Linear alkanes

Straight-chain alkanes are sometimes indicated by the prefix "n-" or "n-"(for "normal") where a non-linear isomer 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 from isobutane. Alternative names for this group used in the petroleum industry are linear paraffins or n-paraffins.

The first eight members of the series (in terms of number of carbon atoms) are named as follows:

methane

CH₄ – one carbon and 4 hydrogen

ethane

C₂H₆ – two carbon and 6 hydrogen

propane

C₃H₈ – three carbon and 8 hydrogen

butane

C₄H₁₀ – four carbon and 10 hydrogen

pentane

C₅H₁₂ – five carbon and 12 hydrogen

hexane

C₆H₁₄ – six carbon and 14 hydrogen

heptane

C₇H₁₆ – seven carbons and 16 hydrogen

octane

C₈H₁₈ – eight carbons and 18 hydrogen

The first four names were derived from methanol, ether, propionic acid and butyric acid. Alkanes with five or more carbon atoms are named by adding the suffix -ane to the appropriate numerical multiplier prefix^[8] with elision of any terminal vowel (-a or -o) from the basic numerical term. Hence, pentane, C₅H₁₂; hexane, C₆H₁₄; heptane, C₇H₁₆; octane, C₈H₁₈; etc. The numeral prefix is generally Greek; however, alkanes with a carbon atom count ending in nine, for example nonane, use the Latin prefix non-.

Branched alkanes

Ball-and-stick model of isopentane (common name) or 2-methylbutane (IUPAC systematic name)

Simple branched alkanes often have a common name using a prefix to distinguish them from linear alkanes, for example n-pentane, isopentane, and neopentane.

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:^[9]

• 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^[10]
• 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

Common name	n-pentane	isopentane	neopentane
IUPAC name	pentane	2-methylbutane	2,2-dimethylpropane
Structure

Saturated cyclic hydrocarbons

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 like pentane (C₅H₁₂), but they are joined up in a five-membered ring. In a similar manner, propane and cyclopropane, butane and cyclobutane, 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 the Cahn–Ingold–Prelog priority rules.^[8]

Trivial/common names

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.^[11][12]

Branched-chain alkanes are called isoparaffins. "Paraffin" is a general term and often does not distinguish between pure compounds and mixtures of isomers, i.e., compounds of the same chemical formula, e.g., pentane and isopentane.

In IUPAC

The following trivial names are retained in the IUPAC system:

• isobutane for 2-methylpropane
• isopentane for 2-methylbutane
• neopentane for 2,2-dimethylpropane.

Non-IUPAC

Some non-IUPAC trivial names are occasionally used:

• cetane, for hexadecane
• cerane, for hexacosane^[13]

Table of alkanes

Boiling point

Melting (blue) and boiling (orange) points of the first 16 n-alkanes in °C.

Alkanes experience intermolecular van der Waals forces. The cumulative effects of these intermolecular forces give rise to greater boiling points of alkanes.^[18]

Two factors influence the strength of the van der Waals forces:

• the number of electrons surrounding the molecule, which increases with the alkane's molecular weight
• the surface area of the molecule

Under standard 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.^[18]

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) and n-butane (butane), which boil at −12 and 0 °C, and 2,2-dimethylbutane and 2,3-dimethylbutane which boil at 50 and 58 °C, respectively.^[18]

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.

Melting points

The melting points of the alkanes follow a similar trend to boiling points for the same reason as outlined above. That is, (all other things being equal) the larger the molecule the higher the melting point. There is one significant difference between boiling points and melting points. Solids have a more rigid and fixed structure than liquids. This rigid structure requires energy to break down. Thus the better put together solid structures will require more energy to break apart. For alkanes, this can be seen from the graph above (i.e., the blue line). The odd-numbered alkanes have a lower trend in melting points than even-numbered alkanes. This is because even-numbered alkanes pack well in the solid phase, forming a well-organized structure which requires more energy to break apart. The odd-numbered alkanes pack less well and so the "looser"-organized solid packing structure requires less energy to break apart.^[19] For a visualization of the crystal structures see.^[20]

The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, again depending on the ability of the alkane in question to pack well in the solid phase.

Conductivity and solubility

Alkanes do not conduct electricity in any way, nor are they substantially polarized by an electric field. For this reason, they do not form hydrogen 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 in entropy). As there is no significant bonding between water molecules and alkane molecules, the second law of thermodynamics suggests that this reduction in entropy should be minimized by minimizing the contact between alkane and water: Alkanes are said to be hydrophobic as they are insoluble in water.

Their solubility in nonpolar solvents is relatively high, a property that is called lipophilicity. 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.^[21]

Molecular geometry

sp³-hybridization in methane.

The molecular structure of the alkanes directly affects their physical and chemical characteristics. It is derived from the electron configuration of carbon, which has four valence 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.

Bond lengths and bond angles

The tetrahedral structure of methane.

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. The bond 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.

Conformation

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 is known as its conformation. In ethane, 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 and staggered 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 as steric hindrance or strain. Strain substantially increases reactivity..^[22]

Spectroscopic properties

Spectroscopic signatures for alkanes are obtainable by the major characterization techniques.^[23]

Acid-base behavior

The acid dissociation constant (pK_a) 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 pure sulfuric acid (H₀ ~ −12), although superacids 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 of antimony pentafluoride (SbF₅) and fluorosulfonic acid (HSO₃F), called magic acid, can protonate alkanes.^[26]

Reactions with oxygen (combustion reaction)

All alkanes react with oxygen in a combustion reaction, although they become increasingly difficult to ignite as the number of carbon atoms increases. The general equation for complete combustion is:

C_nH_2n+2 + (3/2n + 1/2) O₂ → (n + 1) H₂O + n CO₂

or C_nH_2n+2 + (3n + 1/2) O₂ → (n + 1) H₂O + n CO₂

In the absence of sufficient oxygen, carbon monoxide or even soot can be formed, as shown below:

C_nH_2n+2 + (n + 1/2) O₂ → (n + 1) H₂O + n CO

C_nH_2n+2 + (1/2n + 1/2) O₂ → (n + 1) H₂O + n C

For example, methane:

2 CH₄ + 3 O₂ → 4 H₂O + 2 CO

CH₄ + O₂ → 2 H₂O + C

See the alkane heat of formation table for detailed data. The standard enthalpy change of combustion, Δ_cH^⊖, for alkanes increases by about 650 kJ/mol per CH₂ group. Branched-chain alkanes have lower values of Δ_cH^⊖ than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable.

Biodegradation

Some organisms are capable of metalbolizing alkanes.^[27][28] The methane monooxygenases convert methane to methanol. For higher alkanes, cytochrome P450 convert alkanes to alcohols, which are then susceptible to degradation.

Free radical reactions

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 of haloalkanes. The hydrogen atoms of the alkane are progressively replaced by halogen atoms. The reaction of alkanes and fluorine is highly exothermic and can lead to an explosion.^[29] These reactions are an important industrial route to halogenated hydrocarbons. There are three steps:

• Initiation the halogen radicals form by homolysis. Usually, energy in the form of heat or light is required.
• Chain reaction or Propagation 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:^[18]

Monobromination of propane

In the Reed reaction, sulfur dioxide and chlorineconvert hydrocarbons to sulfonyl chlorides under the influence of light.

Under some conditions, alkanes will undergo Nitration.

C-H activation

Certain transition metal complexes promote non-radical reactions with alkanes, resulting in so C–H bond activation reactions.^[30]

Cracking

Cracking breaks larger molecules into smaller ones. This reaction requires heat and catalysts. The thermal cracking process follows a homolytic mechanism with formation of free radicals. The catalytic cracking process involves the presence of acid catalysts (usually solid acids such as silica-alumina and zeolites), which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of ions of opposite charges, usually a carbocation. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C–C scission in position beta (i.e., cracking) and intra- and intermolecular hydrogen transfer or hydride transfer. In both types of processes, the corresponding reactive 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.

Isomerization and reformation

Dragan and his colleague were the first to report about isomerization in alkanes.^[31] Isomerization and reformation are processes in which straight-chain alkanes are heated in the presence of a platinum 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.^[31] In reformation, the alkanes become cycloalkanes or aromatic hydrocarbons, giving off hydrogen as a by-product. Both of these processes raise the octane 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.^[31]

Other reactions

In steam reforming, alkanes react with steam in the presence of a nickel catalyst to give hydrogen and carbon monoxide.

Occurrence of alkanes in the Universe

Methane and ethane make up a tiny proportion of Jupiter's atmosphere

Extraction of oil, which contains many distinct hydrocarbons including alkanes

Alkanes form a small portion of the atmospheres of the outer gas planets such as Jupiter (0.1% methane, 2 ppm ethane), Saturn (0.2% methane, 5 ppm ethane), Uranus (1.99% methane, 2.5 ppm ethane) and Neptune (1.5% methane, 1.5 ppm ethane). Titan (1.6% methane), a satellite of Saturn, was examined by the Huygens probe, which indicated that Titan's atmosphere periodically rains liquid methane onto the moon's surface.^[32] Also on Titan, the Cassini mission has imaged seasonal methane/ethane lakes near the polar regions of Titan. Methane and ethane have also been detected in the tail of the comet 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.^[33] Alkanes have also been detected in meteorites such as carbonaceous chondrites.

Occurrence of alkanes on Earth

Traces of methane gas (about 0.0002% or 1745 ppb) occur in the Earth's atmosphere, produced primarily by methanogenic microorganisms, such as Archaea in the gut of ruminants.^[34]

The most important commercial sources for alkanes are natural gas and oil.^[18] Natural gas contains primarily methane and ethane, with some propane and butane: oil is a mixture of liquid alkanes and other hydrocarbons. 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 an anoxic 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 commercial oil 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 the energy 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 solid methane 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.

Biological occurrence

Methanogenic archaea in the gut of cows produce methane.

Aside from petroleum and natural gas, alkanes occur significantly in nature only as methane, which is produced by some archaea by the process of methanogenesis. These organisms are found in the gut of termites^[35] and cows.^[36] The methane is produced from carbon dioxide or other organic compounds. Energy is released by the oxidation of hydrogen:

CO₂ + 4 H₂ → CH₄ + 2 H₂O

It is probable that our current deposits of natural gas were formed in a similar way.^[37]

RCH2\sCH3}} (R = alkyl)

Another route to alkanes is hydrogenolysis, 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 called hydrodenitrification and hydrodesulfurization:

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. The can be effected by using lithium aluminium hydride, Clemmenson reduction and other specialized routes.

Coal

Coal is a more traditional precursor to alkanes. A wide range of technologies have been intensively practiced for centuries.^[38] Simply heating coal gives alkanes, leaving behind coke. Relevant technologies include the Bergius process and coal liquifaction. Partial combustion of coal and related solid organic compounds generates carbon monoxide, which can be hydrogenated using the Fischer–Tropsch process. This technology allows the synthesize liquid hydrocarbons, including alkanes. This method is used to produce substitutes for petroleum distillates.

Laboratory preparation

Rarely is there any interest in the synthesis of alkanes, since they are usually commercially available and less valued than virtually any precursor. The best-known method is hydrogenation of alkenes. Many C-X bonds can be converted to C-H bonds using lithium aluminium hydride, Clemmenson reduction, and other specialized routes.^[39] Hydrolysis of Alkyl Grignard reagents and alkyl lithium compounds gives alkanes.^[40]

Fuels

The dominant use of alkanes is as fuels. Propane and butane, easily liquified gases, are commonly known as liquified petroleum gas (LPG).^[41] From pentane to octane the alkanes are highly volatile liquids. They are used as fuels in internal 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 causes knocking, than their straight-chain homologues. This propensity to premature ignition is measured by the octane rating of the fuel, where 2,2,4-trimethylpentane (isooctane) has an arbitrary value of 100, and heptane has a value of zero. Apart from their use as fuels, the middle alkanes are also good solvents for nonpolar substances. Alkanes from nonane to, for instance, hexadecane (an alkane with sixteen carbon atoms) are liquids of higher viscosity, less and less suitable for use in gasoline. They form instead the major part of diesel and aviation fuel. Diesel fuels are characterized by their cetane 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.

Precursors to chemicals

By the process of cracking, alkanes can be converted to alkenes. Simple alkenes are precursors to polymers, such as polyethylene and polypropylene. When the cracking is taken to extremes, alkanes can be converted to carbon 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 gives carbon disulfide. Still other chemicals are prepared by reaction with sulfur trioxide and nitric oxide

Other

Some light hydrocarbons are used as aerosol sprays.

Alkanes from hexadecane upwards form the most important components of fuel oil and lubricating 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 as paraffin wax, for example, in candles. This should not be confused however with true wax, which consists primarily of esters.

Alkanes with a chain length of approximately 35 or more carbon atoms are found in bitumen, used, for example, in road surfacing. However, the higher alkanes have little value and are usually split into lower alkanes by cracking.