Higher alkane


Higher alkanes are alkanes with a high number of carbon atoms. It is common jargon. One definition says higher alkanes are alkanes having nine or more carbon atoms. Thus, according to this definition, nonane is the lightest higher alkane. As pure substances, higher alkanes are rarely significant, but they are major components of useful lubricants and fuels.

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.
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.
Even-numbered, long-chain hydrocarbons can also be synthesized through electrolysis and the Wurtz reactions of alkyl bromides.

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. Careful fractionation, first using urea clathrates to remove branched hydrocarbons, and then distillation, produces pure n-hydrocarbons from petroleum.
Regarding synthetic sources, the Fischer-Tropsch 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.

Bioactivity

Some branched higher alkanes are insect pheromones. 7-methyltricosane and 9-methyltricosane are active for ladybird beetles. The emerald ash borer responds to 9-methylpentacosane. Female Asian long-horned beetles Anoplophora glabripennis, which are very damaging, secrete 2-methyldocosane.

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

Alkanes from nonane to hexadecane 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.
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, used in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by cracking.

Names

Some alkanes have non-IUPAC trivial names:
  • cetane, for hexadecane
  • cerane, for hexacosane

    Properties

is the lightest alkane to have a flash point above 25 °C, and is classified as flammable under the US National Library of Medicine.
The properties listed here refer to the straight-chain alkanes.

Nonane to hexadecane

This group of n-alkanes is generally liquid under standard conditions.
NonaneDecaneUndecaneDodecaneTridecaneTetradecanePentadecaneHexadecane
FormulaC9H20C10H22C11H24C12H26C13H28C14H30C15H32C16H34
CAS number
Molar mass 128.26142.29156.31170.34184.37198.39212.42226.45
Melting point −53.5−29.7−25.6−9.6−5.45.99.918.2
Boiling point 150.8174.1195.9216.3235.4253.5270.6286.8
Density 0.717630.730050.740240.748690.756220.762750.768300.77344
Viscosity 0.71390.92561.1851.5031.8802.3352.8633.474
Flash point 314660717999132135
Autoignition temperature 205210205235201
Explosive limits0.9-2.9%0.8-2.6%0.45-6.5%

Heptadecane to tetracosane

From this group on, the n-alkanes are generally solid at standard conditions.
HeptadecaneOctadecaneNonadecaneEicosaneHeneicosaneDocosaneTricosaneTetracosane
FormulaC17H36C18H38C19H40C20H42C21H44C22H46C23H48C24H50
CAS number
Molar mass 240.47254.50268.53282.55296.58310.61324.63338.66
Melting point 2128-3032-3436.740.54248-5052
Boiling point 302317330342.7356.5224 at 2 kPaa380391.3
Density 0.7770.7770.7860.78860.7920.7780.7970.797
Flash point 148166168176

a

Pentacosane to triacontane

PentacosaneHexacosaneHeptacosaneOctacosaneNonacosaneTriacontane
FormulaC25H52C26H54C27H56C28H58C29H60C30H62
CAS number
Molar mass 352.69366.71380.74394.77408.80422.82
Melting point 5456.459.564.563.765.8
Boiling point 401412.2422431.6440.8449.7
Density 0.8010.7780.7800.8070.8080.810

Hentriacontane to hexatriacontane

HentriacontaneDotriacontaneTritriacontaneTetratriacontanePentatriacontaneHexatriacontane
FormulaC31H64C32H66C33H68C34H70C35H72C36H74
CAS number
Molar mass 436.85450.88464.90478.93492.96506.98
Melting point 67.96970-7272.67574-76
Boiling point 458467474285.4 at 0.4 kPa490265 at 130 Pa
Density 0.781 at 68 °C0.8120.8110.8120.8130.814

Heptatriacontane to dotetracontane

HeptatriacontaneOctatriacontaneNonatriacontaneTetracontaneHentetracontaneDotetracontane
FormulaC37H76C38H78C39H80C40H82C41H84C42H86
CAS number
Molar mass 520.99535.03549.05563.08577.11591.13
Melting point 777978848386
Boiling point 504.14510.93517.51523.88530.75536.07
Density 0.8150.8160.8170.8170.8180.819