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Higher alkane
Alkanes having nine or more carbon atoms From Wikipedia, the free encyclopedia
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Higher alkanes are alkanes with a high number of carbon atoms. It is common jargon.[1] One definition says higher alkanes are alkanes having nine or more carbon atoms. Thus, according to this definition, nonane is the lightest higher alkane.[2] As pure substances, higher alkanes are rarely significant, but they are major components of useful lubricants and fuels.[3]
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Synthesis
The preparation of specific long-chain hydrocarbons typically involves manipulations of long chain precursors or the coupling of two medium-chain components. For the first case, fatty acids can be a source of higher alkanes via decarboxylation reaction. Such processes have been investigated as a route to biodiesel.[4]
Fatty acid esters and fatty acid nitriles react with long chain Grignard reagents to give, after suitable workup, long-chain ketones. The Wolff-Kishner Reaction provides a way to remove the ketone functionality, giving long-chain hydrocarbons.[1]
Even-numbered, long-chain hydrocarbons can also be synthesized through electrolysis[5] and the Wurtz reactions of alkyl bromides.
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Occurrence
Higher alkanes can also be isolated and purified from natural or synthetic mixtures. Coal tar is a traditional source of mixtures of long-chain hydrocarbons.[3] Careful fractionation, first using urea clathrates to remove branched hydrocarbons, and then distillation, produces pure n-hydrocarbons from petroleum.[6]
Regarding synthetic sources, the Fischer-Tropsch process (or FT process) produces a mixture of hydrocarbons by the hydrogenation of carbon monoxide. The products obtained are liquid hydrocarbons and waxy solids, mostly n-paraffins. The liquid fraction ranges from C6 to C20, while the solid fraction consists of hydrocarbons above C21.[7]
Bioactivity
Some branched higher alkanes are insect pheromones. 9- and 7-methyltricosanes are active for ladybird beetles (Adalia bipunctata).[8] The emerald ash borer (Agrilus planipennis Fairmaire) responds to 9-methylpentacosane.[9] Female Asian long-horned beetles Anoplophora glabripennis, which are very damaging, secrete 2-methyldocosane.[10]
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Reactions
Higher alkanes in general are relatively inert, just like low molecular weight alkanes they can react with oxygen and start a combustion reaction. They can undergo cracking in the presence of alumina or silica catalysts, forming lower alkanes and alkenes.
Uses
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Alkanes from nonane to hexadecane (those alkanes with nine to sixteen carbon atoms) are liquids of higher viscosity, which are less suitable for use in gasoline. They form instead the major part of diesel, kerosene, and aviation fuel. Diesel fuels are characterised by their cetane number, cetane being an older name for hexadecane. However the higher melting points of these alkanes can cause problems at low temperatures and in polar regions, where the fuel becomes too thick to flow correctly. Mixtures of the normal alkanes are used as boiling point standards for simulated distillation by gas chromatography.[11]
Alkanes from hexadecane upwards form the most important components of fuel oil and lubricating oil. In latter function they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as paraffin wax, used for lubrication, electrical insulation, and candles. Paraffin wax should not be confused with beeswax, which consists primarily of esters.
Alkanes with a chain length of approximately 30 or more carbon atoms are found in bitumen (asphalt), used (for example) in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by cracking.
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Names
Some alkanes have non-IUPAC trivial names:
- cetane, for hexadecane
- cerane, for hexacosane[12]
Properties
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Nonane is the lightest alkane to have a flash point above 25 °C, and is classified as flammable under the US National Library of Medicine. [13]
The properties listed here refer to the straight-chain alkanes (or: n-alkanes).
Nonane to hexadecane
This group of n-alkanes is generally liquid under standard conditions.[3]
Heptadecane to tetracosane
From this group on, the n-alkanes are generally solid at standard conditions.
Pentacosane to triacontane
Hentriacontane to hexatriacontane
Heptatriacontane to dotetracontane
Tritetracontane to octatetracontane
Nonatetracontane to tetrapentacontane
Pentapentacontane to hexacontane
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See also
- Alkene
- Alkyne
- Cycloalkane
- Hydrocarbon
- Paraffin wax, composed mostly of higher linear alkanes
- Polyethylene, a linear alkane of polymeric length
References
External links
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