Hydroxylation theory

The early theory of methane oxidation assumed that carbon and water ware tile initial products of reaction or that hydrogen burned preferentially to carbon. However, in 1861 Kersten0 declared that carbon monoxide and hydrogen were the primary products, and that although some free carbon may form at times, the carbon is normally oxidized to carbon monoxide before the hydrogeu is reacted upon. This idea, later revived by Misterli,7 involves the preferential combustion of carbon and is thus directly opposed to the hydroxylation theory. Tins theory might possibly apply to the case of acetylene combustion, since this hydrocarbou is sufficiently unstable as to explode alone under certain conditions, but cannot hold for the more saturated hydrocarbons which do not explode alone. 

In trying to confirm their theory in regard to the intermediate formation of oxygenated products during combustion, Bone 9 and his co-workers carried out extensive researches and from their work has come the present hydroxylation theory. According to Bone the oxidation of methane takes place in steps, methanol, formaldehyde, formic acid, and carbonic acid being formed in the order named. These various steps are indicated below. The double arrows point out the mam course of reaction, while the single arrows show how the intermediate compounds may decompose. 

The hydroxylation theory has been criticized also by Callendar 16 on the basis of the necessity for splitting of the oxygen molecule, a step not likely to occur readily at the temperatures at which the slow oxidations are conducted. The lack of experimental evidence to support any mechanism involving the ionization of oxygen prior to or at the time of oxidation of a hydrocarbon is an additional factor in opposition to the idea that an alcohol is the primary oxidation product. At explosion temperatures, however, atomic oxygen may be present and effective as such. Actually most of the experimental work on the direct oxidation of methane with elemental oxygen lias shown that water and formaldehyde are among the first reaction products, whereas methanol is not, and several processes 17 claim this reaction to fonn formaldehyde industrially. 

By exploding mixtures of ethane and oxygen in borosilicate bulbs, carbon monoxide, hydrogen, methane, acetylene, and ethylene have been obtained.10 140 As the initial pressure is decreased the amount of unsaturated hydrocarbons and water in the products showed a tendency to increase. The fact that no carbon is produced in these experiments and that water and ethylene are formed lends support to Bone s hydroxylation theory since it is probable that the alcohol formed in the initial step is dehydrated immediately to yield unsaturated hydrocarbon and water. The presence of hydrogen and aldehyde, especially at lower initial pressures, is also indicative of alcohol dissociation. The failure of any ethanol to appear in the product does not preclude its formation and immediate decomposition. It is hardly to be expected that ethanol if formed would exist long enough to pass out of the reaction zone and appear in the product since it is known that at the temperature of the oxidation process ethanol is entirely unstable. 

Wheeler and Blair 14 also formulate a process based on the hydroxylation theory for the slow oxidation of hexane after having noted the formation of large quantities of aldehydes. Callendar 18 has also obtained formaldehyde and acetaldehyde as well as higher aldehydes in the oxidation of hexane. 

The hydroxylation theory of Bone2 and his co-workers has had wide acceptance as far as the oxidation of aliphatic hydrocarbons is concerned. The mechanism postulated involves the successive formation of hydroxyl compounds, which may add oxygen to form additional hydroxyl groups or which may lose water and decompose. In this way methane would first form methanol, then methylene glycol which would be decomposed to formaldehyde and water formaldehyde would be oxidized to formic acid or decomposed to carbon monoxide and hydrogen. The theory, however, is open to a number of criticisms. 

Although considerable controversy lias existed and still exists regarding the exact manner in which a hydrocarbon oxidizes, one fact stands out from the great mass of data that have accumulated and that is that aldehydes appear early and are prominent in the oxidation products. It has been recognized, however, that aldehydes are not the primary products of the encounter between oxygen and hydrocarbon molecules, and it has been proposed that the most probable initial product is peroxidic in type. While the protagonists of the hydroxylation theory concede the attractiveness of certain features of the peroxide theory, they consider that it does not contradict the evidence for the hydroxylation theory.9... 

Mardles 15 passed hexane-air mixtures through hot tubes and reported the presence of active oxygen in the products. He supports the peroxide theory of combustion for this reason and because it offers a better explanation of engine knocking than does the hydroxylation theory. 1-Ic has also proposed that the oxidation occurs in two steps, the first of which is peroxide formation followed by aldehyde formation through decomposition of the active molecule. 

The presence of alcohols lias not been reported in the oxidation of these hydrocarbons, and although this fact does not preclude the intermediate formation of an alcohol as the first step in the oxidation, it does indicate that the mechanism of side chain oxidation is similar to that of aliphatic hydrocarbon oxidation since in this latter case even the proponents of the hydroxylation theory have had difficulty in isolating alcohols. 

The hydroxylation theory seems to have been first used in explaining the combustion of hydrocarbons by van t Hoff. He remarks on the difficulty of oxidation of methane to methyl alcohol as compared with the oxidation of the latter, and gives the following scheme in each stage an oxygen atom combines on one side with carbon and on the other with hydrogen ... 

Armstrong at first inclined to the view that hydrogen burns preferentially to carbon, whilst Stokes thought it more likely that carbon burns preferentially to hydrogen. Armstrong does not seem to have advanced the hydroxylation theory to explain the combustion of hydrocarbons until 1902. ... 

Bone at first thought that carbon monoxide was the primary product of the oxidation of hydrocarbons, but when he obtained formaldehyde and (under pressure) even methyl alcohol by the slow oxidation of methane, he adopted the hydroxylation theory. ... 

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