A variety of positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include methenium or methyl cation CH+ 3, methane cation CH+ 4, and methanium or protonated methane CH+ 5. Some of these have been detected in outer space. Methanium can also be produced as diluted solutions from methane with super acids. Cations with higher charge, such as CH2+ 6 and CH3+ 7, have been studied theoretically and conjectured to be stable.[11] Despite the strength of its C-H bonds, there is intense interest in catalysts that facilitate C–H bond activation in methane (and other low alkanes).[12] Combustion[edit] Methanes heat of combustion is 55.5 MJ/kg.[13] Combustion of methane is a multiple step reaction. The following equations are part of the process, with the net result being: CH4 + 2 O2 → CO2 + 2 H2O (ΔH = −891 kJ/mol (at standard conditions)) CH4+ M* → CH3 + H + M CH4 + O2 → CH3 + HO2 CH4 + HO2 → CH3 + 2 OH CH4 + OH → CH3 + H2O O2 + H → O + OH CH4 + O → CH3 + OH CH3 + O2 → CH2O + OH CH2O + O → CHO + OH CH2O + OH → CHO + H2O CH2O + H → CHO + H2 CHO + O → CO + OH CHO + OH → CO + H2O CHO + H → CO + H2 H2 + O → H + OH H2 + OH → H + H2O CO + OH → CO2 + H H + OH + M → H2O + M* H + H + M → H2 + M* H + O2 + M → HO2 + M* The species M* signifies an energetic third body, from which energy is transferred during a molecular collision. Formaldehyde (HCHO or H 2CO) is an early intermediate (reaction 7). Oxidation of formaldehyde gives the formyl radical (HCO; reactions 8–10), which then give carbon monoxide (CO) (reactions 11, 12 & 13). Any resulting H2 oxidizes to H2O or other intermediates (reaction 14, 15). Finally, the CO oxidizes, forming CO2 (reaction 16). In the final stages (reactions 17–19), energy is transferred back to other third bodies. The overall speed of reaction is a function of the concentration of the various entities during the combustion process. The higher the temperature, the greater the concentration of radical species and the more rapid the combustion process.[14] Reactions with halogens[edit] Methane reacts with halogens given appropriate conditions as follows: X2 + UV → 2 X• X• + CH4 → HX + CH3• CH3• + X2 → CH3X + X• where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation. It is initiated with UV light or some other radical initiator. A chlorine atom is generated from elemental chlorine, which abstracts a hydrogen atom from methane, resulting in the formation of hydrogen chloride. The resulting methyl radical, CH3•, can combine with another chlorine molecule to give methyl chloride (CH3Cl) and a chlorine atom. This chlorine atom can then react with another methane (or methyl chloride) molecule, repeating the chlorination cycle.[15] Similar reactions can produce dichloromethane (CH2Cl2), chloroform (CHCl3), and, ultimately, carbon tetrachloride (CCl4), depending upon reaction conditions and the chlorine to methane ratio.
Posted on: Tue, 05 Aug 2014 04:12:46 +0000
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