Alkane
For saturated hydrocarbons containing one or more rings, see Cycloalkane.An
alkane is an acyclic
saturated hydrocarbon. In other words, an alkane is a long chain of carbon linked together by
single bonds. Alkanes are
aliphatic compounds.
The general formula for alkanes is
CnH2n+2; the simplest possible alkane is therefore
methane, CH
4. The next simplest is
ethane, C
2H
6; the series continues indefinitely. Each carbon atom in an alkane has sp³
hybridization.
Alkanes are also known as
paraffins, or collectively as the
paraffin series. These terms also used for alkanes whose carbon atoms form a single, unbranched chain. Such branched-chain alkanes are called
isoparaffins. Nearly all alkanes are
combustible.
The atoms in alkanes with more than three carbon atoms can be arranged in multiple ways, forming different
isomers. "Normal" alkanes have a linear, unbranched configuration. The number of isomers increases rapidly with the number of carbon atoms; for alkanes with 1 to 12 carbon atoms, the number of isomers equals 1, 1, 1, 2, 3, 5, 9, 18, 35, 75, 159, and 355, respectively .
The names of all alkanes end with
-ane.
Alkanes with unbranched carbon chains
The first four members of the series (in terms of number of carbon atoms) are named as follows::
methane, CH
4:
ethane, C
2H
6:
propane, C
3H
8:
butane, C
4H
10Alkanes with
five or more carbon atoms are named by adding the
suffix -ane to the appropriate
numerical multiplier with elision of a terminal
-a- from the basic numerical term. Hence,
pentane, C
5H
12;
hexane, C
6H
14;
heptane, C
7H
16;
octane, C
8H
18; etc. For a more complete list, see
List of alkanes.
Straight-chain alkanes are sometimes indicated by the prefix
n- (for
normal) to distinguish them from branched-chain alkanes having the same number of carbon atoms. 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-hexane is a
neurotoxin while its branched-chain isomers are not.
Alkanes with branched carbon chains
Branched alkanes are named as follows:
* Identify the longest straight chain of carbon atoms.
* Number the atoms in this chain, starting from 1 at the end nearer to the branching and counting upwards to the other end.
* Examine the groups attached to the chain in order and form their names.
* Form the name by looking at the different attached groups, and writing, for each group, the following:
** The number, or numbers, of the carbon atom, or atoms, where it is attached.
** The prefixes
di-,
tri-,
tetra-, etc. if the group is attached in 2, 3, 4, etc. places, or nothing if it is attached in only one place.
** The name of the attached group.
* The formation of the name is finished by writing down the name of the longest straight chain.
To carry out this algorithm, we must know how to name the substituent groups. This is done by the same method, except that instead of the longest chain of carbon atoms, the longest chain starting from the attachment point is used; also, the numbering is done so that the carbon atom next to the attachment point has the number 1.
For example, the compound
 |
Isobutane.png |
is the only 4-carbon alkane possible, apart from butane. Its formal name is 2-methylpropane.
Pentane, however, has two branched isomers, in addition to its linear, normal form:
 |
Dimethylpropane.png |
2,2-dimethylpropane
and
 |
2-methylbutane.png |
2-methylbutane.
Trivial names
The following nonsystematic names are retained in the
IUPAC system::
isobutane for 2-methylpropane:
isopentane for 2-methylbutane:
neopentane for 2,2-dimethylpropaneThe name
isooctane is very widely used in the
petrochemical industry to refer to
2,2,4-trimethylpentane.
|
Methane and ethane make up a large proportion of Jupiter's atmosphere |
Alkanes occur both on
Earth and in the solar system, however only the first hundred or so, and even then mostly only in traces. The light hydrocarbons, especially
methane and
ethane for example, have been detected both in the tail of the comet
Hyakutake and in some
meteorites such as
carbonaceous chondrites. They also form an important portion of the
atmospheres of the outer gas planets
Jupiter,
Saturn,
Uranus and
Neptune. On
Titan, the satellite of Saturn, it is believed that there were once large oceans of these and longer chain alkanes: smaller seas of liquid ethane are thought still to exist there.
Traces of methane (about 0.0001% or 1 ppm) occur in the Earth's atmosphere, produced primarily by forms of
Archaea. The content in the oceans is negligible due to the low solubility in water: however, at high pressures and low temperatures, methane can co-crystallize with water to form a solid
methane hydrate. Although they cannot be commercially exploited at the present time, the calorific value of the known methane hydrate fields exceeds the energy content of all the natural gas and oil deposits put together—methane extracted from methane hydrate is considered therefore a candidate for future fuels.
|
Extraction of alkanes in Ontario |
Today, the most important commercial sources for alkanes are clearly
natural gas and
oil, which are the only
organic compounds to occur as minerals in nature. Natural gas contains primarily methane and ethane, with some
propane and
butane: oil is a mixture of liquid alkanes and other hydrocarbons. Both were formed when dead marine animals and plants (zooplankton and phytoplankton) 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::C
6H
12O
6 â†' 3CH
4 + 3CO
2These hydrocarbons collected in porous rocks, trapped beneath an impermeable cap rock. In contrast to methane, which is constantly reformed in large quantities, higher alkanes rarely develop to a considerable extent in nature. The present deposits will not be reformed once they are exhausted.
Solid alkanes occur as
evaporation residues from oil, known as
tar. One of the largest natural deposits of solid alkanes is in the
asphalt lake known as the
Pitch Lake in
Trinidad and Tobago.
Alkanes are both important raw materials of the chemical industry and the most important fuels of the world economy.
The starting materials for the processing are always
natural gas and
crude oil. The latter is separated in an
oil refinery by
fractional distillation and processed into many different products, for example
gasoline. The different "fractions" of crude oil have different boiling points and can be isolated and separated quite easily: within the individual fractions the boiling points lie closely together.
The domain of usage of a certain alkane can be determined quite well according to the number of carbon atoms, although the following demarcation is idealized and not perfect. 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.
Propane and
butane can be liquefied at fairly low pressures, and are well known as
liquified petroleum gas (LPG). Propane, for example, is used in the propane gas burner, butane in disposable cigarette lighters (where the pressure is a mere 2
bar). The two alkanes are used as propellants in
aerosol sprays.
From
pentane to
octane the alkanes are highly volatile liquids. They are used as fuels in
internal combustion engines, as they vaporise easily on entry into the combustion chamber without forming droplets which would impair the unifomity 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 charaterised 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
fuel oil and
lubricating oil. In latter function they work at the same time as anti-corrosive agents, as their hydrophobic nature means that water cannot reach the metal surface. Many solid alkanes find use as
paraffin wax, 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.
Numerous ways exist to prepare alkanes in the laboratory. The most well known methods are
hydrogenation of
alkenes and
hydrolysis of
Grignard reagents. Alkanes can also be prepared directly from
alkyl halides in the
Corey-House-Posner-Whitesides reaction. The
Barton-McCombie deoxygenation removes hydroxyl groups from alcohols and the
Clemmensen reduction removes carbonyl groups from aldehydes and ketones to form alkanes.
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-hybridised, 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 109.47° between them.
Bond lengths and bond angles
An alkane molecule has only C–H and C–C single bonds. The former result from the overlap of a sp³-orbital of carbon with the 1s-orbital of a hydrogen; the latter by the overlap of two sp³-orbitals on different carbon atoms. The
bond lengths amount to 1.09×10
âˆ'10 m for a C–H bond and 1.54×10
âˆ'10 m for a C–C bond.
|
The tetrahedral structure of methane. |
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 which represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality.
Conformation
The structural formula and the
bond angles are not usually sufficient to completely describe the geometry of a molecule. There is a further
degree of freedom for each carbon–carbon bond: the
torsion angle between the atoms or groups bound to the atoms at each end of the bond. The spatial arrangement described by the torsion angles of the molecule is known as its
conformation.
|
Newman projections of the two conformations of ethane: eclipsed on the left, staggered on the right. |
Ethane forms the simplest case for studying the conformation of alkanes, as there is only one C–C bond. If one looks down the axis of the C–C bond, then one will see the so-called
Newman projection. The hydrogen atoms on both the front and rear carbon atoms have an angle of 120° between them, resulting from the projection of the base of the tetrahedron onto a flat plane. However the torsion angle between a given hydrogen atom attached to the front carbon and a given hydrogen atom attached to the rear carbon can vary freely between 0° and 360°. This is a consequence of the free rotation about a carbon–carbon single bond. Despite this apparent freedom, only two limiting conformations are important:
eclipsed conformation and
staggered conformation.
The two conformations, also known as
rotamers, differ in energy: The staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation.
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, albeit with short "pauses" at each staggered conformation. The time taken for an ethane molecule to pass from one staggered conformation to the next, equivalent to the rotation of one CH
3-group by 120° relative to the other, is of the order of 10
âˆ'11 seconds.
|
The four conformations of butane. From left to right: fully eclipsed, inclined, partially eclipsed, antiperiplanar. |
The situation with respect to the two C-C bonds in
propane is qualitatively similar to that of ethane: it is more complex, however, for
butane and higher alkanes.
If one takes the central C-C bond of butane as the reference axis, each of the two central carbon atoms is bound to two hydrogen atoms and a methyl group. Four different conformations can be defined by the torsion angle between the two methyl groups and, as in the case of ethane, each has its characteristic energy.
The difference in energy between the fully eclipsed conformation and the
antiperiplanar conformation is about 19 kJ/mol, and is therefore still relatively small at ambient temperature.
The case of higher alkanes is similar: the antiperiplanar conformation is always the most favoured 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 idealised 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.
Physical properties
 |
Melting (blue) and boiling (pink) points of the first 14 n-alkanes in °C. |
The molecular structure, particularly the
surface area of the molecule, determines the boiling point of the alkane: the smaller the surface, the lower the boiling point, as the
van der Waals forces between the molecules are weaker. A reduction of the surface area can be achieved by chain-branching or by a circular structure. This means in practice that alkanes with higher number of carbon atoms usually have higher
boiling points than those with lower numbers of carbon atoms, and that branched-chain alkanes and
cycloalkanes have lower boiling points than their straight-chain homologues. Under
standard conditions, from CH
4 to C
4H
10 alkanes are gaseous; from C
5H
12 to C
17H
36 they are liquids; and after C
18H
38 they are solids. The boiling point increases between 20 and 30 °C per CH
2-group.
The
melting points of the alkanes also rise with the increase in the number of carbon atoms (with only one exception,
propane). However the melting points rise more slowly than the boiling points, in particular for the higher alkanes. In addition, the melting points of alkanes with an odd number of carbon atoms increase faster than the melting points of alkanes with an even number of carbon atoms (see figure): the cause of this phenomenon is the higher
packing density of the alkanes with an even number of carbon atoms.
The melting points of branched-chain alkanes can be either higher or lower than those of the corresponding straight-chain alkanes, depending on the efficiency of molecular packing: this is particularly true for isoalkanes (2-methyl isomers), which often have melting points higher than those of their normal analogues.
Alkanes do not conduct
electricity, 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 minimised by minimising the contact between alkane and water: alkanes are said to be
hydrophobic in that they repel water.
Their solubility in nonpolar solvents is relatively good, a property which is called
lipophilicity. Different alkanes are, for example, miscible in all proportions among themselves.
The density of the alkanes usually increases with increasing number of carbon atoms, but remains less than that of water. Hence, alkanes form the upper layer in an alkane-water mixture.
Chemical properties
Alkanes generally show a relatively low reactivity, because their C–H and C–C bonds are relatively stable and cannot be easily broken. Unlike most other organic compounds, they possess no
functional groups.
They react only very poorly with ionic or other polar substances. The
pKa values of all alkanes are above 60, and so they are practically inert to acids and bases. This inertness is the source of the term
paraffins (
Latin para +
affinis, with the meaning here of "lacking affinity"). In
crude oil the alkane molecules have remained chemically unchanged for millions of years.
However
redox reactions of alkanes, in particular with
oxygen and the
halogens, are possible as the carbon atoms are in a strongly reduced condition; in the case of
methane, the lowest possible
oxidation state for carbon (âˆ'4) is reached. Reaction with oxygen leads to
combustion without any smoke; with halogens,
substitution. For more detailed information, see the
reactions section below. In addition, alkanes have been shown to interact with, and bind to, certain transition metal complexes.
Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as
cracking and
reformation where long-chain alkanes are converted into shorter-chain alkanes and straight-chain alkanes into branched-chain isomers.
In highly branched alkanes, the
bond angles may differ significantly from the optimal value (109.5°) in order to allow the different groups sufficient space. This causes a tension in the molecule, known as
steric hindrance, and can substantially increase the reactivity.
Thermochemistry
Alkanes are stable molecules relative to their constituent elements, which is manifested as a negative
heat of formation. For linear alkanes, each
methylene (CH
2) unit contributes -5 kcal/mol to the overal heat of formation. Branched alkanes are always a little bit more stable than their linear isomers; for example, 2-methylbutane is more stable than
n-pentane by 1.8 kcal/mol, and 2,2-methylpropane is more stable than
n-pentane by 5 kcal/mol.
See the
alkane heat of formation table for detailed data.
Spectroscopic properties
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.
Infrared spectroscopy
The carbon–hydrogen stretching mode gives a strong absorption between 2850 and 2960 cm
âˆ'1, while the carbon–carbon stretching mode absorbes between 800 and 1300 cm
âˆ'1. The carbon–hydrogen bending modes depend on the nature of the group: methyl groups show bands at 1450 cm
âˆ'1 and 1375 cm
âˆ'1, while methylene groups show bands at 1465 cm
âˆ'1 and 1450 cm
âˆ'1. Carbon chains with more than four carbon atoms show a weak absorption at around 725 cm
âˆ'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: δ
C = 8–30 (methyl), 15–55 (methylene), 20–60 (methyne). The carbon-13 resonance of quaternary carbon atoms is characteristically weak, due to the lack of
nuclear Overhauser enhancement and the long
relaxation time: it can be missed in routine spectra.
Mass spectrometry
Alkanes have a high
ionisation energy, and the molecular ion is usually weak. 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 fragment resulting from the loss of a single methyl group (Mâˆ'15) is often absent, and other fragment are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH
2-groups.
Reactions with oxygen
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::2C
nH
2n+2 + (3
n+1)O
2 â†' 2(
n+1)H
2O + 2
nCO
2In the absence of sufficient oxygen,
carbon monoxide or even
soot can be formed, as shown below for
methane::2CH
4 + 3O
2 â†' 2CO + 4H
2O:CH
4 + O
2 â†' C + 2H
2OAlkanes usually burn with a non-luminous flame with very little soot formation.
The
standard enthalpy change of combustion, Î"
cHo, for alkanes increases by about 650 kJ/mol per CH
2 group. Branched-chain alkanes have lower values of Î"
cHo 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
halogens in a so-called
halogenation reaction. The hydrogen atoms of the alkane are progressively replaced, or
substituted, by halogen atoms.
Free radicals are the reactive species which participate in the reaction, which usually leads to a mixture of products. The reaction is highly
exothermic, and can lead to an explosion.
The chain mechanism is as follows, using the chlorination of methane as a typical example::
1. Initiation: splitting of a chlorine molecule to form two chlorine atoms, initiated by
ultraviolet radiation. A chlorine atom has an unpaired electron and acts as a free radical.::Cl
2 â†' 2Cl·:
2. Propagation (two steps): a hydrogen atom is pulled off from methane then the methyl radical pulls a Cl· from Cl
2.::CH
4 + Cl· â†' CH
3· + HCl::CH
3· + Cl
2 â†' CH
3Cl + Cl·:This results in the desired product plus another chlorine radical. This radical will then go on to take part in another propagation reaction causing a chain reaction. If there is sufficient chlorine, other products such as CH
2Cl
2 may be formed.:
3. Termination: recombination of two free radicals:::Cl· + Cl· â†' Cl
2; or::CH
3· + Cl· â†' CH
3Cl; or::CH
3· + CH
3· â†' C
2H
6.:The last possibility in the termination step will result in an impurity in the final mixture; notably this results in an organic molecule with a longer carbon chain than the reactants.
In the case of methane or ethane, all the hydrogen atoms are equivalent and have an equal chance of being replaced. This leads to what is known as a
statistical product distribution. For propane and higher alkanes, the hydrogen atoms which form part of CH
2 (or CH) groups are preferentially replaced.
The reactivity of the different halogens varies considerably: the relative rates are:
fluorine (108) >
chlorine (1) >
bromine (7×10
âˆ'11) >
iodine (2×10
âˆ'22). Hence the reaction of alkanes with fluorine is difficult to control, that with chlorine is moderate to fast, that with bromine is slow and requires high levels of UV irradiation while the reaction with iodine is practically non-existent and
thermodynamically unfavorable.
These reactions are an important industrial route to halogenated hydrocarbons.
Cracking and reforming
"
Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method. The thermal cracking process follows a homolytic mechanism, that is, bonds break symmetrically and thus pairs of
free radicals are formed. 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 carbo
cation and the very unstable
hydride anion. 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 aself-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.
Here is an example of cracking with butane CH
3-CH
2-CH
2-CH
3* 1st possibility (48%): breaking is done on the CH
3-CH
2 bond.
CH
3* / *CH
2-CH
2-CH
3after a certain number of steps, we will obtain an alkane and an
alkene:CH
4 + CH
2=CH-CH
3* 2nd possibility (38%): breaking is done on the CH
2-CH
2 bond.
CH
3-CH
2* / *CH
2-CH
3after a certain number of steps, we will obtain an alkane and an
alkenefrom different types: CH
3-CH
3 + CH
2=CH
2* 3rd possibility (14%): breaking of a C-H bond
after a certain number of steps, we will obtain an
alkene and hydrogen gas: CH
2=CH-CH
2-CH
3 + H
2Other reactions
Alkanes will react with
steam in the presence of a
nickel catalyst to give
hydrogen. Alkanes can by
chlorosulfonated and
nitrated, although both reactions require special conditions. The
fermentation of alkanes to
carboxylic acids is of some technical importance. In the
Reed reaction,
sulfur dioxide,
chlorine and
light convert hydrocarbons to
sulfonyl chlorides.
Methane is explosive in when mixed with air (1–8% CH
4) and is a strong
greenhouse gas: other lower alkanes can also form explosive mixtures with air. The lighter liquid alkanes are highly flammable, although this risk decreases with the length of the carbon chain. Pentane, hexane, heptane and octane are classed as
dangerous for the environment and
harmful. The straight chain isomer of hexane is a
neurotoxin, and therefore rarely used commercially.
Although alkanes occur in nature in various way, they do not rank biologically among the essential materials. Cycloalkanes with 14 to 18 carbon atoms occur in
musk, extracted from
deer of the family
Moschidae. All further information refers to acyclic alkanes.
Bacteria and archaea
Certain types of
bacteria can metabolise alkanes: they prefer even-numbered carbon chains as they are easier to degrade than odd-numbered chains.
On the other hand certain
archaea, the
methanogens, produce large quantites of
methane by the metabolism of
carbon dioxide or other
oxidised organic compounds. The energy is released by the oxidation of
hydrogen::CO
2 + 4H
2 â†' CH
4 + 2H
2O
Methanogens are also the producers of
marsh gas in
wetlands, and release about two billion tonnes of methane per year—the atmospheric content of this gas is produced nearly exclusively by them. The methane output of
cattle and other
herbivores, which can release up to 150 litres per day, and of
termites, is also due to methanogens. They also produce this simplest of all alkanes in the
intestines of
humans. Methanogenic archaea are hence at the end of the
carbon cycle, with carbon being released back into the atmosphere after having been fixed by
photosynthesis. It is probable that our current deposits of
natural gas were formed in a similar way.
Fungi and plants
|
Water forms droplets on a thin film of alkane wax on the skin of the apple. |
Alkanes also play a role, if a minor role, in the biology of the three
eukaryotic groups of organisms:
fungi,
plants and
animals. Some specialised yeasts, e.g.
Candida tropicale,
Pichia sp.,
Rhodotorula sp., can use alkanes as a source of carbon and/or energy. The fungus
Amorphotheca resinae prefers the longer-chain alkanes in
aviation fuel, and can cause serious problems for aircraft in tropical regions.
In plants it is the solid long-chain alkanes that are found; they form a firm layer of wax, the
cuticle, over areas of the plant exposed to the air. This protects the plant against water loss, while preventing the
leaching of important minerals by the rain. It is also a protection against bacteria, fungi and harmful
insects—the latter sink with their legs into the soft waxlike substance and have difficulty moving. The shining layer on fruits such as apples consists of long-chain alkanes. The carbon chains are usually between twenty and thirty carbon atoms in length and are made by the plants from
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.
Animals
Alkanes are found in animal products, although they are less important than unsaturated hydrocarbons. On example is the shark liver oil, which is approximately 14%
pristane (2,6,10,14-tetramethylpentadecane, C
19H
40). Their occurrence is more important in
pheromones, chemical messenger materials, on which above all insects are dependent for communication. With some kinds, as the support beetle
Xylotrechus colonus, primarily
pentacosane (C
25H
52), 3-methylpentaicosane (C
26H
54) and 9-methylpentaicosane (C
26H
54), they are transferred by body contact. With others like the
tsetse fly Glossina morsitans morsitans, the pheromone contains the four alkanes 2-methylheptadecane (C
18H
38), 17,21-dimethylheptatriacontane (C
39H
80), 15,19-dimethylheptatriacontane (C
39H
80) and 15,19,23-trimethylheptatriacontane (C
40H
82), andacts by smell over longer distances, a useful characteristic for
pest control.
Ecological relations
|
Early spider orchid (Ophrys sphegodes) |
One example, in which both plant and animal alkanes play a role, is the ecological relationship between the
sand bee (
Andrena nigroaenea) and the
early spider orchid (
Ophrys sphegodes); the latter is dependent for
pollination on the former. Sand bees use pheromones in order to identify a mate; in the case of
A. nigroaenea, the females emit a mixture of
tricosane (C
23H
48),
pentacosane (C
25H
52) and
heptacosane (C
27H
56) in the ratio 3:3:1, and males are attracted by specifically this odour. 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 attempte to copulate with their imaginary partner: although this endeavour 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 different blooms.
*
Cycloalkane*
Higher alkanes*
Alkene*
Alkyne*
Functional group*
Cracking (chemistry)*
List of alkanes
