Alkane: Difference between revisions

In

(C₁₄H₃₀).

The International Union of Pure and Applied Chemistry (IUPAC) defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula Template:Carbon_nTemplate:Hydrogen_2n+2, and therefore consisting entirely of hydrogen atoms and saturated carbon atoms". However, some sources use the term to denote any saturated hydrocarbon, including those that are either monocyclic (i.e. the cycloalkanes) or polycyclic,^[2] despite their having a distinct general formula (i.e. cycloalkanes are C_nH_2n).

In an alkane, each carbon atom is sp³-hybridized with 4 sigma bonds (either C–C or C–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 its carbon skeleton or carbon backbone. The number of carbon atoms may be considered as the size of the alkane.

One group of the

(SATP), for which the number of carbon atoms in the carbon backbone is greater than about 17. With their repeated –CH₂ units, the alkanes constitute a homologous series of organic compounds in which the members differ in molecular mass by multiples of 14.03 u (the total mass of each such methylene-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 by methanogenic bacteria 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 much biological activity. They can be viewed as molecular trees upon which can be hung the more active/reactive functional groups of biological molecules.

The alkanes have two main commercial sources: petroleum (crude oil)^[3] and natural gas.

An

group is an alkane-based molecular fragment that bears one open valence for bonding. They are generally abbreviated with the symbol for any organyl group, R, although Alk is sometimes used to specifically symbolize an alkyl group (as opposed to an alkenyl group or aryl group).

Structure and classification

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Saturated hydrocarbons are hydrocarbons having only single covalent bonds between their carbons. They can be:

• linear (general formula C
  _nH
  _2n+2) wherein the carbon atoms are joined in a snake-like structure;
• branched (general formula C
  _nH
  _2n+2, n > 3) wherein the carbon backbone splits off in one or more directions; the simplest branched alkane is 2-methyl-propane, or more commonly known as isobutane, which has three carbon atoms as backbone, and another carbon (a methyl group) linked to the second atom of the backbone, so as to form a "T" shaped molecule. The 'iso-' prefix comes from the fact that a hydrogen atom misses from the central carbon atom of the isopropyl radical (most known for its isopropyl alcohol compound, or rubbing alcohol);
• cyclic (general formula C
  _nH
  _2n, n > 3) wherein the carbon backbone is linked so as to form a loop.

According to the definition by

(loopless) ones, corresponding to k = 0.

Isomerism

n-butane and isobutane are the two C₄H₁₀ isomers; cyclobutane and methylcyclopropane are the two C₄H₈ alkane isomers.
Bicyclo[1.1.0]butane is the only C₄H₆ alkane and has no alkane isomer; tetrahedrane

(below) is the only C₄H₄ alkane and so has no alkane isomer.

Alkanes with more than three carbon atoms can be arranged in various ways, forming structural 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 the n-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]

• C₁: methane only
• C₂: ethane only
• C₃: propane only
• C₄: 2 isomers: butane and isobutane
• C₅: 3 isomers: pentane, isopentane, and neopentane
• C₆: 5 isomers: hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane
• C₇: 9 isomers:
  2,2,3-trimethylbutane
• C₁₂: 355 isomers
• C₃₂: 27,711,253,769 isomers
• C₆₀: 22,158,734,535,770,411,074,184 isomers, many of which are not stable.

Branched alkanes can be

]

Nomenclature

The IUPAC 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".^[5]

In 1866,

It is difficult or impossible to find compounds with more than one IUPAC name. This is because shorter chains attached to longer chains are prefixes and the convention includes brackets. Numbers in the name, referring to which carbon a group is attached to, should be as low as possible so that 1- is implied and usually omitted from names of organic compounds with only one side-group. Symmetric compounds will have two ways of arriving at the same name.^{[citation needed]}

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, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., "n-butane" rather than simply "butane" to distinguish it from isobutane. Alternative names for this group are: linear paraffins or n-paraffins.^{[citation needed]}

The first six 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

The first four names were

.

Branched alkanes

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:^[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

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.^{[citation needed]}

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.^{[citation needed]}

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.^[9]

Trivial/common names

The trivial (non-

]

It is almost certain that the term 'paraffin' stems from the petrochemical industry. Branched-chain alkanes are called isoparaffins. The use of the term "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.^{[citation needed]}

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^[12]

Physical properties

All alkanes are colorless.[13]^[14] Alkanes with the lowest molecular weights are gasses, those of intermediate molecular weight are liquids, and the heaviest are waxy solids.^{[citation needed]}

Table of alkanes

Alkane	Formula	Boiling point [°C]	Melting point [°C]	Density [kg/m3] (at 20 °C)	Isomers
Methane	CH4	-162	−182	0.656 (gas)	1
Ethane	C2H6	−89	−183	1.26 (gas)	1
Propane	C3H8	−42	−188	2.01 (gas)	1
Butane	C4H10	0	−138	2.48 (gas)	2
Pentane	C5H12	36	−130	626 (liquid)	3
Hexane	C6H14	69	−95	659 (liquid)	5
Heptane	C7H16	98	−91	684 (liquid)	9
Octane	C8H18	126	−57	703 (liquid)	18
Nonane	C9H20	151	−54	718 (liquid)	35
Decane	C10H22	174	−30	730 (liquid)	75
Undecane	C11H24	196	−26	740 (liquid)	159
Dodecane	C12H26	216	−10	749 (liquid)	355
Tridecane	C13H28	235	-5.4	756 (liquid)	802
Tetradecane	C14H30	253	5.9	763 (liquid)	1858
Pentadecane	C15H32	270	10	769 (liquid)
Hexadecane	C16H34	287	18	773 (liquid)
Heptadecane	C17H36	303	22	777 (solid)
Octadecane	C18H38	317	28	781 (solid)
Nonadecane	C19H40	330	32	785 (solid)
Icosane	C20H42	343	37	789 (solid)
Triacontane	C30H62	450	66	810 (solid)
Tetracontane	C40H82	525	82	817 (solid)
Pentacontane	C50H102	575	91	824 (solid)
Hexacontane	C60H122	625	100	829 (solid)
Heptacontane	C70H142	653	109	869 (solid)

Boiling point

Alkanes experience intermolecular van der Waals forces. Stronger intermolecular van der Waals forces give rise to greater boiling points of alkanes.^[15]

There are two determinants for 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

A straight-chain alkane will have a boiling point higher than a branched-chain alkane due to the greater surface area in contact, thus the greater van der Waals forces, between adjacent molecules. For example, compare

]

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.[16]

Melting points

The

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: This is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of the linear analogues.^{[citation needed]}

Conductivity and solubility

Alkanes do not conduct electricity in any way, nor are they substantially

]

Their solubility in nonpolar solvents is relatively high, a property that is called lipophilicity. Alkanes are, for example, miscible in all proportions among themselves.^{[citation needed]}

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.^{[citation needed]}

Molecular geometry

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 always sp³-hybridized, that is to say that the valence electrons are said to be in four equivalent orbitals derived from the combination of the 2s orbital and the three 2p orbitals. These orbitals, which have identical energies, are arranged spatially in the form of a tetrahedron, the angle of cos⁻¹(−1/3) ≈ 109.47° between them.^{[citation needed]}

Bond lengths and bond angles

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.^{[citation needed]}

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 correspond with the reality.^{[citation needed]}

Conformation

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The structural formula and the

.

.

The two conformations differ in energy: the staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation (the least stable).

This difference in energy between the two conformations, known as the

torsion energy

, is low compared to the thermal energy of an ethane molecule at ambient temperature. There is constant rotation about the C–C bond. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one CH₃ group by 120° relative to the other, is of the order of 10⁻¹¹ seconds.

The case of

higher alkanes

is more complex but based on similar principles, with the antiperiplanar conformation always being the most favored around each carbon–carbon bond. For this reason, alkanes are usually shown in a zigzag arrangement in diagrams or in models. The actual structure will always differ somewhat from these idealized forms, as the differences in energy between the conformations are small compared to the thermal energy of the molecules: Alkane molecules have no fixed structural form, whatever the models may suggest.

Spectroscopic properties

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Virtually all organic compounds contain carbon–carbon, and carbon–hydrogen bonds, and so show some of the features of alkanes in their spectra. Alkanes are notable for having no other groups, and therefore for the absence of other characteristic spectroscopic features of a functional group like –OH, –CHO, –COOH etc.

Infrared spectroscopy

The carbon–hydrogen stretching mode gives a strong absorption between 2850 and 2960 cm⁻¹, while the carbon–carbon 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. Carbon chains with more than four carbon atoms show a weak absorption at around 725 cm⁻¹.

NMR spectroscopy

The proton resonances of alkanes are usually found at

, and can be missed in weak samples, or samples that have not been run for a sufficiently long time.

Mass spectrometry

Alkanes have a high

. The fragment resulting from the loss of a single methyl group (M − 15) is often absent, and other fragments are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH₂ groups.

Chemical properties

Alkanes are only weakly reactive with most chemical compounds. The

]

]

In highly branched alkanes, the bond angle may differ significantly 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.

However, in general and perhaps surprisingly, when branching is not extensive enough to make highly disfavorable 1,2- and 1,3-alkyl–alkyl steric interactions (worth ~3.1 kcal/mol and ~3.7 kcal/mol in the case of the eclipsing conformations of butane and pentane, respectively) unavoidable, the branched alkanes are actually 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.[19] Due to the subtlety of this effect, the exact reasons for this rule have been vigorously debated in the chemical literature and is yet unsettled. Several explanations, including stabilization of branched alkanes by electron correlation,^[20] destabilization of linear alkanes by steric repulsion,^[21] stabilization by neutral hyperconjugation,^[22][19] and/or electrostatic effects^[23] have been advanced as possibilities. The controversy is related to the question of whether the traditional explanation of hyperconjugation is the primary factor governing the stability of alkyl radicals.^[24][21]

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 → 2 CO + 4 H₂O

CH₄ + 3/2 O₂ → CO + 2 H₂O

See the

for detailed data. The , 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.

Reactions with halogens

Alkanes react with

]

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 a statistical mixture: 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:^[15]

Cracking

Cracking breaks larger molecules into smaller ones. This can be done with a thermal or catalytic method. The thermal cracking process follows a

]

Isomerization and reformation

Dragan and his colleague were the first to report about isomerization in alkanes.

Other reactions

Alkanes will react with

Occurrence

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Occurrence of alkanes in the Universe

Alkanes form a small portion of the

.

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.^[29]

The most important commercial sources for alkanes are natural gas and

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

.

Methane is also present in what is called

renewable energy source

.

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

Acyclic alkanes occur in nature in various ways.

Bacteria and archaea

Certain types of bacteria can metabolize alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.^[30]

On the other hand, certain

:

CO₂ + 4 H₂ → CH₄ + 2 H₂O

Methanogens are also the producers of

Fungi and plants

Alkanes also play a role, if a minor role, in the biology of the three eukaryotic groups of organisms: fungi, plants and animals. Some specialized yeasts, e.g., Candida tropicale, Pichia sp., Rhodotorula sp., can use alkanes as a source of carbon or energy. The fungus Amorphotheca resinae prefers the longer-chain alkanes in aviation fuel, and can cause serious problems for aircraft in tropical regions.^[34]

In plants, the solid long-chain alkanes are found in the plant cuticle and epicuticular wax of many species, but are only rarely major constituents.^[35] They protect the plant against water loss, prevent the leaching 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^[35] and are made by the plants by decarboxylation of even-numbered fatty acids. The exact composition of the layer of wax is not only species-dependent but changes also with the season and such environmental factors as lighting conditions, temperature or humidity.^[35]

More volatile short-chain alkanes are also produced by and found in plant tissues. The

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.[38]

Animals

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%

Ecological relations

One example, in which both plant and animal alkanes play a role, is the ecological relationship between the

(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.

Production

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Petroleum refining

As stated earlier, the most important source of alkanes is natural gas and

and processed into many products.

Fischer–Tropsch

The

petroleum distillates

.

Laboratory preparation

There is usually little need for alkanes to be synthesized in the laboratory, since they are usually commercially available. Also, alkanes are generally unreactive chemically or biologically, and do not undergo

gives the conjugate acid, n-butane as a side-product:

C₄H₉Li + H₂O → C₄H₁₀ + LiOH

However, at times it may be desirable to make a section of a molecule into an alkane-like functionality (

:

RCH=CH₂ + H₂ → RCH₂CH₃     (R =

alkyl

)

Alkanes or alkyl groups can also be prepared directly from

removes hydroxyl groups from alcohols e.g.

and the Clemmensen reduction^{[42][43][44][45]} removes carbonyl groups from aldehydes and ketones to form alkanes or alkyl-substituted compounds e.g.:

Preparation of alkanes from other organic compounds

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Alkanes can be prepared from a variety of organic compounds.These include alkenes, alkynes, haloalkanes, alcohols, aldehydes and ketones and carboxylic acids.

From alkenes and alkynes

When alkenes and alkynes are subjected to hydrogenation reaction by treating them with hydrogen in the presence of palladium or platinum or nickel catalyst, they produce alkanes. In this reaction powdered catalyst is preferred to increase the surface area so that adsorption of hydrogen on the catalyst increases. In this reaction the hydrogen gets attached on the catalyst to form a hydrogen-catalyst bond which leads to weakening of H-H bond, thereby leading to the addition of hydrogen on alkenes and alkynes. The reaction is exothermic because the product alkane is stable as it has more sigma bonds than the reactant alkenes and alkynes due to conversion of pi bond to sigma bonds.^[46]

From haloalkanes

Alkanes can be produced from haloalkanes using different methods.

Wurtz reaction

When haloalkane is treated with sodium in dry ether, alkane with double the number of carbon atoms is obtained. This reaction proceeds through free radical intermediate and has possibility of alkene formation in case of tertiary haloalkanes and vicinal dihalides.

Corey-House-Synthesis

When haloalkane is treated with dialkyl lithium cuprite, which is otherwise known as Gilman's reagent, any higher alkane is obtained.

Reaction with metal hydride

When haloalkanes are treated with metal hydride ,eg, sodium hydride and lithium aluminium hydride.

Frankland reaction

When haloalkane is treated with zinc in ester, alkane is obtained.

Fittig reaction

When aryl halide is treated with sodium in dry ether, it forms biphenyl.

Ullmann biaryl synthesis

When aryl halide is treated with copper, it forms biphenyl.

Wurtz-Fittig reaction

When aryl halide is treated with haloalkane, we get alkyl benzene.

Applications

The applications of alkanes depend on the number of carbon atoms. The first four alkanes are used mainly for heating and cooking purposes, and in some countries for electricity generation. Methane and ethane are the main components of natural gas; they are normally stored as gases under pressure. It is, however, easier to transport them as liquids: This requires both compression and cooling of the gas.

.

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.

Alkanes from hexadecane upwards form the most important components of

.

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.

Some synthetic polymers such as polyethylene and polypropylene are alkanes with chains containing hundreds or thousands of carbon atoms. These materials are used in innumerable applications, and billions of kilograms of these materials are made and used each year.

Environmental transformations

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Alkanes are chemically very inert apolar molecules which are not very reactive as organic compounds. This inertness yields serious ecological issues if they are released into the environment. Due to their lack of functional groups and low water solubility, alkanes show poor bioavailability for microorganisms.^[48]

There are, however, some microorganisms possessing the metabolic capacity to utilize n-alkanes as both carbon and energy sources.^[49] Some bacterial species are highly specialised in degrading alkanes; these are referred to as hydrocarbonoclastic bacteria.^[50]

Hazards

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Methane is flammable, explosive and dangerous to inhale; because it is a colorless, odorless gas, special caution must be taken around methane.^[51] Ethane is also extremely flammable, explosive, and dangerous to inhale.^[52] Both of them may cause suffocation.^[51][52] Propane, too, is flammable and explosive,^[53] and may cause drowsiness or unconsciousness if inhaled.^[53] Butane presents the same hazards as propane.^[54]

Alkanes also pose a threat to the environment. Branched alkanes have a lower biodegradability than unbranched alkanes.^[55] Methane is considered to be the greenhouse gas that is most dangerous to the environment,^[56] although the amount of methane in the atmosphere is relatively low.^[56]

See also

• Alkene
• Alkyne
• Cycloalkane
• Higher alkanes

References

Further reading

• Virtual Textbook of Organic Chemistry
• A visualization of the crystal structures of alkanes up to nonan
• Interesting Chemicals - Alkanes (paraffins)