: Compound Relative Rate Pentane 1 Dimethyl ether 25 1 Methyl n-propyl ether 175 7 1 Methyl ethyl ether 175 7 1.1 1 Di-n-propyl ether 1300 50 7 7 1 Diethyl ether 2500 100 14 14 2 The rate of oxidation of di-iso-propyl ether is much lower than that of diethyl ether at 170°C.; in fact, it was not possible to make kinetic measurements with di-iso-propyl ether because no appreciable reaction occurs over a wide range of temperatures and pressures below those at which cool flames are observed. The similarity of the formal kinetics of the low-temperature oxidation of ethers to those of the unsubstituted paraffins, chloro-compounds, amines and ketons indicates that the oxidation of ethers occurs by the same basic mechanism proposed for the hydrocarbons. If the relative ease of oxidation of the ethers, compared to pentane, can be accounted for by the inductive effect of substituents according to the hypothesis outlined above, then the R′O group is electron attractive and exerts a −I affect. The inductive action of the alkoxy group deduced from oxidation measurements agreed with the −I effect of CH3O determined by independent means. Furthermore, a comparison of the relative rates of oxidation of the ethers themselves shows that the −I effect of the ethoxy and propoxy group is greater than that of the methoxy. The stabilising influence of methyl groups, and their inherent resistance to oxidation, so prominent in the behaviour of paraffins, are confirmed by the stability of dimethyl and di-iso-propyl ethers when compared with other ethers. Ethers do not show the progressive and marked increase in the rate of oxidation with increasing length of the normal carbon chain which has been observed with the other organic compounds previously studied. It appears that the inductive action of the alkoxy group has to some extent saturated the response of the molecule, and the effect of the introduction of -O- is next reinforced by lengthening of the carbon chain. Early in the investigation it was found that relatively large quantities of peroxides were formed during the oxidation of ethers. According to current theories, compounds of this type play an important part, not only in the low-temperature, slow oxidation, but also in the cool flame reaction and it was thought that the oxidation reactions of ethers might be eminently suitable for a study of the cool flame mechanism. The most satisfactory hypothesis of cool flame formation postulates that peroxides play a dual role; on the one hand, they may dissociate and lead to the branching chain reaction characteristic of the slow oxidation, or on the other hand, above a critical temperature they may largely decompose by an alternate route which does not lead to branching but gives rise to the cool flame. This mechanism gives a satisfactory interpretation of the results on cool flames in butanone-oxygen mixtures (Bardwell, J, and Hinshelwood, C.N.; Proc. Roy. Soc. 1950, A201, 26.). Although the slow oxidation of ethers resembles that of other compounds, the cool flane reaction is quite different. In contrast to the precipitous decline in the concentration of peroxides of the ROOR″ type previously observed after the cool flame in butanone-oxygen mixtures, the concentration of peroxides derived from ethers is practically unaffected by the passage of the cool flame. In fact, peroxides derived from diethyl ether exist at temperatures and pressures considerably higher than those at which cool flames appear. Evidently peroxides or this type do not play the prominent port in the cool flame reaction of ethers that they seem to play in the butanone cool flame. A test of cool flame mechanisms with the ethers must await further information.