Alkanes

Alkanes(also known as paraffins) are acyclic saturated hydrocarbons, consisting of carbon and hydrogen atoms arranged in a linear or branched chain. Alkanes are considered to be the simplest organic compounds, because in their structure only simple carbon-hydrogen and carbon-carbon bonds are found, and the carbon atoms are in the sp3 hybridization state. Alkanes have the general formula CnH2n+2, and this class of hydrocarbons begins with the simplest organic compound, methane (CH4), for which n = 1. So, the first representative of the homologous series is methane, and the next three terms of the homologous series are: ethane (C2H6), propane ( C3H8) and butane (C4H10).

Alkanes are often regarded as basic compounds in organic chemistry, since most organic compounds can be regarded as derivatives of alkanes, into which various functional groups are grafted. The name alkyl group, generally symbolized R, refers to that group that comes structurally from the chain of an alkane. Higher alkanes (those with a chain consisting of more than 17 carbon atoms) represent a separate category of alkanes, as they are solid compounds under standard conditions of temperature and pressure. Alkanes are not very reactive and have limited biological activity. The main commercial sources of alkanes are oil and natural gas.

Nomenclature

The IUPAC nomenclature for alkanes is based on the identification of hydrocarbon chains. Linear, unbranched, saturated chains are named with the help of numerical prefixes of Greek origin, indicating the number of carbon atoms, to which the suffix -an is added. In 1866, August Wilhelm von Hofmann suggested the systematic nomenclature based on the user of the vowels a, e, i, o and u, for creating the suffixes -an, -en, -in, -on, -un for hydrocarbons with the general formulas CnH2n+2, CnH2n, CnH2n−2, CnH2n−4, CnH2n−6.

Below is a table of the prefixes used to name the first alkanes, and some suitable higher alkanes as examples:

The number of carbon atoms 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 20 30 40 50
Prefix Met Et Prop But Pent Hex Hept Oct Non Dec Undec Dodec Tridec Tetradec Pentadec Hexadec Eicos Triacont Tetracont Pentacont

Linear alkanes

Linear alkanes are often named with the prefix n-, which comes from normal. Although this prefix can be omitted, it should be used when it is necessary to differentiate between a particular linear alkane and its chain isomers (eg, n-hexane and 2- or 3-methylpentane). The first four alkanes of this series have special names (methane, ethane, propane, butane). The names have been retained for their historical value, coming from methanol, ether, propionic acid, and butyric acid. The following linear alkanes are named according to the rules already discussed, by adding the prefix indicating the number of carbon atoms and the suffix -an (pentane, hexane, heptane, etc.).

Branched alkanes

Branched alkanes with a simple structure often have common names, the distinction being made with the help of a prefix; for example, the isomers of n-pentane are isopentane and neopentane. However, for those with a more complex structure, the IUPAC nomenclature is used.

Isomerism and radicals

Being simple compounds, alkanes show few types of isomerism, however, even in their case quite interesting situations can arise. Alkanes have chain isomers, conformational isomers (or conformers), and some can even be chiral, such as 3-methylhexane.

Chain isomerism

Alkanes that have at least four carbon atoms show structural isomers, as the chain can be shaped in different ways for the same given molecular chemical formula. The simplest isomer of such an alkane is called a normal-alkane (often denoted n-alkane) and is that isomer that shows the linear chain, without any branching (by branching we mean any alkyl group grafted onto the longest chain). When branching occurs, the isomers are called iso-alkanes. The number of possible isomers for an alkane increases in direct proportion to the number of carbon atoms in the chain. For example, for acyclic alkanes we have:

C1:methane is the only one
C2:ethane is the only one
C3:propane is the only one
C4:we have 2 isomers, n-butane and isobutane
C5:we have 3 isomers, n-pentane, isopentane and neopentane
C6:we have 5 isomers, n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane and 2,3-dimethylbutane
C12:we have 355 isomers
C32:we have 27,711,253,769 isomers
C60:we have 22,158,734,535,770,411,074,184 isomers

Conformational isomerism

Structural formulas and bond angles are often not sufficient to fully describe the geometry of a molecule. There is a certain degree of freedom for each carbon-carbon bond: the torsion angle between the atoms or groups of atoms connected to the atoms at each end of the bond. The spatial arrangement described by molecular torsion angles is known as conformation.

Ethane is the simplest compound in the series of alkanes that exhibits conformational isomerism, since there is only one carbon-carbon bond in its molecule. If, hypothetically, one could look towards the axis of the C-C bond, one would see what is described by a Newman projection. Between the hydrogen atoms bound to the carbon atoms (near and far) there is an angle of 120°, which can also be seen if the molecule is projected on a plane. However, the torsion angles between any hydrogen atom bonded to a nearby carbon atom and a hydrogen atom bonded to a distant carbon atom can vary between 0° and 360°. This is a consequence of the fact that the single carbon-carbon bond exhibits free rotation. Despite this apparent free rotation, in reality there are only two important conformations: eclipsed and intercalated. These conformations show different energies, the lowest being for the intercalated one of 12.6 kJ/mol, indicating that it is more stable than the eclipsed conformation.

The difference between the energies of the two conformations, known as torsional energies, is small compared to the thermal energy of the ethane molecule under normal conditions. There is continuous rotation about the carbon-carbon bond, and the time required (in the case of ethane) to go from the intercalated conformation to the next conformation, equivalent to rotating one CH3 group 120° relative to each other, is on the order of 10−11 seconds.

Radicals

By removing one or more hydrogen atoms from an alkane molecule, a hydrocarbon radical is obtained. Conventionally, the valence line (CH3-) is used to represent radicals; this symbolizes the odd electron and not an electron pair as in regular writing. The naming of hydrocarbon radicals, obtained by removing hydrogen atoms from a single carbon atom, is done by replacing the suffix -an with the suffix -il for monovalent radicals (obtained by removing a single hydrogen atom), -ylidene for divalent ones: CH4 methane CH3- methyl -CH2- methylidene / methylene - CH- methylidine / methine; R-CH-alkylidene.

The name of divalent radicals, obtained by removing hydrogen atoms from different carbon atoms is formed by adding the suffix -idiyl to the name of the alkane. The monovalent radicals of alkanes are generically called alkyl radicals.

Methods of obtaining

Industrial methods

One of the main industrial sources of alkanes is natural gas and oil. Separation of alkanes occurs in oil refineries following the fractional distillation refining process.

One of the processes is known as the Bergius process, which involves the direct liquefaction of coal into a mixture of liquid hydrocarbons, called synthetic petroleum.
Another very useful process is the Fischer-Tropsch process, useful for the synthesis of liquid hydrocarbons, including liquid alkanes, with carbon monoxide and hydrogen as precursors.

Laboratory methods

Laboratory synthesis of alkanes is usually not necessary because they can be commercialized. Also, due to their low reactivity, they do not undergo many functional group interconversion reactions. Often, the methods by which alkanes are obtained in the laboratory are actually reactions in which alkanes are useless by-products. Below are some of the most well-known and relevant methods of obtaining alkanes.

An example would be the use of n-butyllithium as a base, when in the reaction with water its conjugate acid, n-butane, is obtained together with lithium hydroxide: C4H9Li + H2O → C4H10 + LiOH
Another possible method is the hydrogenation reaction (Senderen reaction and Sabatier reaction of hydrogenation of carbon dioxide[14]) which can be applied to several classes of unsaturated hydrocarbons (such as alkenes and alkynes). Hydrogenation reactions are always carried out with the help of a metal catalyst, nickel, platinum or palladium (R denotes an alkyl residue)
R-CH=CH2 + H2 → R-CH2-CH3 (hydrogenation of alkenes) R-C≡CH + 2H2 → R-CH2-CH3 (hydrogenation of alkynes)
The preparation of alkanes can be carried out directly following the Corey-House synthesis, which involves the reaction between an alkyl halide and an alkyl lithium dicuprate (the alkyl residues can be identical or different): Li+[R–Cu–R]– + R'–X → R–R' + "RCu" + Li+X–
There are three reduction reactions of aldehydes and ketones by which alkanes can be obtained: the Clemmensen reduction (performed with zinc metal and hydrochloric acid), the Wolff-Kishner reduction (performed with hydrazine in a basic medium), and the Mozingo reduction (performed with a dithiol, with cyclic dithioacetal intermediate which is reduced with Raney nickel): alcani
Reduction of other functional groups, such as alcohol or carboxyl, is done with red phosphorus.
The Wurtz reaction, whereby two alkyl halides (or the same halide, depending on the nature of the resulting alkane) are reacted in the presence of sodium; the obtained alkane contains the two alkyl groups from the reactants linked: alcani
Kolbe electrolysis is a decarboxylative dimerization reaction between two carboxylic acids (or two carboxylate ions) by which an alkane formed from the two residues corresponding to the acids is obtained: alcani
The Barton–McCombie deoxygenation reaction is a radical substitution reaction that helps convert an alcohol into the corresponding alkane: alcani

Properties

All alkanes are colorless and odorless compounds. Those with low molecular mass are gaseous (methane, ethane, propane and butane), those with intermediate mass are liquid (starting with pentane), and the higher ones are solid (paraffins).

Alkane Formula Boiling point[°C] Melting point[°C] Density[g/cm3] (at 20 °C)
Methane CH4 -162 -182 0,000656 (gas)
Ethan C2H6 −89 −183 0,00126 (gas)
Propane C3H8 −42 −188 0,00201 (gas)
Butane C4H10 0 −138 0,00248 (gas)
Pentane C5H12 36 −130 0,626(liquid)
Hexane C6H14 69 −95 0,659(liquid)
Heptane C7H16 98 −91 0,684(liquid)
Octane C8H18 126 −57 0,703(liquid)
Nonane C9H20 151 −54 0,718(liquid)
Decane C10H22 174 −30 0,730(liquid)
Undecane C11H24 196 -26 0,740(liquid)
Dodecane C12H26 216 −10 0,749(liquid)
Pentadecane C15H32 270 9,95 0,769(liquid)
Hexadecane C16H34 287 18 0,773(liquid)
Heptadecane C17H36 303 21,97 0,777(liquid)
Icosane C20H42 343 37 solid
Triacontane C30H62 450 66 solid
Tetracontane C40H82 525 82 solid
Pentacontane C50H102 575 91 solid
Hexacontane C60H10122 625 100 solid

Boiling point

Alkane molecules are involved in intermolecular van der Waals forces. The longer the chain, the stronger the interactions of this type, and thus the boiling points of the alkanes also increase.

There are two properties that determine the strength with which van der Waals interactions are manifested:

the number of electrons around the molecule, which increases with the molecular mass of the alkane
the total surface area of ​​the molecule

At standard temperature and pressure, the terms CH4 through C4H10 are gaseous compounds, C5H12 through C17H36 are liquid compounds, and starting with C18H38 are solid compounds. Considering that the boiling point of alkanes is mainly determined by molecular mass, it is not surprising that the boiling point has an approximately linear dependence on molecular mass. According to empirical determinations, the boiling point of an alkane can be said to be about 20–30 °C higher than for the lower term.

A straight chain alkane will have a higher boiling point than a branched alkane with the same number of carbon atoms. This fact is due to the larger contact surface between molecules in the first case, which leads to the possibility of forming more van der Waals forces between adjacent molecules.

Melting point

The melting points of branched alkanes can be either higher or lower than those of the corresponding normal-alkanes, but this strictly depends on the ability of the respective alkane to arrange itself structurally in the solid phase. This theory is particularly true for isoalkanes, which typically have higher melting points than their linear structural analogs.

Chemical properties

Alkanes, as the name paraffins (meaning without affinity) suggest, are very little reactive, and their reactivity is limited to weak chemical interactions only with ionic and polar compounds. The values ​​of acidity constants (pKa) for all alkanes are greater than 60, since they are practically inert to acids or bases). A good example of the lack of reactivity of alkanes is the fact that in oil these compounds have remained unchanged for several million years.

However, redox reactions of alkanes, in particular those with oxygen and halogens, are feasible, since the carbon atoms in the hydrocarbon chain are in a low oxidation state (ie in reduced form). In the case of methane, the carbon atom has the lowest oxidation state, with the value −4. Oxidation reactions with oxygen from air can lead to the formation of various products, and vary depending on the reaction conditions. If we have a sufficient and stoichiometric amount of oxygen, a simple combustion or burning reaction will occur, producing carbon dioxide and water. Radical halogenation reactions take place in the presence of halogens, and halogenated compounds or halo(geno)alkanes are obtained as products. Furthermore, it has been shown that alkanes can interact and bind with certain transition metal complexes in C-H bond activation processes.

The chemical species that play an important role in exemplifying the chemistry of alkanes are free radicals and molecules with unpaired electrons, being involved in catalytic conversion cracking and reforming reactions, that is, in the transformation of higher alkanes into lower alkanes and linear alkanes into branched alkanes.

Natural spread

In the universe

Alkanes make up a small fraction of the atmospheres of the gaseous outer planets Jupiter (0.1% methane, 2 ppm ethane), Saturn (0.2% methane, 5 ppm ethane), Uranus (1.99% methane, 2.5 ppm ethane) and Neptune (1.5% methane, 1.5 ppm ethane). Titan (1.6% methane), one of the satellites of the planet Saturn, was examined by the Huygens probe, and as a result of the determinations it was concluded that there is a periodic rain of liquid methane on the surface of the satellite. Also on Titan, the Cassini mission captured seasonal methane/ethane lakes near the satellite's polar regions. Methane and ethane respectively were detected in the tail of Comet Hyakutake. Chemical analysis indicated that these compounds had an approximately equal distribution, so it is believed that the icy part of the comet formed in interstellar space, far from the Sun, because otherwise these compounds would have evaporated easily. Alkanes were detected and in some meteorites.

On earth

The very small amount of gaseous methane (approximately 0.0002% or 1745 ppb) in the terrestrial atmosphere is largely produced by methanogenic microorganisms, through the process of methanogenesis. An example of such organisms are species from the Archaea domain (archaebacteria).

The most important commercial sources of alkanes are natural gas and oil. Natural gas contains mostly methane and ethane, with smaller amounts of propane and butane, and oil is a mixture of liquid alkanes and other hydrocarbons. These hydrocarbons were obtained when aquatic species of animals and plants (zooplankton and phytoplankton) died and sank to the bottom of the seas. By covering them with sediments and due to the oxygen-deprived environment, over millions of years and at a high temperature and pressure, their compaction took place into their current form. Natural gas resulted, through such a chemical process:

C6H12O6 → 3 CH4 + 3 CO2

Methane is also present in biogas, a mixture of gases produced by animals and decaying matter, and this is a possible source of renewable energy.

Alkanes have a very low solubility in water, so it can be considered that their presence in the oceans is negligible. However, at high temperatures and pressures (such as the ocean floor), methane can co-crystallize with water, forming a clathrate-type solid (methane hydrate). Although this phenomenon cannot be commercially exploited with today's technology , the amount of energy that can be released from all existing clathrate methane would be greater than the amount of energy that can be released from all existing deposits of natural gas and oil. Thus, it can be said that methane clathrate could be a viable fuel source in the future. Several extraction methods have recently begun to be developed.

In living organisms

Bacteria and archaebacteria

Certain types of bacteria are capable of metabolizing alkanes: they prefer chains with an even number of carbon atoms, being easier to degrade than those with an odd number of carbon atoms.

On the other hand, certain species of archaebacteria, the methanogenic ones, produce large amounts of methane as a result of the metabolism of carbon dioxide or other organic compounds in an oxidized state. Energy is released by the oxidation of hydrogen:

CO2 + 4 H2 → CH4 + 2 H2O

Fungi and plants

Alkanes also play a role, albeit a minor one, in the biology of fungi and plant species. Some yeast species, such as tropical Candida, Pichia sp., Rhodotorula sp., can use alkanes as a carbon or energy source. The fungus Amorphotheca resinae prefers long-chain alkanes, which can cause serious problems due to the degradation of aviation fuel in tropical areas.

Animals

Some alkanes are found in animal products, although they generally contain more unsaturated hydrocarbons. An example is shark liver oil, which contains about 14% pristane (2,6,10,14-tetramethylpentadecane, C19H40). Alkanes are important as pheromones, the chemical messengers that insects depend on for communication. In some species, such as Xylotrechus colonus, pentacosane (C25H52), 3-methylpentaicosan (C26H54) and 9-methylpentaicosan (C26H54) are transferred by direct contact. In the titmouse, Glossina morsitans morsitans, the pheromone contains four alkanes: 2-methylheptadecane (C18H38), 17,21-dimethylheptatriacontane (C39H80), 15,19-dimethylheptatriacontane (C39H80) and 15,19,23-trimethylheptatriacontane (C40H82).

Danger

Methane is a flammable and explosive gas. Inhaling it is also dangerous. As it is a colorless and odorless gas, special protective measures must be taken. Ethane is itself highly flammable, dangerous by inhalation and explosive. Both gases can cause suffocation. Similarly, propane is also flammable and explosive, and can induce drowsiness or cause loss of consciousness through inhalation. Butane has the same dangers as propane.