Chemical Properties of Methane

Methane is a saturated hydrocarbon, incapable of additive reactions. It undergoes metathesis, however, with chlorine, except when perfectly dry, yielding successively methyl chloride (CH₃Cl), methylene chloride (CH₂Cl₂), chloroform (CHCl₃), and carbon tetrachloride (CCl₄). Fluorine acts upon methane more, and bromine less vigorously, than chlorine, but iodine has no action upon this hydrocarbon.

Methane is very stable towards heat, but various and conflicting views have been held as to the manner of decomposition of this and other simple hydrocarbons. It has been shown, however, by Bone and Coward that methane is decomposed by heat into its elements without the production of ethylene, acetylene, etc.; that decomposition is inappreciable at 700° C., and that the rate of decomposition is about sixty times greater at 985° C. than at 785° C. Moreover the decomposition took place at the surface of the hot tube through which the gas was passed, and the carbon deposited was peculiarly hard and lustrous.

Methane is decomposed into its elements by electric sparks or by the electric arc. According to Berthelot, however, acetylene and other hydrocarbons of high molecular weight are formed.

Methane burns in air or oxygen, forming water and carbon dioxide. According to Phillips the lowest temperature at which the combustion of methane and oxygen can take place, that is when the mixed gases are passed over pallatised asbestos, is 404°-414° C.; but Den ham finds this temperature to lie between 514° C. and 546° C. The mechanism of the combustion of methane and other hydrocarbons has been studied by Bone and Wheeler. These observers found that the slow reaction of methane with oxygen at temperatures between 300° C. and 400° C. resulted in the simultaneous oxidation of carbon and hydrogen, thus: 2CH₄ + 3O₂ = 2CO + 4H₂O; but that formaldehyde appeared as an intermediate and carbon dioxide as a final product. The following stages of oxidation are consequently recognised:

(i) CH₄ + O₂ = O:CH₂ + H₂O;
(ii a) (O:CH₂) + O₂ = CO₂ + H₂O,
(ii b) 2(O:CH₂) + O₂ = 2CO + 2H₂O.

Reaction (ii b) may involve the intermediate production of formic acid, thus:

2O:CH₂ + O₂ = 2O:H-C-OH = 2CO + 2H₂O.

Armstrong has put forward the view that the slow oxidation of a hydrocarbon such as methane is a series of hydroxylations in which water plays a part and hydrogen peroxide is produced, thus:

CH₄ + O₂ + H₂O = CH₃OH + H₂O₂
CH₃OH + O₂ + H₂O = CH₂(OH)₂ + H₂O₂;
CH₂(OH)₂ = CH₂:O + H₂O;
CH₂(OH)₂ + O₂ + H₂O = CH(OH)₃ + H₂O₂;
CH(OH)₃ = CH:OOH + H₂O;
CH(OH)₃ + O₂ + H₂O = C(OH)₄ + H₂O₂;
C(OH)₄ = CO₂ + 2H₂O;

Thus gaseous oxidation becomes analogous to oxidation in the liquid state. Bone and Andrew have shown, however, that the presence of water is not necessary to the combustion of a hydrocarbon, which may be supposed to be directly hydroxylated by oxygen.

When a mixture of methane and oxygen is exploded at constant volume and an initial pressure of from 10 to 50 atmospheres, the following is the course of the reaction:

CH₄ + O₂ → H₂C(OH)₂ → H₂C:O + H₂O → H₂ + CO + H₂O.

The result of the explosion of the mixture (CH₄ + O₂ + 2H₂) shows that the affinity of methane for oxygen in explosions is from twenty to thirty times that of hydrogen.

Methane forms an explosive mixture with air or oxygen. The intensity of the explosion increases from zero to a maximum, and diminishes again to zero within certain limits. The rates of propagation of the explosion have been studied by Mallard and Le Chatelier, Emich, and Parker; and Teclu has found that the limits of explosibility of a mixture of methane and air are 3.20-3.67 and 7.46-7.88 per cent, of methane. Below the lower limit the mixture does not burn; above the higher limit it does not explode. Burrell and Robertson, however, find that the lowest limit for the complete propagation of flame in a mixture of methane and air is between 3.75 and 4 per cent, of methane, and that the lowest ignition temperature for mixtures of methane and air is 500° C.