Hydrocarbon

Hydrocarbons are refined at oil refineries such as this one.

In chemistry, a hydrocarbon is any chemical compound that consists only of the elements carbon (C) and hydrogen (H). They all contain a carbon backbone, called a carbon skeleton, and have hydrogen atoms attached to that backbone. (Often the term is used as a shortened form of the term aliphatic hydrocarbon.) Most hydrocarbons are combustible.

Examples

The simplest hydrocarbon is methane (swamp/marsh gas), a hydrocarbon with one carbon atom and four hydrogen atoms: CH₄. Ethane is a hydrocarbon (more specifically, an alkane) consisting of two carbon atoms held together with a single bond, each with three hydrogen atoms bonded: C₂H₆. Propane has three carbon atoms (C₃H₈) and butane has four carbons (C₄H₁₀).

A model of an arene called Benzene

A model of the alkane known as Hexane

A model of the alkyne called Ethyne

Three types of hydrocarbons

There are essentially three types of hydrocarbons:

1. aromatic hydrocarbons, which have at least one aromatic ring
2. saturated hydrocarbons, also known as alkanes, which don't have any double, triple or aromatic bonds
3. unsaturated hydrocarbons, which have one or more double or triple bonds between carbon atoms, are divided into:
   • alkenes
   • alkynes

The number of hydrogen atoms

The number of hydrogen atoms in hydrocarbons can be determined, if the number of carbon atoms is known, by using these following equations:

• Alkanes: C_nH_2n+2
• Alkenes: C_nH_2n (assuming only one double bond)
• Alkynes: C_nH_2n-2 (assuming only one triple bond)
• Cyclic hydrocarbons: C_nH_2n

Each of these hydrocarbons must follow the 4-hydrogen rule which states that all carbon atoms must have the maximum number of hydrogen atoms that it can hold (the limit is four). A carbon atom has a bonding capacity of 4 and therfore must make 4 bonds, whether it be with hydrogen or an ajoining carbon atom. Note, an extra bond removes 2 hydrogen atoms and only saturated hydrocarbons can attain the full four. This is because of the unique positions of the carbon's four electrons.

Molecular graph

Usually carbon backbone is represented as molecular graph in which only carbon atoms are represented as vertices and bonds as edges. Molecular graphs contain the structure of the hydrocarbon in which missing hydrogen atoms can be added in a unique way. Hydrocarbons are extensively studied in mathematical chemistry.

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 biofuels, as well as plastics, paraffin, waxes, solvents and oils. In urban pollution, these components—along with NOx and sunlight—all contribute to the formation of tropospheric ozone.

The concentration of hydrocarbon vapours can be harmful if inhaled.

Burning hydrocarbons

Hydrocarbons are one of Earth's most important natural 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. Hydrocarbons are all substances with low entropy (meaning they hold a lot of energy potential), which can be released and harnessed by burning them. 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 in pipes around the building heating every room. 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 in to 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 which is toxic because it will bind with hemoglobin more readily than oxygen, so if it is breathed in, oxygen can not be absorbed, causing suffocation. Incomplete combustion also has a byproduct of carbon in the form of soot.

Coal reserves will last for decades and possibly beyond 2100. Mostly in response to climate concerns, clean coal technology is currently under development. For example, the UK and 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. By some, hydrocarbons in the atmosphere are considered to be dangerous air pollutants.

External links

• The Methane Molecule
• Dismissal of the Claims of a Biological Connection for Natural Petroleum.
• Poten & Partners: Glossary of Hydrocarbon Terms

See also

• Abiogenic petroleum origin
• Energy storage
• Fractional distillation
• Functional group
• Alkanes

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