Methane

Methane (pronounced METH-ayn in the US, MEE-thayn in the UK) is a chemical compound with the formula CH₄—one carbon atom bonded to four hydrogen atoms. It belongs to the group-14 hydrides, is the simplest alkane, and makes up the bulk of natural gas. Because methane is so plentiful on Earth, it is an appealing fuel economically, but storing and capturing it is tricky since it remains a gas under standard temperature and pressure. In our atmosphere, methane is transparent to visible light but traps infrared radiation, functioning as a greenhouse gas. It is an organic hydrocarbon and one of the simplest organic compounds.

Methane occurs naturally both underground and beneath the seafloor, formed through geological and biological processes. The largest methane reservoir lies under the seafloor as methane clathrates. When it reaches the surface and enters the air, it is called atmospheric methane. Methane has also been found on other planets, such as Mars, which is significant for astrobiology research.

Structurally, methane is a tetrahedral molecule with four identical C–H bonds. Its electronic structure involves two bonding molecular orbitals from the overlap of carbon and hydrogen valence orbitals. The lowest-energy orbital comes from carbon’s 2s orbital overlapping with the in-phase combination of the four hydrogen 1s orbitals; above that is a triply degenerate orbital overlapping with various linear combinations of hydrogen 1s orbitals. This “three-over-one” bonding matches photoelectron spectroscopic measurements.

At standard temperature and pressure, methane is odorless, colorless, and transparent. It does absorb visible light, especially at the red end, due to overtone bands, but this is only noticeable over very long light paths—which gives Uranus and Neptune their blue or bluish-green colors as light passes through methane-rich atmospheres and scatters back. The familiar smell of household natural gas comes from added odorants, typically blends containing tert-butylthiol, for safety. Methane boils at −161.5 °C under one atmosphere of pressure. As a gas, it is flammable in air at concentrations between 5.4% and 17% at standard pressure.

Solid methane exists in several forms, nine of which are known. Cooling methane at normal pressure produces methane I, which crystallizes in a cubic system (space group Fm3m). In this form, hydrogen atoms are not fixed in place—methane molecules can rotate freely, making it a plastic crystal.

The main chemical reactions of methane are combustion, steam reforming to syngas, and halogenation. These reactions are generally hard to control. Like other hydrocarbons, methane is an extremely weak acid, with a pKa in DMSO estimated at 56. It cannot be deprotonated in solution, but its conjugate base exists in forms like methyllithium. Several positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures, including the methyl cation (CH₃⁺), methane cation (CH₄⁺), and protonated methane (CH₅⁺). Some have been detected in outer space. Methanium can also be produced in dilute solutions from methane with superacids. Higher-charge cations like CH₆²⁺ and CH₇³⁺ have been studied theoretically and are thought to be stable. Despite the strength of its C–H bonds, there is strong interest in catalysts that activate these bonds in methane and other lower alkanes.

Methane’s heat of combustion is 55.5 MJ/kg. Combustion follows a multiple-step reaction: CH₄ + 2 O₂ → CO₂ + 2 H₂O (ΔcH = −891 kJ/mol under standard conditions; −802 kJ/mol if water vapor forms). Peters’ four-step chemistry is a simplified model for methane burning. Partial oxidation to methanol is carried out by certain enzymes, like methane monooxygenase. Some bacteria oxidize methane using nitrite as the oxidant in the absence of oxygen—a process called anaerobic oxidation of methane. Industrial-scale oxidation has never been commercially successful, even with limited oxygen. Homogeneous and heterogeneous catalytic systems have been demonstrated, but none are economical; they typically generate protected products to avoid overoxidation, such as the Catalytica system, copper zeolites, and iron zeolites stabilizing alpha-oxygen active sites.

Under the right conditions, methane reacts with halogen radicals: •X + CH₄ → HX + •CH₃, then •CH₃ + X₂ → CH₃X + •X, where X is fluorine, chlorine, bromine, or iodine. This free radical halogenation is triggered by UV light or radical initiators like peroxides, producing a halogen atom. A two-step chain reaction follows: the halogen atom pulls a hydrogen from methane, forming a hydrogen halide and a methyl radical (•CH₃), which then reacts with a halogen molecule to create a halomethane and a new halogen atom. Similar reactions can replace additional hydrogen atoms, yielding dihalomethane, trihalomethane, and tetrahalomethane depending on conditions.