An alkane in organic chemistry is a saturated hydrocarbon without cycles, that is, an acyclic hydrocarbon in which the molecule has the maximum possible number of hydrogen atoms and so has no double bonds. Alkanes are also often known as paraffins, or collectively as the paraffin series; these terms, however, are also used to apply only to alkanes whose carbon atoms form a single, unbranched chain; when this is done, branched-chain alkanes are called isoparaffins. Alkanes are aliphatic compounds.
The general formula for alkanes is CnH2n+2; the simplest possible alkane is therefore methane, CH4. The next simplest is ethane, C2H6; the series continues indefinitely. Each carbon atom in an alkane has sp3 hybridization.
- 1 Isomerism
- 2 Nomenclature of alkanes
- 3 Occurrence
- 4 Purification and use
- 5 Preparation
- 6 Molecular geometry
- 7 Properties
- 8 Reactions
- 9 Hazards
- 10 Alkanes in nature
- 11 See also
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 (sequence A000602 in OEIS).
Nomenclature of alkanes
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:
Alkanes 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, C5H12; hexane, C6H14; heptane, C7H16; octane, C8H18; 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 one end 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 File: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:
The following nonsystematic names are retained in the IUPAC system:
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 are of great importance for other heavenly bodies: they are found, for example, 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.
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 developed when dead sea animals were covered with sediments to the exclusion of oxygen and converted over many millions years at high temperatures and high pressure to the respective natural substances. Natural gas resulted thereby for example from the following reaction:
- C6H12O6 → 3CH4 + 3CO2
They collected themselves in porous rocks, which were sealed by impermeable layers above. 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.
Purification and use
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 componants 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 the 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 number 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 nember, 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 to 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.
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 can be considered 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 an 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 which represent the bonds as being at right angles to one another, while both common and useful, do not correspond with the reality.
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, then the so-called Newman projection results: the circle represents the two carbon atoms, one behind the other, and the bonds to hydrogen are represented by the straight lines. 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:
- In the eclipsed conformation, corresponding to a torsion angle of 0°, 120° or 240°, the hydrogen atoms attached to the front carbon are directly in front of those attached to the rear carbon.
- In the staggered conformation, corresponding to a torsion angle of 60°, 180° or 300°, the hydrogen atoms attached to the front carbon are exactly in between those attached to the rear carbon.
The two conformations, also known as rotomers, differ in energy: the staggered conformation is 12.6 kJ/mol lower in energy (more stable) than the eclipsed conformation. The explanation for this difference in energy has been the subject of debate, with two main theories predominating:
- in the eclipsed conformation, the electrostatic repulsion between the electrons in the carbon–hydrogen bonds is maximised.
- in the staggered conformation, the hyperconjugation (a form of delocalisation) of the valence electrons is maximised.
These two explanations are not contradictory or exclusive; the latter is thought to be the more important for ethane itself.
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 CH3-group by 120° relative to the other, is of the order of 10−11 seconds.
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 fully eclipsed or synperiplanar conformation has a torsion angle of 0°. It is the configuration with the highest energy.
- The inclined conformation has a torsion angle of 60° (or 300°). It is a local energy minimum.
- The partially eclipsed conformation has a torsion angle of 120° (or 240°). It is a local energy maximum.
- The antiperiplanar conformation has a torsion angle of 180°. The two methyl groups are as far from each other as is possible, and this configuration has the lowest 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 alaknes 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.
The conformations of other organic molecules are based on those of alkanes, and are discussed in the relevant articles.
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 CH4 to C4H10 alkanes are gaseous; from C5H12 to C17H36 they are liquids; and after C18H38 they are solids. The boiling point increases between 20 and 30 °C per CH2-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 coexistance 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.
Alkanes generally show a relatively low reactivity, because their C–H and C–C bonds are relatively stable and cannot be easily broken. Unlike all 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 (−4) is reached. Reaction with oxygen leads to combustion; with halogens, substitution.
Free radicals, molecules with unpaired electrons, play a large role in most reactions of alkanes, such as cracking and reformation where long-chain alknes are converted into shorter-chain alkanes and straight-chain alknaes into branched-chain isomers.
In highly brached 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 hinderance, and can substantially increase the reactivity.
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 characreistic spectroscopic features.
The carbon–hydrogen stretching mode gives a strong absorption between 2850 and 2960 cm−1, while the carbon–carbon stretching mode absorbes 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.
The proton resonances of alkanes are usually found at δH = 0–1. The carbon-13 resonances depend on the number of hydrogen atoms attached to the carbon: δC = 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.
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 (M−15) is often absent, and other fragment are often spaced by intervals of fourteen mass units, corresponding to sequential loss of CH2-groups.
Reactions with oxygen
- 2CnH2n+2 + (3n+1)O2 → 2(n+1)H2O + 2nCO2
- 2CH4 + 3O2 → 2CO + 4H2O
- CH4 + O2 → C + 2H2O
Alkanes usually burn with a non-luminous flame with very little soot formation.
The standard enthalpy change of combustion, ΔcH
o, for alkanes increases by about 650 kJ/mol per CH2 group. Branched-chain alkanes have lower values of ΔcH o 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.
- Cl2 → 2Cl·
- 2. Propagation (two steps): a hydrogen atom is pulled off from methane then the methyl radical pulls a Cl· from Cl2.
- CH4 + Cl· → CH3· + HCl
- CH3· + Cl2 → CH3Cl + 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 CH2Cl2 may be formed.
- 3. Termination: recombination of two free radicals:
- Cl· + Cl· → Cl2; or
- CH3· + Cl· → CH3Cl; or
- CH3· + CH3· → C2H6.
- The last possibilty 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 CH2 (or CH) groups are preferentially replaced.
The reactivity of the different halogens varies considerably: the relative rates are: fluorine (108) > chlorine (1) > bromine (7×10−11) > iodine (2×10−22). Hence the reaction of alkanes with fluorine is difficult to control, that with chlorine is moderate to fast, that with bormine is slow and requires high levels of UV irradiation while the reaction with iodine is practically non-existent and thermodynamically unfavorable.
This reactions is 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 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.
Here is an example of cracking with butane CH3-CH2-CH2-CH3
- 1st possibility (48%): breaking is done on the CH3-CH2 bond.
CH3* / *CH2-CH2-CH3
after a certain number of steps, we will obtain an alkane and an alkene: CH4 + CH2=CH-CH3
- 2nd possibility (38%): breaking is done on the CH2-CH2 bond.
CH3-CH2* / *CH2-CH3
after a certain number of steps, we will obtain an alkane and an alkene from different types: CH3-CH3 + CH2=CH2
- 3rd possibility (14%): breaking of a C-H bond
after a certain number of steps, we will obtain an alkene and hydrogen gas: CH2=CH-CH2-CH3 + H2
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.
Methane is explosive in when mixed with air (1–8% CH4) 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 and heptane are classed as dangerous for the environment and harmful: octane is also classed as harmful. The straight chain isomer of hexane is a neurotoxin, and therefore rarely used commercially.
Alkanes in nature
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-nembered chains.
On the other hand certain archaea, the methanogens, produce large quantites of methane by the metabolism of carbon dioxide or other oxidised oranic compounds. The energy is released by the oxidation of hydrogen:
- CO2 + 4H2 → CH4 + 2H2O
Methanogens are also the producers marsh gas in wetlands, and release about two billion tonnes of methane per year—the atmospheric content of this gas was produced practically 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
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.
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, C19H40). 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 (C25H52), 3-methylpentaicosane (C26H54) and 9-methylpentaicosane (C26H54), they are transferred by body contact. With others like the tsetse fly Glossina morsitans morsitans, the pheromone contians 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, a useful characteristic for pest control.
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. Also, sand bees use pheromones for partner identification; in the case of A. nigroaenea, the females use a mixture of tricosane (C23H48), pentacosane (C25H52) and heptacosane (C27H56) in the ratio 3:3:1, and males are attracted by specifically this odour. The orchid takes advantage of this circumstance—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 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 disseminated after the frustrated departure of the male to different blooms.
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