Hydrocarbon

  In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon or hydrogen are referred to as "Pure" hydrocarbons, whereas other hydrocarbons with bonded compounds or impurities of sulphur or nitrogen, are referred to as "impure", and remain somewhat erroneously referred to as hydrocarbons.

Hydrocarbons are referred to as consisting of a "backbone" or "skeleton" composed entirely of carbon and hydrogen and other bonded compounds, and lack a functional group that generally facilitates combustion without adverse effects. The majority of hydrocarbons found naturally occur in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen which, when bonded can catenate to form seemingly limitless chains.^[1][2]

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Types of hydrocarbons

The classifications for hydrocarbons defined by IUPAC nomenclature of organic chemistry are as follows:

1. Saturated hydrocarbons (alkanes) are the most simple of the hydrocarbon species and are composed entirely of single bonds and are saturated with hydrogen; they are the basis of petroleum fuels and are either found as linear or branched species of unlimited number. The general formula for saturated hydrocarbons is C_nH_{2n + 2} (assuming non-cyclic structures).
2. Unsaturated hydrocarbons have one or more double or triple bonds between carbon atoms. Those with one double bond are called alkenes, with the formula C_nH_2n (assuming non-cyclic structures). Those containing triple bonds are called alkynes.
3. Cycloalkanes are hydrocarbons containing one or more carbon rings to which hydrogen atoms are attached. The general formula for a saturated hydrocarbon containing one ring is C_nH_2n
4. Aromatic hydrocarbons, also known as arenes which have at least one aromatic ring

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).

General properties

Because of differences in molecular structure, the empirical formula remains different between hydrocarbons; in linear, or "straight-run" alkanes, alkenes and alkynes, the amount of bonded hydrogen lessens in alkenes and alkynes due to the "self-bonding" or catenation of carbon preventing entire saturation of the hydrocarbon by the formation of double or triple bonds.

This inherent ability of hydrocarbons to bond to themselves is referred to as catenation, and allows hydrocarbon to form more complex molecules, such as cyclohexane, and in rarer cases, arenes such as benzene. This ability comes from the fact that bond character between carbon atoms is entirely non-polar, in that the distribution of electrons between the two elements is somewhat even due to the same electronegativity values of the elements (~0.30), and does not result in the formation of an electrophile.

Generally, with catenation comes the loss of the total amount of bonded hydrocarbons and an increase in the amount of energy required for bond cleavage due to strain exerted upon the molecule; in molecules such as cyclohexane, this is referred to as ring strain, and occurs due to the "destabilized" spatial electron configuration of the atom.

In simple chemistry, as per valence bond theory, the carbon atom must follow the "4-hydrogen rule", which states that the maximum number of atoms available to bond with carbon is equal to the number of electrons that are attracted into the outer shell of carbon. In terms of shells, carbon consists of an incomplete outer shell, which comprises 4 electrons, and thus has 4 electrons available for covalent or dative bonding.

Simple hydrocarbons and their variations

Burning hydrocarbons

Hydrocarbons are one of the Earth's most important energy resources. 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 burnt. Often this energy is used directly as heat such as in home heaters, which use either oil 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 electric energy in power plants. Hydrocarbons (usually coal) are burnt and the energy released in this way is used to turn water into steam, which is used to turn a turbine that generates energy.

In an ideal reaction, the waste would be only water and carbon dioxide, but because the coal is not pure or clean there are often many toxic byproducts such as mercury and arsenic. Also, incomplete combustion causes the production of carbon monoxide (CO) which is toxic to humans due to its tendency to bind to hemoglobin molecules in the bloodstream. Once bound, CO does not allow oxygen to be carried by hemoglobin and can result in hypoxia. Incomplete combustion also has a byproduct of carbon in the form of soot.

As methane only releases one carbon dioxide for two water molecules, it is considered the cleanest fuel.

Mostly in response to climate concerns, clean coal technology is currently under development. For example, the UK and the People's Republic of China have signed an agreement to develop such technology with carbon dioxide emissions capture and storage in both China and the EU by 2020. Similar research is being conducted in the U.S. and other countries. Increasing evidence links the use of Hydrocarbons in the form of fossil fuels to environmental pollution and Global Warming.

Petroleum

Main article: Petroleum

Liquid geologically-extracted hydrocarbons are referred to as petroleum (literally "rock oil") or mineral oil, while gaseous geologic hydrocarbons are referred to as natural gas. All are significant sources of fuel and raw materials as a feedstock for the production of organic chemicals and are commonly found in the Earth's subsurface using the tools of petroleum geology.

The extraction of liquid hydrocarbon fuel from a number of sedimentary basins has been integral to modern energy development. Hydrocarbons are mined from tar sands, oil shale and potentially extracted from sedimentary methane hydrates. These reserves require distillation and upgrading to produce synthetic crude and petroleum.

Oil reserves in sedimentary rocks are the principal source of hydrocarbons for the energy, transport and petrochemical industries. Hydrocarbons are of prime economic importance because they encompass the constituents of the major fossil fuels (coal, petroleum, natural gas, etc.) and plastics, paraffin, waxes, solvents and oils. In urban pollution, these components--along with NOx and sunlight--all contribute to the formation of tropospheric ozone.

Hydrocarbon vapours can be harmful if inhaled.

Notes

1. ^ Clayden, J., Greeves, N., et al. (2000), p21
2. ^ McMurry, J. (2000), p75-81

References

1. McMurry, J. (2000). "Organic Chemistry" 5^th ed. Brooks/Cole: Thomson Learning
2. Clayden, J., Greeves, N., et al. (2000) "Organic Chemistry" Oxford

See also

• Abiogenic petroleum origin
• Energy storage
• Fractional distillation
• Functional group
• Hydrocarbon mixtures
• Petroleum

References

http://www.hydrocarbononline.com/

http://www.hydrocarbonprocessing.com/