In organic chemistry,
an alkane, or paraffin (a still-used historical name that also
has other meanings),
is a saturated hydrocarbon. Alkanes consist only of hydrogen and carbon atoms and all bonds are single bonds.
Acyclic alkanes have the general chemical formula CnH2n+2. Alkanes
belong to a homologous series of organic compounds in which the members differ by a molecular mass
of 14.03u (mass of a methanediyl group,
—CH2—, one carbon atom of mass 12.01u, and two hydrogen atoms of
mass ≈1.01u each). There are two main commercial sources: crude oil
and natural gas.
Each carbon atom has 4 bonds (either
C-H or C-C bonds),
and each hydrogen atom is joined to a carbon atom (H-C bonds). A series of
linked carbon atoms is known as the carbon skeleton
or carbon backbone. The number of carbon atoms is used to define the size of
the alkane (e.g., C2-alkane).
An alkyl
group, generally abbreviated with the
symbol R, is a functional group or side-chain that, like an alkane, consists solely of
single-bonded carbon and hydrogen atoms, for example a methyl or ethyl group.
The simplest possible alkane (the
parent molecule) is methane, CH4. There is no limit to the number of carbon
atoms that can be linked together, the only limitation being that the molecule
is acyclic, is saturated, and is a hydrocarbon.
Saturated oils and waxes are examples of larger alkanes where the number of carbons
in the carbon backbone is greater than 10.
Alkanes are not very reactive and
have little biological activity. All alkanes are colourless and odourless. Alkanes can be
viewed as a molecular tree upon which can be hung the more biologically
active/reactive portions (functional groups)
of the molecule.
Structure classification
Saturated
hydrocarbons 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 - cyclic (general
formula C
nH
2n, n > 2) wherein the carbon backbone is linked so as to form a loop.
According
to the definition by IUPAC, the former two are
alkanes, whereas the third group is called cycloalkanes. Saturated
hydrocarbons can also combine any of the linear, cyclic (e.g., polycyclic) and
branching structures, and they are still alkanes (no general formula) as long
as they are acyclic (i.e., having no
loops).They also have single covalent bonds between their carbons.
Isomerism
Alkanes
with more than three carbon atoms can be arranged in various different 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:
- C1: methane only
- C2: ethane only
- C3: propane only
- C4: 2 isomers: n-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
- 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 chiral. For example 3-methylhexane and its higher homologues are chiral due to
their stereogenic
center
at carbon atom number 3. In addition to these isomers, the chain of carbon
atoms may form one or more loops. Such compounds are called cycloalkanes.
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".
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. Now, the first three
name hydrocarbons with single, double and triple bonds; "-one"
represents a ketone; "-ol"
represents an alcohol or OH group; "-oxy-" means an ether and refers to oxygen between two
carbons, so that methoxy-methane is the IUPAC name for dimethyl ether.
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.
Linear alkanes
Straight-chain
alkanes are sometimes indicated by the prefix 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-hexane or 2- or
3-methylpentane.
The
members of the series (in terms of number of carbon atoms) are named as
follows:
methane, CH4 -
one carbon and four hydrogen
ethane, C2H6
- two carbon and six 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 derived from methanol, ether, propionic acid and butyric acid, respectively.
Alkanes with five or more carbon atoms are named by adding the suffix -ane to the
appropriate numerical multiplier prefix 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 prefix is generally Greek, however alkanes with a carbon atom count
ending in nine, for example nonane, use the Latin prefix non-.
For a more complete list, see List of alkanes.
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:
- 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 root chain so that sum of the numbers assigned to each side group will be as low as possible
- 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
Cyclic alkanes
So-called
cyclic alkanes are, in the technical sense, not alkanes, but
cycloalkanes. They are hydrocarbons just like alkanes, but 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, 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 similar to substituted alkanes — the cycloalkane ring is
stated, and the substituents are according to their position on the ring, with
the numbering decided by Cahn-Ingold-Prelog rules.
Trivial 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.
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
with the same chemical
formula,
e.g., pentane and isopentane.
Examples
The
following trivial names are retained in the IUPAC system:
- isobutane for 2-methylpropane
- isopentane for 2-methylbutane
- neopentane for 2,2-dimethylpropane.
Physical properties
All
alkanes are colourless and odourless.
Table of alkanes
Alkane
|
Formula
|
Boiling
point [°C]
|
Melting
point [°C]
|
Density
[g·cm−3] (at 20 °C)
|
CH4
|
-162
|
-182
|
Gas
|
|
C2H6
|
-89
|
-183
|
Gas
|
|
C3H8
|
-42
|
-188
|
Gas
|
|
C4H10
|
0
|
-138
|
Gas
|
|
C5H12
|
36
|
-130
|
0.626
(liquid)
|
|
C6H14
|
69
|
-95
|
0.659
(liquid)
|
|
C7H16
|
98
|
-91
|
0.684
(liquid)
|
|
C8H18
|
126
|
-57
|
0.703
(liquid)
|
|
C9H20
|
151
|
-54
|
0.718
(liquid)
|
|
C10H22
|
174
|
-30
|
0.730
(liquid)
|
|
C11H24
|
196
|
-26
|
0.740
(liquid)
|
|
C12H26
|
216
|
-10
|
0.749
(liquid)
|
|
C16H34
|
287
|
18
|
0.773
(liquid)
|
|
C20H42
|
343
|
37
|
Solid
|
|
C30H62
|
450
|
66
|
Solid
|
|
C40H82
|
525
|
82
|
Solid
|
|
C50H102
|
575
|
91
|
Solid
|
|
C60H122
|
625
|
100
|
Solid
|
Boiling point
Alkanes
experience inter-molecular van
der Waals forces.
Stronger inter-molecular van der Waals forces give rise to greater boiling
points of alkanes.
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
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 almost a 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.
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 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. For the latter case,
two molecules 2,3-dimethylbutane can "lock" into each other better
than the cross-shaped 2,2-dimethylbutane, hence the greater van der Waals
forces.
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 point
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 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-number alkanes pack less well and so the "looser"
organized solid packing structure requires less energy to break apart.
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.
Conductivity and solubility
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 minimized by minimizing 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 that 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.
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(−⅓)
≈ 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 1.09×10−10 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 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.
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, 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 (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
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 different 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 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
Alkanes
have a high ionization
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 (M−15) is often absent, and
other fragment are often spaced by intervals of fourteen mass units,
corresponding to sequential loss of CH2-groups.
Chemical properties
Alkanes
are only weakly reactive with ionic and other polar substances. The acid dissociation constant (pKa) values of all alkanes
are above 60, hence they are practically inert to acids and bases (see: carbon acids). 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.
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 (−4) is reached.
Reaction with oxygen (if present in sufficient quantity to satisfy the
reaction stoichiometry) leads to combustion
without any smoke, producing carbon dioxide and water. Free radical halogenation reactions occur with halogens, leading
to the production of haloalkanes. In addition,
alkanes have been shown to interact with, and bind to, certain transition metal
complexes in (See: carbon-hydrogen bond activation).
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 angle 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.
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
+ (1.5n+0.5)O2 → (n+1)H2O + nCO2
or
CnH2n+2 + ((3n+1)/2)O2 → (n+1)H2O
+ nCO2
For
example methane
2CH4
+ 3O2 → 2CO + 4H2O
CH4
+ 1.5O2 → CO + 2H2O
See
the alkane heat of
formation table
for detailed data. The standard enthalpy change of combustion, ΔcHo,
for alkanes increases by about 650 kJ/mol per CH2 group.
Branched-chain alkanes have lower values of ΔcHo
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 free
radical halogenation reaction. The hydrogen atoms of the alkane are
progressively replaced by halogen atoms. Free-radicals are the reactive
species that participate in the reaction, which usually leads to a mixture of
products. The reaction is highly exothermic, and can lead to an
explosion.
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 step 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: [In the Figure
below, the Statistical Distribution should be 25% and 75%]
Cracking
Cracking
breaks larger molecules into smaller ones. This can be done with a thermal or
catalytic method. 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 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 a
self-propagating chain mechanism. The chain of reactions is eventually
terminated by radical or ion recombination.
Isomerization and reformation
Dragan
and his colleague were the first to report about isomerization in alkanes. 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. 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.
Other reactions
Alkanes
will react with steam in the presence of a
nickel catalyst to give hydrogen. Alkanes can be 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.
Nucleophilic Abstraction can be used to separate an alkane from
a metal. Alkyl groups can be transferred from one compound to another by transmetalation reactions.
Occurrence
Occurrence of alkanes in the Universe
Alkanes
form a small portion of the atmospheres of the outer gas planets such as Jupiter (0.1% methane,
0.0002% ethane), Saturn (0.2% methane,
0.0005% ethane), Uranus (1.99% methane,
0.00025% 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. 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. 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.
The
most important commercial sources for alkanes are natural gas and oil. 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
→ 3CH4 + 3CO2
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.
Methane
is also present in what is called biogas, produced by animals
and decaying matter, which is a possible 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.
On
the other hand, certain archaea, the methanogens, produce large
quantities of methane by the metabolism of
carbon
dioxide
or other oxidized organic compounds.
The energy is released by the oxidation of hydrogen:
CO2
+ 4H2 → CH4 + 2H2O
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 liters 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
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 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, the solid long-chain alkanes are found in the plant cuticle and epicuticular wax of many species, but
are only rarely major constituents. 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
twenty-seven and thirty-three carbon atoms in length 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.
More
volatile short-chain alkanes are also produced by and found in plant tissues.
The Jeffrey
pine
is noted for producing exceptionally high levels of n-heptane in its resin, for
which reason its distillate was designated as the zero point for one octane rating. Floral scents have
also long been known to contain volatile alkane components, and n-nonane is a significant
component in the scent of some roses. Emission of gaseous
and volatile alkanes such as ethane, pentane, and hexane by plants has also
been documented at low levels, though they are not generally considered to be a
major component of biogenic air pollution.
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.
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% pristane
(2,6,10,14-tetramethylpentadecane, C19H40). They are
important as pheromones, chemical messenger
materials, on which insects depend for communication. In some species, e.g. the
support beetle Xylotrechus
colonus,
pentacosane (C25H52),
3-methylpentaicosane (C26H54) and 9-methylpentaicosane (C26H54)
are transferred by body contact. With others like the tsetse fly Glossina
morsitans morsitans, the pheromone contains the four alkanes
2-methylheptadecane (C18H38),
17,21-dimethylheptatriacontane (C39H80),
15,19-dimethylheptatriacontane (C39H80) and
15,19,23-trimethylheptatriacontane (C40H82), and acts by
smell over longer distances. Waggle-dancing honey bees produce and release
two alkanes, tricosane and pentacosane.
Ecological relations
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 (C23H48),
pentacosane (C25H52)
and heptacosane (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 different blooms.
Production
Petroleum refining
As
stated earlier, the most important source of alkanes is natural gas and crude oil. Alkanes are
separated in an oil
refinery
by fractional
distillation
and processed into many different products.
Fischer-Tropsch
The
Fischer-Tropsch
process
is a method to synthesize liquid hydrocarbons, including alkanes, from carbon monoxide and hydrogen. This
method is used to produce substitutes for 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
non-reactive chemically or biologically, and do not undergo functional group interconversions cleanly. When
alkanes are produced in the laboratory, it is often a side-product of a
reaction. For example, the use of n-butyllithium as a strong base gives the conjugate
acid, n-butane as a side-product:
C4H9Li
+ H2O → C4H10 + LiOH
However,
at times it may be desirable to make a portion of a molecule into an alkane
like functionality (alkyl group) using the
above or similar methods. For example, an ethyl group is an alkyl group;
when this is attached to a hydroxy group, it gives ethanol, which is not an
alkane. To do so, the best-known methods are hydrogenation of alkenes:
Alkanes
or alkyl groups can also be prepared directly from alkyl halides in the Corey-House-Posner-Whitesides reaction. The Barton-McCombie deoxygenation removes hydroxyl
groups from alcohols e.g.
and
the Clemmensen
reduction removes carbonyl
groups from aldehydes and ketones to form alkanes or alkyl-substituted
compounds e.g.:
Applications
The
applications of a certain alkane can be determined quite well according to 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.
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 and as a fuel
for cars, butane in disposable
cigarette lighters. The two alkanes are used as propellants in aerosol sprays.
From
pentane to octane the alkanes are
reasonably 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 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 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.
Some
synthetic polymers such as polyethylene and polypropylene are alkanes with
chains containing hundreds of 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
When
released in the environment, alkanes don't undergo rapid biodegradation,
because they have no functional groups (like hydroxyl or carbonyl) that are needed by
most organisms in order to metabolize the compound.
However,
some bacteria can metabolize some alkanes (especially those linear and short),
by oxidizing the terminal carbon
atom. The product is an alcohol, that could be next
oxidized to an aldehyde, and finally to a carboxylic acid. The resulting fatty acid could be metabolized
through the fatty
acid degradation
pathway.
Hazards
Methane
is explosive when mixed with air (1 – 8% CH4). 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.
Considerations
for detection /risk control:
- Methane is lighter than air (possibility of accumulation under roofs)
- Ethane is slightly heavier than air (possibility of pooling at ground levels / pits)
- Propane is heavier than air (possibility of pooling at ground levels / pits)
- Butane is heavier than air (possibility of pooling at ground levels / pits)
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