TheInternational Union of Pure and Applied Chemistry (IUPAC) defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formulaCnH2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denoteany saturated hydrocarbon, including those that are either monocyclic (i.e. thecycloalkanes) orpolycyclic, despite them having a distinct general formula (e.g. cycloalkanes areCnH2n).
In an alkane, each carbon atom issp3-hybridized with 4sigma bonds (either C–C orC–H), and each hydrogen atom is joined to one of the carbon atoms (in a C–H bond). The longest series of linked carbon atoms in a molecule is known as itscarbon skeleton or carbon backbone. The number of carbon atoms may be considered as the size of the alkane.
One group of thehigher alkanes arewaxes, solids atstandard ambient temperature and pressure (SATP), for which the number of carbon atoms in the carbon backbone is greater than 16.With their repeated –CH2 units, the alkanes constitute ahomologous series of organic compounds in which the members differ inmolecular mass by multiples of 14.03 u (the total mass of each suchmethylene bridge unit, which comprises a single carbon atom of mass 12.01 u and two hydrogen atoms of mass ~1.01 u each).
Methane is produced bymethanogenic archaea and some long-chain alkanes function as pheromones in certain animal species or as protective waxes in plants and fungi. Nevertheless, most alkanes do not have muchbiological activity. They can be viewed as molecular trees upon which can be hung the more active/reactivefunctional groups of biological molecules.
The alkanes have two main commercial sources:petroleum (crude oil) andnatural gas.
Analkyl group is an alkane-based molecular fragment that bears one open valence for bonding. They are generally abbreviated with the symbol for anyorganyl group, R, although Alk is sometimes used to specifically symbolize an alkyl group (as opposed to an alkenyl group or aryl group).
Ordinarily, the C–C single bond distance is 1.53ångströms (1.53×10−10 m).[3] Saturated hydrocarbons can be linear, branched, orcyclic. The third group is sometimes calledcycloalkanes.[1] Very complicated structures are possible by combining linear, branched, cyclic alkanes.
Tetrahedrane is the only C4H4 alkane and also has no alkane isomer.
Alkanes with more than threecarbon atoms can be arranged in various ways, formingstructural isomers. The simplest isomer of an alkane is the one in which the carbon atoms are arranged in a single chain with no branches. This isomer is sometimes called then-isomer (n for "normal", although it is not necessarily the most common). However, the chain of carbon atoms may also be branched at one or more points. The number of possible isomers increases rapidly with the number of carbon atoms. For example, for acyclic alkanes:[4]
Branched alkanes can bechiral. For example,3-methylhexane and its higherhomologues are chiral due to theirstereogenic center at carbon atom number 3. The above list only includes differences of connectivity, not stereochemistry. In addition to the alkane isomers, the chain of carbon atoms may form one or more rings. Such compounds are calledcycloalkanes, and are also excluded from the above list because changing the number of rings changes themolecular formula. For example,cyclobutane andmethylcyclopropane are isomers of each other (C4H8), but are not isomers of butane (C4H10).
Branched alkanes are more thermodynamically stable than their linear (or less branched) isomers. For example, the highly branched 2,2,3,3-tetramethylbutane is about 1.9 kcal/mol more stable than its linear isomer,n-octane.[5]
TheIUPAC nomenclature (systematic way of naming compounds) for alkanes is based on identifying hydrocarbon chains. Unbranched, saturated hydrocarbon chains are named systematically with a Greek numerical prefix denoting the number of carbons and the suffix "-ane".[6]
In 1866,August Wilhelm von Hofmann suggested systematizing nomenclature by using the whole sequence of vowels a, e, i, o and u to create suffixes -ane, -ene, -ine (or -yne), -one, -une, for thehydrocarbons CnH2n+2, CnH2n, CnH2n−2, CnH2n−4, CnH2n−6.[7] In modern nomenclature, the first three specifically name hydrocarbons with single, double and triple bonds;[8] while "-one" now represents aketone.
Straight-chain alkanes are sometimes indicated by the prefixn- (for "normal") where a non-linearisomer exists. Although this is not strictly necessary and is not part of the IUPAC naming system, the usage is still common in cases where one wishes to emphasize or distinguish between the straight-chain and branched-chain isomers, e.g., "n-butane" rather than simply "butane" to differentiate it fromisobutane. Alternative names for this group used in the petroleum industry arelinear paraffins orn-paraffins.
The first eight members of the series (in terms of number of carbon atoms) are named as follows:
Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for examplen-pentane,isopentane, andneopentane.
IUPAC naming conventions can be used to produce a systematic name.
The key steps in the naming of more complicated branched alkanes are as follows:[10]
Identify the longest continuous chain of carbon atoms.
Name this longest root chain using standard naming rules.
Name each side chain by changing the suffix of the name of the alkane from "-ane" to "-yl".
Number the longest continuous chain in order to give the lowest possible numbers for the side-chains.[11]
Number and name the side chains before the name of the root chain.
If there are multiple side chains of the same type, use prefixes such as "di-" and "tri-" to indicate it as such, and number each one.
Add side chain names in alphabetical (disregarding "di-" etc. prefixes) order in front of the name of the root chain.
Comparison of nomenclatures for three isomers of C5H12
Though technically distinct from the alkanes, this class of hydrocarbons is referred to by some as the "cyclic alkanes." As their description implies, they contain one or more rings.
Simple cycloalkanes have a prefix "cyclo-" to distinguish them from alkanes. Cycloalkanes are named as per their acyclic counterparts with respect to the number of carbon atoms in their backbones, e.g.,cyclopentane (C5H10) is a cycloalkane with 5 carbon atoms just likepentane (C5H12), but they are joined up in a five-membered ring. In a similar manner,propane andcyclopropane,butane andcyclobutane, etc.
Substituted cycloalkanes are named similarly to substituted alkanes – the cycloalkane ring is stated, and the substituents are according to their position on the ring, with the numbering decided by theCahn–Ingold–Prelog priority rules.[9]
The trivial (non-systematic) name for alkanes is "paraffins". Together, alkanes are known as the "paraffin series". Trivial names for compounds are usually historical artifacts. They were coined before the development of systematic names, and have been retained due to familiar usage in industry. Cycloalkanes are also called naphthenes.[12][13]
Branched-chain alkanes are calledisoparaffins. "Paraffin" is a general term and often does not distinguish between pure compounds and mixtures ofisomers, i.e., compounds of the samechemical formula, e.g.,pentane andisopentane.
In IUPAC
The following trivial names are retained in the IUPAC system:
All alkanes are colorless.[15][16] Alkanes with the lowest molecular weights are gases, those of intermediate molecular weight are liquids, and the heaviest are waxy solids.[17][18]
Melting (blue) and boiling (orange) points of the first 16n-alkanes in °C.
Alkanes experiences intermolecularvan der Waals forces. The cumulative effects of these intermolecular forces give rise to greater boiling points of alkanes.[19]
Two factors influence the strength of the van der Waals forces:
the number of electrons surrounding themolecule, which increases with the alkane's molecular weight
the surface area of the molecule
Understandard conditions, from CH4 to C4H10 alkanes are gaseous; from C5H12 to C17H36 they are liquids; and after C18H38 they are solids. As the boiling point of alkanes is primarily determined by weight, it should not be a surprise that the boiling point has an almost linear relationship with the size (molecular weight) of the molecule. As a rule of thumb, the boiling point rises 20–30 °C for each carbon added to the chain; this rule applies to other homologous series.[19]
A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, and thus greater van der Waals forces, between adjacent molecules. For example, compare isobutane (2-methylpropane) andn-butane, which boil at −12 and 0 °C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58 °C, respectively.[19]
On the other hand, cycloalkanes tend to have higher boiling points than their linear counterparts due to the locked conformations of the molecules, which give a plane of intermolecular contact.
Themelting points of the alkanes follow a similar trend toboiling points for the same reason as outlined above. That is, (all other things being equal) the larger the molecule the higher the melting point. However, alkanes' melting points follow a more complex pattern, due to variations in the properties of their solid crystals.[a]
One difference in crystal structure that even-numbered alkanes (from hexane onwards) tend to form denser-packed crystals compared to their odd-numbered neighbors. This causes them to have a greaterenthalpy of fusion (amount of energy required to melt them), raising their melting point.[20] A second difference in crystal structure is that even-numbered alkanes (from octane onwards) tend to form more rotationally ordered crystals compared to their odd-numbered neighbors. This causes them to have a greaterentropy of fusion (increase in disorder from the solid to the liquid state), lowering their melting point.[21]
While these effects operate in opposing directions, the first effect tends to be slightly stronger, leading even-numbered alkanes to have slightly higher melting points than the average of their odd-numbered neighbors.
This trend does not apply to methane, which has an unusually high melting point, higher than both ethane and propane. This is because it has a very low entropy of fusion, attributable to its high molecular symmetry and the rotational disorder in solid methane near its melting point (Methane I).[21]
The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on these two factors. More symmetric alkanes tend towards higher melting points, due to enthalpic effects when they form ordered crystals, and entropic effects when they form disordered crystals (e.g.neopentane).[21]
Alkanes do not conduct electricity in any way, nor are they substantiallypolarized by anelectric field. For this reason, they do not formhydrogen bonds and are insoluble in polar solvents such as water. Since the hydrogen bonds between individual water molecules are aligned away from an alkane molecule, the coexistence of an alkane and water leads to an increase in molecular order (a reduction inentropy). As there is no significant bonding between water molecules and alkane molecules, thesecond law of thermodynamics suggests that this reduction in entropy should be minimized by minimizing the contact between alkane and water: Alkanes are said to behydrophobic as they are insoluble in water.
Their solubility in nonpolar solvents is relatively high, a property that is calledlipophilicity. Alkanes are, for example, miscible in all proportions among themselves.
The density of the alkanes usually increases with the number of carbon atoms but remains less than that of water. Hence, alkanes form the upper layer in an alkane–water mixture.[22]
The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from theelectron configuration ofcarbon, which has fourvalence electrons. The carbon atoms in alkanes are described as sp3 hybrids; that is to say that, to a good approximation, the valence electrons are in orbitals directed towards the corners of a tetrahedron which are derived from the combination of the 2s orbital and the three 2p orbitals. Geometrically, the angle between the bonds are cos−1(−1/3) ≈ 109.47°. This is exact for the case of methane, while larger alkanes containing a combination of C–H and C–C bonds generally have bonds that are within several degrees of this idealized value.
An alkane has only C–H and C–C single bonds. The former result from the overlap of an sp3 orbital of carbon with the 1s orbital of a hydrogen; the latter by the overlap of two sp3 orbitals on adjacent carbon atoms. Thebond lengths amount to 1.09 × 10−10 m for a C–H bond and 1.54 × 10−10 m for a C–C bond.
The spatial arrangement of the bonds is similar to that of the four sp3 orbitals—they are tetrahedrally arranged, with an angle of 109.47° between them. Structural formulae that represent the bonds as being at right angles to one another, while both common and useful, do not accurately depict the geometry.
Newman projections of two of many conformations of ethane: eclipsed on the left, staggered on the right.Ball-and-stick models of the two rotamers of ethane
The spatial arrangement of the C-C and C-H bonds are described by the torsion angles of the molecule, known as itsconformation. Inethane, the simplest case for studying the conformation of alkanes, there is nearly free rotation about a carbon–carbon single bond. Two limiting conformations are important:eclipsed conformation andstaggered conformation. The staggered conformation is 12.6 kJ/mol (3.0 kcal/mol) lower in energy (more stable) than the eclipsed conformation (the least stable). In highly branched alkanes, the bond angle may differ from the optimal value (109.5°) to accommodate bulky groups. Such distortions introduce a tension in the molecule, known assteric hindrance or strain. Strain substantially increases reactivity.[23]
The C-H stretching mode gives strong absorptions between 2850 and 2960 cm−1 and weaker bands for the C-C stretching mode absorbs between 800 and 1300 cm−1. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450 cm−1 and 1375 cm−1, while methylene groups show bands at 1465 cm−1 and 1450 cm−1.[25] Carbon chains with more than four carbon atoms show a weak absorption at around 725 cm−1.
The proton resonances of alkanes are usually found atδH = 0.5–1.5. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon:δC = 8–30 (primary, methyl, –CH3), 15–55 (secondary, methylene, –CH2–), 20–60 (tertiary, methyne, C–H) and quaternary. The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack ofnuclear Overhauser effect and the longrelaxation time, and can be missed in weak samples, or samples that have not been run for a sufficiently long time.
Since alkanes have highionization energies, theirelectron impact mass spectra show weak currents for their molecular ions. The fragmentation pattern can be difficult to interpret, but in the case of branched chain alkanes, the carbon chain is preferentially cleaved at tertiary or quaternary carbons due to the relative stability of the resultingfree radicals. The mass spectra for straight-chain alkanes is illustrated by that fordodecane: the fragment resulting from the loss of a single methyl group (M − 15) is absent, fragments are more intense than the molecular ion and are spaced by intervals of 14 mass units, corresponding to loss of CH2 groups.[26]
Alkanes are only weakly reactive with most chemical compounds. They only react with the strongest of electrophilic reagents by virtue of their strong C–H bonds (~100 kcal/mol) and C–C bonds (~90 kcal/mol). They are also relatively unreactive toward free radicals. This inertness is the source of the termparaffins (with the meaning here of "lacking affinity"). Incrude oil the alkane molecules have remained chemically unchanged for millions of years.
Theacid dissociation constant (pKa) values of all alkanes are estimated to range from 50 to 70, depending on the extrapolation method, hence they are extremely weak acids that are practically inert to bases (see:carbon acids). They are also extremely weak bases, undergoing no observable protonation in puresulfuric acid (H0 ~ −12), althoughsuperacids that are at least millions of times stronger have been known to protonate them to give hypercoordinate alkanium ions (see:methanium ion). Thus, a mixture ofantimony pentafluoride (SbF5) andfluorosulfonic acid (HSO3F), calledmagic acid, can protonate alkanes.[27]
All alkanes react withoxygen in acombustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:
See thealkane heat of formation table for detailed data.Thestandard enthalpy change of combustion, ΔcH⊖, for alkanes increases by about 650 kJ/mol per CH2 group. Branched-chain alkanes have lower values of ΔcH⊖ than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable.
Some organisms are capable of metabolizing alkanes.[28][29] Themethane monooxygenases convert methane tomethanol. For higher alkanes,cytochrome P450 convert alkanes to alcohols, which are then susceptible to degradation.
Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes.Free radical halogenation reactions occur with halogens, leading to the production ofhaloalkanes. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. The reaction of alkanes and fluorine is highlyexothermic and can lead to an explosion.[30] These reactions are an important industrial route to halogenated hydrocarbons. There are three steps:
Initiation the halogen radicals form byhomolysis. Usually, energy in the form of heat or light is required.
Chain reaction orPropagation then takes place—the halogen radical abstracts a hydrogen from the alkane to give an alkyl radical. This reacts further.
Chain termination where the radicals recombine.
Experiments have shown that all halogenation produces a mixture of all possible isomers, indicating that all hydrogen atoms are susceptible to reaction. The mixture produced, however, is not statistical: Secondary and tertiary hydrogen atoms are preferentially replaced due to the greater stability of secondary and tertiary free-radicals. An example can be seen in the monobromination of propane:[19]
Cracking breaks larger molecules into smaller ones. This reaction requires heat and catalysts. The thermal cracking process follows ahomolytic mechanism with formation offree radicals. The catalytic cracking process involves the presence ofacidcatalysts (usually solid acids such assilica-alumina andzeolites), which promote aheterolytic (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually acarbocation. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C–C scission in positionbeta (i.e., cracking) andintra- andintermolecular hydrogen transfer orhydride transfer. In both types of processes, the correspondingreactive intermediates (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.[citation needed]
Dragan and his colleague were the first to report about isomerization in alkanes.[32] Isomerization and reformation are processes in which straight-chain alkanes are heated in the presence of aplatinum catalyst. In isomerization, the alkanes become branched-chain isomers. In other words, it does not lose any carbons or hydrogens, keeping the same molecular weight.[32] In reformation, the alkanes becomecycloalkanes oraromatic hydrocarbons, giving off hydrogen as a by-product. Both of these processes raise theoctane number of the substance. Butane is the most common alkane that is put under the process of isomerization, as it makes many branched alkanes with high octane numbers.[32]
Methane andethane make up a tiny proportion ofJupiter's atmosphereExtraction of oil, which contains many distincthydrocarbons including alkanes
Alkanes form a small portion of theatmospheres of the outer gas planets such asJupiter (0.1% methane, 2 ppm ethane),Saturn (0.2% methane, 5 ppm ethane),Uranus (1.99% methane, 2.5 ppm ethane) andNeptune (1.5% methane, 1.5 ppm ethane).Titan (1.6% methane), a satellite of Saturn, was examined by theHuygens probe, which indicated that Titan's atmosphere periodically rains liquid methane onto the moon's surface.[33] Also on Titan, the Cassini mission has imaged seasonal methane/ethane lakes near the polar regions of Titan.Methane andethane have been detected in the tail of thecomet Hyakutake. Chemical analysis showed that the abundances of ethane and methane were roughly equal, which is thought to imply that its ices formed in interstellar space, away from the Sun, which would have evaporated these volatile molecules.[34] Alkanes have been detected inmeteorites such ascarbonaceous chondrites.
Traces of methane gas (about 0.0002% or 1745 ppb) occur in the Earth's atmosphere, produced primarily bymethanogenic microorganisms, such asArchaea in the gut of ruminants.[35]
The most important commercial sources for alkanes are natural gas andoil.[19] Natural gas contains primarily methane and ethane, with somepropane andbutane: oil is a mixture of liquid alkanes and otherhydrocarbons. These hydrocarbons were formed when marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in ananoxic environment and converted over many millions of years at high temperatures and high pressure to their current form. Natural gas resulted thereby for example from the following reaction:
C6H12O6 → 3 CH4 + 3 CO2
These hydrocarbon deposits, collected in porous rocks trapped beneath impermeable cap rocks, comprise commercialoil fields. They have formed over millions of years and once exhausted cannot be readily replaced. The depletion of these hydrocarbons reserves is the basis for what is known as theenergy crisis.
Alkanes have a low solubility in water, so the content in the oceans is negligible; however, at high pressures and low temperatures (such as at the bottom of the oceans), methane can co-crystallize with water to form a solidmethane clathrate (methane hydrate). Although this cannot be commercially exploited at the present time, the amount of combustible energy of the known methane clathrate fields exceeds the energy content of all the natural gas and oil deposits put together. Methane extracted from methane clathrate is, therefore, a candidate for future fuels.
Aside from petroleum and natural gas, alkanes occur significantly in nature only as methane, which is produced by somearchaea by the process ofmethanogenesis. These organisms are found in the gut of termites[36] and cows.[37] Themethane is produced fromcarbon dioxide or other organic compounds. Energy is released by the oxidation ofhydrogen:
CO2 + 4 H2 → CH4 + 2 H2O
It is probable that our current deposits of natural gas were formed in a similar way.[38]
Certain types of bacteria can metabolize alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.[39]
Alkanes play a negligible role in higher organisms, with rare exception.Some yeasts, e.g.,Candida tropicale,Pichia sp.,Rhodotorula sp., can use alkanes as a source of carbon or energy. The fungusAmorphotheca resinae prefers the longer-chain alkanes inaviation fuel, and can cause serious problems for aircraft in tropical regions.[40]
In plants, the solid long-chain alkanes are found in theplant cuticle andepicuticular wax of many species, but are only rarely major constituents.[41] They protect the plant against water loss, prevent theleaching of important minerals by the rain, and protect against bacteria, fungi, and harmful insects. The carbon chains in plant alkanes are usually odd-numbered, between 27 and 33 carbon atoms in length,[41] and are made by the plants bydecarboxylation of even-numberedfatty acids. The exact composition of the layer of wax is not only species-dependent but also changes with the season and such environmental factors as lighting conditions, temperature or humidity.[41]
TheJeffrey pine is noted for producing exceptionally high levels ofn-heptane in its resin, for which reason its distillate was designated as the zero point for oneoctane rating. Floral scents have also long been known to contain volatile alkane components, andn-nonane is a significant component in the scent of someroses.[42] Emission of gaseous and volatile alkanes such asethane,pentane, andhexane by plants has also been documented at low levels, though they are not generally considered to be a major component of biogenic air pollution.[43]
Edible vegetable oils also typically contain small fractions of biogenic alkanes with a wide spectrum of carbon numbers, mainly 8 to 35, usually peaking in the low to upper 20s, with concentrations up to dozens of milligrams per kilogram (parts per million by weight) and sometimes over a hundred for the total alkane fraction.[44]
Alkanes are found in animal products, although they are less important than unsaturated hydrocarbons. One example is the shark liver oil, which is approximately 14%pristane (2,6,10,14-tetramethylpentadecane, C19H40). They are important aspheromones, chemical messenger materials, on which insects depend for communication. In some species, e.g. the support beetleXylotrechus colonus,pentacosane (C25H52), 3-methylpentaicosane (C26H54) and 9-methylpentaicosane (C26H54) are transferred by body contact. With others like thetsetse flyGlossina morsitans morsitans, the pheromone contains the four alkanes 2-methylheptadecane (C18H38), 17,21-dimethylheptatriacontane (C39H80), 15,19-dimethylheptatriacontane (C39H80) and 15,19,23-trimethylheptatriacontane (C40H82), and acts by smell over longer distances.Waggle-dancinghoney bees produce and release two alkanes, tricosane and pentacosane.[45]
One example, in which both plant and animal alkanes play a role, is the ecological relationship between thesand bee (Andrena nigroaenea) and theearly spider orchid (Ophrys sphegodes); the latter is dependent forpollination on the former. Sand bees use pheromones in order to identify a mate; in the case ofA. nigroaenea, the females emit a mixture oftricosane (C23H48),pentacosane (C25H52) andheptacosane (C27H56) in the ratio 3:3:1, and males are attracted by specifically this odor. The orchid takes advantage of this mating arrangement to get the male bee to collect and disseminate its pollen; parts of its flower not only resemble the appearance of sand bees but also produce large quantities of the three alkanes in the same ratio as female sand bees. As a result, numerous males are lured to the blooms and attempt to copulate with their imaginary partner: although this endeavor is not crowned with success for the bee, it allows the orchid to transfer its pollen,which will be dispersed after the departure of the frustrated male to other blooms.
Another route to alkanes ishydrogenolysis, which entails cleavage of C-heteroatom bonds using hydrogen. In industry, the main substrates are organonitrogen and organosulfur impurities, i.e. the heteroatoms are N and S. The specific processes are calledhydrodenitrification andhydrodesulfurization:
R3N + 3 H2 → 3 RH + H3N
R2S + 2 H2 → 2 RH + H2S
Hydrogenolysis can be applied to the conversion of virtually any functional group into hydrocarbons. Substrates include haloalkanes, alcohols, aldehydes, ketones, carboxylic acids, etc. Both hydrogenolysis and hydrogenation are practiced in refineries. They can be effected by usinglithium aluminium hydride,Clemmenson reduction and other specialized routes.
Coal is a more traditional precursor to alkanes. A wide range of technologies have been intensively practiced for centuries.[46] Simply heating coal gives alkanes, leaving behindcoke. Relevant technologies include theBergius process andcoal liquefaction. Partial combustion of coal and related solid organic compounds generatescarbon monoxide, which can be hydrogenated using theFischer–Tropsch process. This technology allows the synthesis of liquid hydrocarbons, including alkanes. This method is used to produce substitutes forpetroleum distillates.
The dominant use of alkanes is as fuels.Propane andbutane, easily liquified gases, are commonly known asliquified petroleum gas (LPG).[49] Frompentane tooctane the alkanes are highly volatile liquids. They are used as fuels ininternal combustion engines, as they vaporize easily on entry into the combustion chamber without forming droplets, which would impair the uniformity of the combustion. Branched-chain alkanes are preferred as they are much less prone to premature ignition, which causesknocking, than their straight-chain homologues. This propensity to premature ignition is measured by theoctane rating of the fuel, where2,2,4-trimethylpentane (isooctane) has an arbitrary value of 100, andheptane has a value of zero. Apart from their use as fuels, the middle alkanes are also goodsolvents for nonpolar substances. Alkanes fromnonane to, for instance,hexadecane (an alkane with sixteen carbon atoms) are liquids of higherviscosity, less and less suitable for use in gasoline. They form instead the major part ofdiesel andaviation fuel. Diesel fuels are characterized by theircetane number, cetane being an old 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.
By the process ofcracking, alkanes can be converted toalkenes. Simple alkenes are precursors to polymers, such aspolyethylene andpolypropylene. When the cracking is taken to extremes, alkanes can be converted tocarbon black, which is a significant tire component.
Chlorination of methane gives chloromethanes, which are used as solvents and building blocks for complex compounds. Similarly treatment of methane with sulfur givescarbon disulfide. Still other chemicals are prepared by reaction withsulfur trioxide andnitric oxide.
Alkanes from hexadecane upwards form the most important components offuel oil andlubricating oil. In the 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 asparaffin wax, for example, incandles. This should not be confused however with truewax, which consists primarily ofesters.
Alkanes with a chain length of approximately 35 or more carbon atoms are found inbitumen, used, for example, in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes bycracking.
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