Hydrocarbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon.[1]:620 Hydrocarbons are examples of group 14 hydrides. Hydrocarbons from which one hydrogen atom has been removed are functional groups called hydrocarbyls.[2] Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structures, it is difficult to generalize further. Most anthropogenic emissions of hydrocarbons are from the burning of fossil fuels including fuel production and combustion. Natural sources of hydrocarbons such as ethylene, isoprene, and monoterpenes come from the emissions of vegetation.[3]

Ball-and-stick model of the methane molecule, CH4. Methane is part of a homologous series known as the alkanes, which contain single bonds only.

Types

As defined by IUPAC nomenclature of organic chemistry, the classifications for hydrocarbons are:

  1. Saturated hydrocarbons are the simplest of the hydrocarbon species. They are composed entirely of single bonds and are saturated with hydrogen. The formula for acyclic saturated hydrocarbons (i.e., alkanes) is CnH2n+2.[1]:623 The most general form of saturated hydrocarbons is CnH2n+2(1-r), where r is the number of rings. Those with exactly one ring are the cycloalkanes. Saturated hydrocarbons are the basis of petroleum fuels and are found as either linear or branched species. Substitution reaction is their characteristics property (like chlorination reaction to form chloroform). Hydrocarbons with the same molecular formula but different structural formulae are called structural isomers.[1]:625 As given in the example of 3-methylhexane and its higher homologues, branched hydrocarbons can be chiral.[1]:627 Chiral saturated hydrocarbons constitute the side chains of biomolecules such as chlorophyll and tocopherol.[4]
  2. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with double bond are called alkenes. Those with one double bond have the formula CnH2n (assuming non-cyclic structures).[1]:628 Those containing triple bonds are called alkyne. Those with one triple bond have the formula CnH2n−2.[1]:631
  3. Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring. 10% of total nonmethane organic carbon emission are aromatic hydrocarbons from the exhaust of gasoline-powered vehicles.[5]

Hydrocarbons can be gases (e.g. methane and propane), liquids (e.g. hexane and benzene), waxes or low melting solids (e.g. paraffin wax and naphthalene) or polymers (e.g. polyethylene, polypropylene and polystyrene).

The term 'aliphatic' refers to non-aromatic hydrocarbons. Saturated aliphatic hydrocarbons are sometimes referred to as 'paraffins'. Aliphatic hydrocarbons containing a double bond between carbon atoms are sometimes referred to as 'olefins'.

Simple hydrocarbons and their variations
Variations on hydrocarbons based on the number of carbon atoms
Number of
carbon atoms		 Alkane (single bond) Alkene (double bond) Alkyne (triple bond) Cycloalkane Alkadiene
1 Methane————
2 EthaneEthene (ethylene)Ethyne (acetylene)——
3 PropanePropene (propylene)Propyne (methylacetylene)CyclopropanePropadiene (allene)
4 ButaneButene (butylene)ButyneCyclobutaneButadiene
5 PentanePentenePentyneCyclopentanePentadiene (piperylene)
6 HexaneHexeneHexyneCyclohexaneHexadiene
7 HeptaneHepteneHeptyneCycloheptaneHeptadiene
8 OctaneOcteneOctyneCyclooctaneOctadiene
9 NonaneNoneneNonyneCyclononaneNonadiene
10 DecaneDeceneDecyneCyclodecaneDecadiene
11 UndecaneUndecballeneUndecyneCycloundecaneUndecadiene
12 DodecaneDodeceneDodecyneCyclododecaneDodecadiene

Usage
Oil refineries are one way hydrocarbons are processed for use. Crude oil is processed in several stages to form desired hydrocarbons, used as fuel and in other products.
Tank wagon 33 80 7920 362-0 with hydrocarbon gas at Bahnhof Enns (2018).

The predominant use of hydrocarbons is as a combustible fuel source. Methane is the predominant component of natural gas. The C6 through C10 alkanes, alkenes and isomeric cycloalkanes are the top components of gasoline, naphtha, jet fuel and specialized industrial solvent mixtures. With the progressive addition of carbon units, the simple non-ring structured hydrocarbons have higher viscosities, lubricating indices, boiling points, solidification temperatures, and deeper color. At the opposite extreme from methane lie the heavy tars that remain as the lowest fraction in a crude oil refining retort. They are collected and widely utilized as roofing compounds, pavement composition (bitumen), wood preservatives (the creosote series) and as extremely high viscosity shear-resisting liquids.

Some large-scale nonfuel applications of hydrocarbons begins with ethane and propane, which are obtained from petroleum and natural gas. These two gases are converted either to syngas[6] or to ethylene and propylene.[7][8] These two alkenes are precursors to polymers, including polyethylene, polystyrene, acrylates,[9][10][11] polypropylene, etc. Another class of special hydrocarbons is BTX, a mixture of benzene, toluene, and the three xylene isomers.[12] Global consumption of benzene, estimated at more than 40,000,000 tons (2009).[13]

Hydrocarbons are also prevalent in nature. Some eusocial arthropods, such as the Brazilian stingless bee, Schwarziana quadripunctata, use unique hydrocarbon "scents" in order to determine kin from non-kin. The chemical hydrocarbon composition varies between age, sex, nest location, and hierarchal position.[14]

There is also potential to harvest hydrocarbons from plants like Euphorbia lathyri and Euphorbia tirucalli as an alternative and renewable energy source for vehicles that use diesel.[15] Furthermore, endophytic bacteria from plants that naturally produce hydrocarbons have been used in hydrocarbon degradation in attempts to deplete hydrocarbon concentration in polluted soils.[16]

Reactions

The noteworthy feature of hydrocarbons is their inertness, especially for saturated members. Otherwise, three main types of reactions can be identified:

Free-radical reactions
Main article: Substitution reaction

Substitution reactions only occur in saturated hydrocarbons (single carbon–carbon bonds). Such reactions require highly reactive reagents, such as chlorine and fluorine. In the case of chlorination, one of the chlorine atoms replaces a hydrogen atom. The reactions proceed via free-radical pathways.

CH4 + Cl2 → CH3Cl + HCl
CH3Cl + Cl2 → CH2Cl2 + HCl

all the way to CCl4 (carbon tetrachloride)

C2H6 + Cl2 → C2H5Cl + HCl
C2H4Cl2 + Cl2 → C2H3Cl3 + HCl

all the way to C2Cl6 (hexachloroethane)

Substitution

Of the classes of hydrcarbons, aromatic compounds uniquely (or nearly so) undergo substitution reactions. The chemical process practiced on the largest scale is an example: the reaction of benzene and ethylene to give ethylbenzene.

Addition reactions
Main article: Addition reaction

Addition reactions apply to alkenes and alkynes. In this reaction a variety of reagents add "across" the pi-bond(s). Chlorine, hydrogen chloride, water, and hydrogen are illustrative reagents. Alkenes and some alkynes also undergo polymerization, alkene metathesis, and alkyne metathesis.

Oxidation
Main article: Combustion

Hydrocarbons are currently the main source of the world's electric energy and heat sources (such as home heating) because of the energy produced when they are combusted.[17] Often this energy is used directly as heat such as in home heaters, which use either petroleum or natural gas. The hydrocarbon is burnt and the heat is used to heat water, which is then circulated. A similar principle is used to create electrical energy in power plants.

Common properties of hydrocarbons are the facts that they produce steam, carbon dioxide and heat during combustion and that oxygen is required for combustion to take place. The simplest hydrocarbon, methane, burns as follows:

CH4 + 2 O2 → 2 H2O + CO2 + energy

In inadequate supply of air, carbon monoxide gas and water vapour are formed:

2 CH4 + 3 O2 → 2 CO + 4 H2O

Another example is the combustion of propane:

C3H8 + 5 O2 → 4 H2O + 3 CO2 + energy

And finally, for any linear alkane of n carbon atoms,

CnH2n+2 + 3n + 1/2 O2 → (n + 1) H2O + n CO2 + energy.

Partial oxidation characterizes the reactions of alkenes and oxygen. This process is the basis of rancidification and paint drying.

Origin
Natural oil spring in Korňa, Slovakia.

The vast majority of hydrocarbons found on Earth occur in petroleum, coal, and natural gas. Petroleum (literally "rock oil" – petrol for short) and coal are generally thought to be products of decomposition of organic matter. In contrast to petroleum, is coal, which is richer in carbon and poorer in hydrogen. Natural gas is the product of methanogenesis.[18][19]

A seemingly limitless variety of compounds comprise petroleum, hence the necessity of refineries. These hydrocarbons consist of saturated hydrocarbons, aromatic hydrocarbons, or combinations of the two. Missing in petroleum are alkenes and alkynes. Their production requires refineries. Petroleum-derived hydrocarbons are mainly consumed for fuel, but they are also the source of virtually all synthetic organic compounds, including plastics and pharmaceuticals. Natural gas is consumed almost exclusively as fuel. Coal is used as a fuel and as a reducing agent in metallurgy.

Abiological hydrocarbons

A small fraction of hydrocarbon found on earth is thought to be abiological.[20]

Some hydrocarbons also are widespread and abundant in the solar system. Lakes of liquid methane and ethane have been found on Titan, Saturn's largest moon, confirmed by the Cassini-Huygens Mission.[21] Hydrocarbons are also abundant in nebulae forming polycyclic aromatic hydrocarbon (PAH) compounds.[22]

Bioremediation

Bioremediation of hydrocarbon from soil or water contaminated is a formidable challenge because of the chemical inertness that characterize hydrocarbons (hence they survived millions of years in the source rock). Nonetheless, many strategies have been devised, bioremediation being prominent. The basic problem with bioremediation is the paucity of enzymes that act on them. Nonetheless the area has received regular attention.[23] Bacteria in the gabbroic layer of the ocean's crust can degrade hydrocarbons; but the extreme environment makes research difficult.[24] Other bacteria such as Lutibacterium anuloederans can also degrade hydrocarbons.[25] Mycoremediation or breaking down of hydrocarbon by mycelium and mushrooms is possible.[26][27]

Safety
Main article: Hydrocarbon poisoning

Hydrocarbons are generally of low toxicity, hence the widespread use of gasoline and related volatile products. Aromatic compounds such as benzene are narcotic and chronic toxins and are carcinogenic. Certain rare polycyclic aromatic compounds are carcinogenic. Hydrocarbons are highly flammable.

Environmental impact

Burning hydrocarbons as fuel, which produces carbon dioxide and water, is a major contributor to anthropogenic global warming. Hydrocarbons are introduced into the environment through their extensive use as fuels and chemicals as well as through leaks or accidental spills during exploration, production, refining, or transport of fossil fuels. Anthropogenic hydrocarbon contamination of soil is a serious global issue due to contaminant persistence and the negative impact on human health.[28]

When soil is contaminated by hydrocarbons, it can have a significant impact on its microbiological, chemical, and physical properties. This can serve to prevent, slow down or even accelerate the growth of vegetation depending on the exact changes that occur. Crude oil and natural gas are the two largest sources of hydrocarbon contamination of soil.[29]

See also

References
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