Methane

{{#if:OdorlessOdorlessColorless gas655.6 μg mL⁻¹22.7 mg L⁻¹1.0914 nmol Pa⁻¹ kg⁻¹90.741|! style="background: #F8EABA; text-align: center;" colspan="2" | Properties

Methane
Stereo, skeletal formula of methane with some measurements added
Ball and stick model of methane	Spacefill model of methane
IUPAC name Methane (substitutive) Tetrahydridocarbon (additive)
Identifiers
CAS number	74-82-8 7pxY
PubChem	297
ChemSpider	291 7pxY
EC number	200-812-7
UN number	1971
KEGG	C01438 7pxN
MeSH	Methane
ChEBI	CHEBI:16183 7pxY
ChEMBL	CHEMBL17564 7pxY
RTECS number	PA1490000
Beilstein Reference	1718732
Gmelin Reference	59
3DMet	B01450
Jmol-3D images	Image 1
SMILES C <p><p style="word-wrap: break-word;">
InChI InChI=1S/CH4/h1H4 7pxY Key: VNWKTOKETHGBQD-UHFFFAOYSA-N 7pxY
Molecular formula	CH4
Molar mass	16.04 g mol−1
Structure
Molecular shape	Tetrahedron
Dipole moment	0 D
Thermochemistry
Std enthalpy of formation ΔfHo298	−74.87 kJ mol−1
Std enthalpy of combustion ΔcHo298	−891.1–−890.3 kJ mol−1
Standard molar entropy So298	186.25 J K−1 mol−1
Specific heat capacity, C	35.69 J K−1 mol−1
Hazards
GHS pictograms	The flame pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)
GHS signal word	DANGER
GHS hazard statements	H220
GHS precautionary statements	P210
EU Index	601-001-00-4
EU classification	Flammable F+
R-phrases	R12
S-phrases	(S2), S16, S33
NFPA 704	75px 4 1 0
Flash point	−188 °C
Autoignition temperature	537 °C
Explosive limits	5–15%
Related compounds
Related alkanes	Methyl iodide Diiodomethane Iodoform Carbon tetraiodide Ethane Ethyl iodide
Related compounds	Guanidine Pimagedine
14pxN (verify) (what is: 10pxY/10pxN?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Methane (pronounced /ˈmɛθeɪn/ or /ˈmiːθeɪn/) is a chemical compound with the chemical formula CH₄. It is the simplest alkane, the main component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel. However, because it is a gas at normal conditions, methane is difficult to transport from its source.

Methane is a relatively potent greenhouse gas. The concentration of methane in the Earth's atmosphere in 1998, expressed as a mole fraction, was 1745 nmol/mol (parts per billion, ppb), up from 700 nmol/mol in 1750. By 2008, however, global methane levels, which had stayed mostly flat since 1998, had risen to 1800 nmol/mol.

Properties and bonding

Methane is a tetrahedral molecule with four equivalent C-H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) resulting from the overlap of the valence orbitals on C and H. The lowest energy MO is the result of the overlap of the 2s orbital on carbon with the in-phase combination of the 1s orbitals on the four hydrogen atoms. Above this level in energy is a triply degenerate set of MOs that involve overlap if the 2p orbitals on carbon with various linear combinations of the 1s orbitals on hydrogen. The resulting "three-over-one" bonding scheme is consistent with photoelectron spectroscopic measurements.

At room temperature and standard pressure, methane is a colorless, odorless gas. The familiar smell of natural gas as used in homes is a safety measure achieved by the addition of an odorant, often methanethiol or ethanethiol. Methane has a boiling point of −161 °C (−257.8 °F) at a pressure of one atmosphere. As a gas it is flammable only over a narrow range of concentrations (5–15%) in air. Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres).

Chemical reactions

Main reactions with methane are: combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control. Partial oxidation to methanol, for example, is challenging because the reaction typically progresses all the way to carbon dioxide and water even with incomplete amounts of oxygen. The enzymes methane monooxygenase can produce methanol from methane, but they cannot be used for industrial scale reactions.

Acid-base reactions

Like other hydrocarbons, methane is a very weak acid. Its pKa in DMSO is estimated to be 56. It cannot be deprotonated in solution, but the conjugate base with methyllithium is known. Protonation of methane can be achieved with super acids to give CH₅⁺, sometimes called the methanium ion. 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).

Combustion

In the combustion of methane, several steps are involved. An early intermediate is formaldehyde (HCHO or H₂CO). Oxidation of formaldehyde gives the formyl radical (HCO), which then give carbon monoxide (CO):

CH₄ + O₂ → CO + H₂ + H₂O

The resulting H₂ oxidizes to H₂O, releasing heat. This reaction occurs very quickly, usually in significantly less than a millisecond.

2 H₂ + O₂ → 2 H₂O

Finally, the CO oxidizes, forming CO₂ and releasing more heat. This process is generally slower than the other chemical steps, and typically requires a few to several milliseconds to occur.

2 CO + O₂ → 2 CO₂

The result of the above is the following total equation:

CH₄ + 2 O₂ → CO₂ + 2 H₂O (ΔH = −891 kJ/mol (at standard conditions))

Reactions with halogens

Methane reacts with halogens given appropriate conditions as follows:

CH₄ + X₂ → CH₃X + HX

where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation, beginning with the attachment of Cl· radicals on methane to produce CH₃·, which combines with a second Cl· to give methyl chloride (CH₃Cl). Similar reactions will produce dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and, ultimately, carbon tetrachloride (CCl₄). The energy required to start this reaction comes from UV radiation or heating.

Uses

Methane is used in industrial chemical processes and may be transported as a refrigerated liquid (liquefied natural gas, or LNG). While leaks from a refrigerated liquid container are initially heavier than air due to the increased density of the cold gas, the gas at ambient temperature is lighter than air. Gas pipelines distribute large amounts of natural gas, of which methane is the principal component.

Fuel

Main article: natural gas

Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than any other hydrocarbon but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol, of which 12.0 g/mol is carbon) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons. In many cities, methane is piped into homes for domestic heating and cooking purposes. In this context it is usually known as natural gas, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot.

Methane in the form of compressed natural gas is used as a vehicle fuel and is claimed to be more environmentally friendly than other fossil fuels such as gasoline/petrol and diesel.

Research into adsorption methods of methane storage for this purpose has been conducted.

Developing technologies

Research is being conducted by NASA on methane's potential as a rocket fuel.

Methane emitted from coal mines has been converted to electricity. It is used for fuel.

Chemical feedstock

Although there is great interest in converting methane into useful or more easily liquified compounds, the only practical processes are relatively unselective. In the chemical industry, methane is converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. This endergonic process (requiring energy) utilizes nickel catalysts and requires high temperatures, around 700–1100 °C:

CH₄ + H₂O → CO + 3 H₂

Related chemistries are exploited in the Haber-Bosch Synthesis of ammonia from air, which is reduced with natural gas to a mixture of carbon dioxide, water, and ammonia.

Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.

Production

Biological routes

Main article: methanogenesis

Naturally occurring methane is mainly produced by the process of methanogenesis. This multistep process is used by microorganisms as an energy source. The net reaction is:

CO₂ + 8 H⁺ + 8 e^- → CH₄ + 2 H₂O

The final step in the process is catalysed by the enzyme methyl-coenzyme M reductase. Methanogenesis is a form of anaerobic respiration used by organisms that occupy landfill, ruminants (e.g., cattle), and the guts of termites.

It is uncertain if plants are a source of methane emissions.

Industrial routes

Methane can be produced by hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer-Tropsch process. This technology is practiced on a large scale to produce longer chain molecules than methane.

Natural gas is so abundant that the intentional production of methane is relatively rare. The only large scale facility of this kind is the Great Plains Synfuels plant, started in 1984 in Beulah, North Dakota as a way to develop abundant local resources of low grade lignite, a resource which is otherwise very hard to transport for its weight, hash content, low calorific value and propensity to spontaneous combustion during storage and transport.

Occurrence

Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore. It is the major component of natural gas, about 87% by volume. The major source of methane is extraction from geological deposits known as natural gas fields, with coal seam gas extraction becoming a major source (see Coal bed methane extraction, a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from an non-mineable coal seams). It is associated with other hydrocarbon fuels, and sometimes accompanied by helium and nitrogen. The gas at shallow levels (low pressure) forms by anaerobic decay of organic matter and reworked methane from deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those that contain oil generate natural gas.

It is generally transported in bulk by pipeline in its natural gas form, or LNG carriers in its liquefied form; few countries transport it by truck.

Alternative sources

Apart from gas fields, an alternative method of obtaining methane is via biogas generated by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste (including landfills), or any other biodegradable feedstock, under anaerobic conditions. Rice fields also generate large amounts of methane during plant growth. Methane hydrates/clathrates (ice-like combinations of methane and water on the sea floor, found in vast quantities) are a potential future source of methane. Cattle belch methane accounts for 16% of the world's annual methane emissions to the atmosphere. One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. Early research has found a number of medical treatments and dietary adjustments that help slightly limit the production of methane in ruminants. A more recent study, in 2009, found that at a conservative estimate, at least 51% of global greenhouse gas emissions were attributable to the life cycle and supply chain of livestock products, meaning all meat, dairy, and by-products, and their transportation.

Paleoclimatology research published in Current Biology suggests that flatulence from dinosaurs may have warmed the Earth.

Atmospheric methane

Main article: Atmospheric methane

File:AIRS Methane.png

2011 methane concentration in the upper troposphere

Methane is created near the Earth's surface, primarily by microorganisms by the process of methanogenesis. It is carried into the stratosphere by rising air in the tropics. Uncontrolled build-up of methane in the atmosphere is naturally checked — although human influence can upset this natural regulation — by methane's reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor. It has a net lifetime of about 10 years, and is primarily removed by conversion to carbon dioxide and water

Methane also affects the degradation of the ozone layer.

In addition, there is a large (but unknown) amount of methane in methane clathrates in the ocean floors as well as the Earth's crust. Most methane is the result of biological process called methanogenesis.

In 2010, methane levels in the Arctic were measured at 1850 nmol/mol, a level over twice as high as at any time in the previous 400,000 years. Historically, methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known as ice ages, and between 600 to 700 nmol/mol during the warm interglacial periods. It has a high global warming potential: 72 times that of carbon dioxide over 20 years, and 25 times over 100 years, and the levels are rising.

Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 compared to CO₂ over a 100-year period (although accepted figures probably represents an underestimate). This means that a methane emission will have 25 times the effect on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72. The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases (these gases don't include water vapor which is by far the largest component of the greenhouse effect). Usually, excess methane from landfills and other natural producers of methane is burned so CO₂ is released into the atmosphere instead of methane, because methane is a more effective greenhouse gas. Recently, methane emitted from coal mines has been successfully utilized to generate electricity.

Clathrates

Arctic methane release from permafrost and methane clathrates is an expected consequence and further cause of global warming.

Safety

Methane is not toxic; however, it is extremely flammable and may form explosive mixtures with air. Methane is violently reactive with oxidizers, halogens, and some halogen-containing compounds. Methane is also an asphyxiant and may displace oxygen in an enclosed space. Asphyxia may result if the oxygen concentration is reduced to below about 16% by displacement, as most people can tolerate a reduction from 21% to 16% without ill effects. The concentration of methane at which asphyxiation risk becomes significant is much higher than the 5–15% concentration in a flammable or explosive mixture. Methane off-gas can penetrate the interiors of buildings near landfills and expose occupants to significant levels of methane. Some buildings have specially engineered recovery systems below their basements to actively capture this gas and vent it away from the building.

See also

• 2007 Zasyadko mine disaster
• Abiogenic petroleum origin
• Aerobic methane production
• Anaerobic digestion
• Anaerobic respiration
• Arctic methane release
• Biogas
• Coal Oil Point seep field
• Energy density
• Greenhouse gas
• Halomethane, halogenated methane derivatives
• List of alkanes
• Methanation
• Methane clathrate, form of water ice that contains methane
• Methanogen, archaea that produce methane as a metabolic by-product
• Methanogenesis, the formation of methane by microbes
• Methanotroph, bacteria that are able to grow using methane as their only source of carbon and energy
• Methyl group, a functional group similar to methane
• Organic gas
• Thomas Gold

Appendix: extraterrestrial methane

Methane has been detected or is believed to exist in several locations of the solar system. In most cases, it is believed to have been created by abiotic processes. Possible exceptions are Mars and Titan.

• Moon – traces are outgassed from the surface
• Mars – the atmosphere contains 10 nmol/mol methane. In January 2009, NASA scientists announced that they had discovered that the planet often vents methane into the atmosphere in specific areas, leading some to speculate this may be a sign of biological activity going on below the surface. In June, 2012, scientists reported that measuring the ratio of hydrogen and methane levels on Mars may help determine the likelihood of life on Mars. According to the scientists, "...low H₂/CH₄ ratios (less than approximately 40) indicate that life is likely present and active." Other scientists have recently reported methods of detecting hydrogen and methane in extraterrestrial atmospheres.
• Jupiter – the atmosphere contains about 0.3% methane
• Saturn – the atmosphere contains about 0.4% methane
  • Iapetus
  • Titan — the atmosphere contains 1.6% methane and thousands of methane lakes have been detected on the surface In the upper atmosphere the methane is converted into more complex molecules including acetylene, a process that also produces molecular hydrogen. There is evidence that acetylene and hydrogen are recycled into methane near the surface. This suggests the presence either of an exotic catalyst, or an unfamiliar form of methanogenic life.
  • Enceladus – the atmosphere contains 1.7% methane
• Uranus – the atmosphere contains 2.3% methane
  • Ariel – methane is believed to be a constituent of Ariel's surface ice
  • Miranda
  • Oberon – about 20% of Oberon's surface ice is composed of methane-related carbon/nitrogen compounds
  • Titania – about 20% of Titania's surface ice is composed of methane-related organic compounds
  • Umbriel – methane is a constituent of Umbriel's surface ice
• Neptune – the atmosphere contains 1.6% methane
  • Triton – Triton has a tenuous nitrogen atmosphere with small amounts of methane near the surface.
• Pluto – spectroscopic analysis of Pluto's surface reveals it to contain traces of methane
  • Charon – methane is believed present on Charon, but it is not completely confirmed
• Eris – infrared light from the object revealed the presence of methane ice
• Comet Halley
• Comet Hyakutake – terrestrial observations found ethane and methane in the comet
• Extrasolar planet HD 189733b – This is the first detection of an organic compound on a planet outside the solar system. Its origin is unknown, since the planet's high temperature (700 °C) would normally favor the formation of carbon monoxide instead.
• Interstellar clouds

Further reading

• The Climate Threat We Can Beat, in May/June 2012 Foreign Affairs with David G. Victor, Charles F. Kennel, Veerabhadran Ramanathan (paid website only while current issue)

External links

• Gavin Schmidt, Methane: A Scientific Journey from Obscurity to Climate Super-Stardom, NASA Goddard, September 2004
• Methane thermodynamics
• International Chemical Safety Card 0291
• Methane Hydrates
• Safety data for methane
• Methane-eating bug holds promise for cutting greenhouse gas, Media Release, GNS Science, New Zealand
• Catalytic conversion of methane to more useful chemicals and fuels
• Methane as a Savior of the Dairy Industry

af:Metaan