Alkane

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Chemical structure of methane, the simplest alkane Methane-2D-stereo.svg
Chemical structure of methane, the simplest alkane

In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. [1] Alkanes have the general chemical formula CnH2n+2. The alkanes range in complexity from the simplest case of methane (CH4), where n = 1 (sometimes called the parent molecule), to arbitrarily large and complex molecules, like pentacontane (C50H102) or 6-ethyl-2-methyl-5-(1-methylethyl) octane, an isomer of tetradecane (C14H30).

Contents

The International Union of Pure and Applied Chemistry (IUPAC) defines alkanes as "acyclic branched or unbranched hydrocarbons having the general formula CnH2n+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, despite their having a distinct general formula (i.e. cycloalkanes are CnH2n).

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 higher alkanes are waxes, solids at standard ambient temperature and pressure (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) and natural gas.

An alkyl 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

Ordinarily the C-C single bond distance is 1.53 ångströms (1.53×10−10 m). [2] Saturated hydrocarbons can be linear, branched, or cyclic. The third group is sometimes called cycloalkanes. [1] Very complicated structures are possible by combining linear, branch, cyclic alkanes.

Isomerism

C4 alkanes and cycloalkanes (left to right): n-butane and isobutane are the two C4H10 isomers; cyclobutane and methylcyclopropane are the two C4H8 isomers. Saturated C4 hydrocarbons ball-and-stick (C4H10 C4H8).png
C4 alkanes and cycloalkanes (left to right): n-butane and isobutane are the two C4H10 isomers; cyclobutane and methylcyclopropane are the two C4H8 isomers.
Saturated C4 hydrocarbons ball-and-stick (C4H6).png
Bicyclo[1.1.0]butane is the only C4H6 alkane and has no alkane isomer.
Tetrahedrane-3D-balls.png
Tetrahedrane is the only C4H4 alkane and also 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: [3]

Branched alkanes can be chiral. For example, 3-methylhexane and its higher homologues are chiral due to their stereogenic 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 called cycloalkanes, and are also excluded from the above list because changing the number of rings changes the molecular formula. For example, cyclobutane and methylcyclopropane 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. [4]

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, 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 the hydrocarbons CnH2n+2, CnH2n, CnH2n−2, CnH2n−4, CnH2n−6. [6] In modern nomenclature, the first three specifically name hydrocarbons with single, double and triple bonds; [7] while "-one" now represents a ketone.

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) Isopentane-numbered-3D-balls.png
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]

Comparison of nomenclatures for three isomers of C5H12
Common namen-pentaneisopentaneneopentane
IUPAC namepentane2-methylbutane2,2-dimethylpropane
Structure Pentane-2D-Skeletal.svg Isopentane-2D-skeletal.svg Neopentane-2D-skeletal.png

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:

Non-IUPAC

Some non-IUPAC trivial names are occasionally used:

Physical properties

All alkanes are colorless. [14] [15] Alkanes with the lowest molecular weights are gases, those of intermediate molecular weight are liquids, and the heaviest are waxy solids. [16] [17]

Table of alkanes

AlkaneFormulaBoiling point [note 1]
[°C]
Melting point [note 1]
[°C]
Density [note 1]
[kg/m3] (at 20 °C)
Isomers [note 2]
Methane CH4−162−1820.656 (gas)1
Ethane C2H6−89−1831.26 (gas)1
Propane C3H8−42−1882.01 (gas)1
Butane C4H100−1382.48 (gas)2
Pentane C5H1236−130626 (liquid)3
Hexane C6H1469−95659 (liquid)5
Heptane C7H1698−91684 (liquid)9
Octane C8H18126−57703 (liquid)18
Nonane C9H20151−54718 (liquid)35
Decane C10H22174−30730 (liquid)75
Undecane C11H24196−26740 (liquid)159
Dodecane C12H26216−10749 (liquid)355
Tridecane C13H28235−5.4756 (liquid)802
Tetradecane C14H302535.9763 (liquid)1858
Pentadecane C15H3227010769 (liquid)4347
Hexadecane C16H3428718773 (liquid)10,359
Heptadecane C17H3630322777 (solid)24,894
Octadecane C18H3831728781 (solid)60,523
Nonadecane C19H4033032785 (solid)148,284
Icosane C20H4234337789 (solid)366,319
Triacontane C30H6245066810 (solid)4,111,846,763
Tetracontane C40H8252582817 (solid)62,481,801,147,341
Pentacontane C50H10257591824 (solid)1,117,743,651,746,953,270
Hexacontane C60H122625100829 (solid)2.21587345357704×1022
HeptacontaneC70H142653109869 (solid)4.71484798515330×1026
  1. 1 2 3 Physical properties of the straight-chain isomer
  2. Total number of constitutional isomers for this molecular formula

Boiling point

Melting (blue) and boiling (orange) points of the first 16 n-alkanes in degC. AlkaneBoilingMeltingPoint.png
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:

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

sp -hybridization in methane. Ch4 hybridization.svg
sp -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. Ch4-structure.png
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. Newman projection ethane.png
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 Ethane-rotamers-3D-balls.png
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]

Infrared spectroscopy

The C-H stretching mode gives a 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. [24] 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 δH = 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 of nuclear Overhauser effect and the long relaxation time, and can be missed in weak samples, or samples that have not been run for a sufficiently long time.

Mass spectrometry

Since alkanes have high ionization energies, their electron 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 resulting free radicals. The mass spectra for straight-chain alkanes is illustrated by that for dodecane: 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. [25]

Chemical properties

Alkanes are only weakly reactive with most chemical compounds. They only reacts 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 term paraffins (with the meaning here of "lacking affinity"). In crude oil the alkane molecules have remained chemically unchanged for millions of years.

Acid-base behavior

The acid 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 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:

CnH2n+2 + (3/2n + 1/2) O2 → (n + 1) H2O + n CO2
or CnH2n+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:

CnH2n+2 + (n + 1/2)  O2 → (n + 1) H2O + n  CO
CnH2n+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, 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.

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:

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 Monobromination of propane.png
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.[ citation needed ]

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

Occurrence of alkanes in the Universe

Methane and ethane make up a tiny proportion of Jupiter's atmosphere Jupiter.jpg
Methane and ethane make up a tiny proportion of Jupiter's atmosphere
Extraction of oil, which contains many distinct hydrocarbons including alkanes Oil well.jpg
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. Rotbuntes Rind.jpg
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]

Applications

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.

Hazards

Alkanes are highly flammable, but they have low toxicities. Methane "is toxicologically virtually inert." Alkanes can be asphyxiants and narcotic. [38]

See also

Related Research Articles

<span class="mw-page-title-main">Aliphatic compound</span> Hydrocarbon compounds without aromatic rings

In organic chemistry, hydrocarbons are divided into two classes: aromatic compounds and aliphatic compounds. Aliphatic compounds can be saturated like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds, whether straight or branched, and which contain no rings of any type, are always aliphatic. Cyclic compounds can be aliphatic if they are not aromatic.

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

<span class="mw-page-title-main">Functional group</span> Group of atoms giving a molecule characteristic properties

In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the rest of the molecule's composition. This enables systematic prediction of chemical reactions and behavior of chemical compounds and the design of chemical synthesis. The reactivity of a functional group can be modified by other functional groups nearby. Functional group interconversion can be used in retrosynthetic analysis to plan organic synthesis.

<span class="mw-page-title-main">Hydrocarbon</span> Organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may be similar to that of gasoline or lighter fluid. They occur in a diverse range of molecular structures and phases: they can be gases, liquids, low melting solids or polymers.

In chemistry, a structural isomer of a compound is another compound whose molecule has the same number of atoms of each element, but with logically distinct bonds between them. The term metamer was formerly used for the same concept.

<span class="mw-page-title-main">Cycloalkane</span> Saturated alicyclic hydrocarbon

In organic chemistry, the cycloalkanes are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring, and all of the carbon-carbon bonds are single. The larger cycloalkanes, with more than 20 carbon atoms are typically called cycloparaffins. All cycloalkanes are isomers of alkenes.

In organic chemistry, an alkyl group is an alkane missing one hydrogen. The term alkyl is intentionally unspecific to include many possible substitutions. An acyclic alkyl has the general formula of −CnH2n+1. A cycloalkyl group is derived from a cycloalkane by removal of a hydrogen atom from a ring and has the general formula −CnH2n−1. Typically an alkyl is a part of a larger molecule. In structural formulae, the symbol R is used to designate a generic (unspecified) alkyl group. The smallest alkyl group is methyl, with the formula −CH3.

<span class="mw-page-title-main">Open-chain compound</span> Type of organic molecule with a linear structure

In chemistry, an open-chain compound or acyclic compound is a compound with a linear structure, rather than a cyclic one. An open-chain compound having no side groups is called a straight-chain compound. Many of the simple molecules of organic chemistry, such as the alkanes and alkenes, have both linear and ring isomers, that is, both acyclic and cyclic. For those with 4 or more carbons, the linear forms can have straight-chain or branched-chain isomers. The lowercase prefix n- denotes the straight-chain isomer; for example, n-butane is straight-chain butane, whereas i-butane is isobutane. Cycloalkanes are isomers of alkenes, not of alkanes, because the ring's closure involves a C-C bond. Having no rings, all open-chain compounds are aliphatic.

In chemical nomenclature, the IUPAC nomenclature of organic chemistry is a method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). It is published in the Nomenclature of Organic Chemistry. Ideally, every possible organic compound should have a name from which an unambiguous structural formula can be created. There is also an IUPAC nomenclature of inorganic chemistry.

<span class="mw-page-title-main">Pentane</span> Alkane with 5 carbon atoms

Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the n-pentane isomer, in which case pentanes refers to a mixture of them; the other two are called isopentane (methylbutane) and neopentane (dimethylpropane). Cyclopentane is not an isomer of pentane because it has only 10 hydrogen atoms where pentane has 12.

In organic chemistry, a substituent is one or a group of atoms that replaces atoms, thereby becoming a moiety in the resultant (new) molecule.

In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism.

In organic chemistry, a homologous series is a sequence of compounds with the same functional group and similar chemical properties in which the members of the series can be branched or unbranched, or differ by molecular formula of CH2 and molecular mass of 14u. This can be the length of a carbon chain, for example in the straight-chained alkanes (paraffins), or it could be the number of monomers in a homopolymer such as amylose. A homologue is a compound belonging to a homologous series.

<span class="mw-page-title-main">Conformational isomerism</span> Different molecular structures formed only by rotation about single bonds

In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds. While any two arrangements of atoms in a molecule that differ by rotation about single bonds can be referred to as different conformations, conformations that correspond to local minima on the potential energy surface are specifically called conformational isomers or conformers. Conformations that correspond to local maxima on the energy surface are the transition states between the local-minimum conformational isomers. Rotations about single bonds involve overcoming a rotational energy barrier to interconvert one conformer to another. If the energy barrier is low, there is free rotation and a sample of the compound exists as a rapidly equilibrating mixture of multiple conformers; if the energy barrier is high enough then there is restricted rotation, a molecule may exist for a relatively long time period as a stable rotational isomer or rotamer. When the time scale for interconversion is long enough for isolation of individual rotamers, the isomers are termed atropisomers. The ring-flip of substituted cyclohexanes constitutes another common form of conformational isomerism.

<span class="mw-page-title-main">Eclipsed conformation</span> Molecular form in which substituents on two adjacent atoms are closest together

In chemistry an eclipsed conformation is a conformation in which two substituents X and Y on adjacent atoms A, B are in closest proximity, implying that the torsion angle X–A–B–Y is 0°. Such a conformation can exist in any open chain, single chemical bond connecting two sp3-hybridised atoms, and it is normally a conformational energy maximum. This maximum is often explained by steric hindrance, but its origins sometimes actually lie in hyperconjugation.

<span class="mw-page-title-main">Ring strain</span> Instability in molecules with bonds at unnatural angles

In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealized value of approximately 109°. Because of their high strain, the heat of combustion for these small rings is elevated.

<span class="mw-page-title-main">Neopentane</span> Chemical compound

Neopentane, also called 2,2-dimethylpropane, is a double-branched-chain alkane with five carbon atoms. Neopentane is a flammable gas at room temperature and pressure which can condense into a highly volatile liquid on a cold day, in an ice bath, or when compressed to a higher pressure.

<span class="mw-page-title-main">Binary silicon-hydrogen compounds</span>

Silanes are saturated chemical compounds with the empirical formula SixHy. They are hydrosilanes, a class of compounds that includes compounds with Si−H and other Si−X bonds. All contain tetrahedral silicon and terminal hydrides. They only have Si−H and Si−Si single bonds. The bond lengths are 146.0 pm for a Si−H bond and 233 pm for a Si−Si bond. The structures of the silanes are analogues of the alkanes, starting with silane, SiH4, the analogue of methane, continuing with disilane Si2H6, the analogue of ethane, etc. They are mainly of theoretical or academic interest.

Group 14 hydrides are chemical compounds composed of hydrogen atoms and group 14 atoms.

<span class="mw-page-title-main">2,3-Dimethylpentane</span> Chemical compound

2,3-Dimethylpentane is an organic compound of carbon and hydrogen with formula C
7
H
16
, more precisely CH
3
CH(CH
3
)
CH(CH
3
)
CH
2
CH
3
: a molecule of pentane with methyl groups –CH
3
replacing hydrogen atoms on carbon atoms 2 and 3. It is an alkane, a fully saturated hydrocarbon; specifically, one of the isomers of heptane.

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Further reading