Hydrocarbon

Hydrocarbons are obtained by refining petroleum at refineries such as this one.

In chemical terms, a hydrocarbon is any compound that is constituted of just the elements carbon (C) and hydrogen (H). Each hydrocarbon molecule consists of a carbon backbone, or "carbon skeleton," with hydrogen atoms attached to that backbone.

Hydrocarbons are among the Earth's most important natural resources. They are currently the main source of the world’s electric energy and heat energy (such as for home heating) because they produce a large amount of heat when burnt. The gasoline that serves as fuel for automobiles consists primarily of hydrocarbons. In addition, chemists use many hydrocarbons as building blocks for the synthesis of other organic compounds.

Natural occurrence and extraction

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. They 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 and oil shale. These reserves require distillation and upgrading to produce synthetic crude and petroleum. A future source appears to be sedimentary methane hydrates.

Types of hydrocarbons

A model of the alkane known as hexane.

A model of the alkyne called ethyne.

A model of an arene called benzene.

There are essentially three types of hydrocarbons:

Saturated hydrocarbons, also known as alkanes: In each molecule of an alkane, the chemical bonds that join the carbon atoms are single covalent bonds. If the alkane molecule includes a ring of carbon atoms (all connected by single covalent bonds), it is called a cycloalkane.
Unsaturated hydrocarbons, which are subdivided into two groups:
- alkenes: Each molecule of an alkene contains at least one double covalent bond between carbon atoms.
- alkynes: Each molecule of an alkyne contains at least one triple covalent bond between carbon atoms.
Aromatic hydrocarbons, or arenes: Each molecule of an aromatic hydrocarbon contains at least one aromatic ring, in which the bonds between carbon atoms are aromatic bonds.

Some simple hydrocarbons

The simplest hydrocarbon is methane, the main constituent of natural gas. Its chemical formula, CH₄, indicates that each molecule of methane consists of four hydrogen atoms attached to one carbon atom. Ethane, with the formula C₂H₆, is a hydrocarbon (more specifically, an alkane) in which each molecule has two carbon atoms held together with a single covalent bond, and three hydrogen atoms are bound to each carbon atom. Each molecule of propane (C₃H₈) has three carbon atoms, and each molecule of butane (C₄H₁₀) has four carbons.

General chemical formulas

If a hydrocarbon molecule contains "n" carbon atoms, and the type of hydrocarbon is known, its general chemical formula can be written as follows:

Alkane: C_nH_2n+2.
Cycloalkane: C_nH_2n (assuming a single ring, with all the carbon atoms in the ring structure).
Alkene: C_nH_2n (assuming only one double bond in each molecule).
Alkyne: C_nH_2n-2 (assuming only one triple bond in each molecule).
Aromatic ring: C_nH_n (assuming a single ring, with all the carbon atoms in the ring structure).

The above chemical formulas are based on the understanding that each carbon atom forms four covalent bonds, including bonds with hydrogen atoms and other carbon atoms. A double covalent bond counts as two bonds; a triple covalent bond counts as three bonds. A carbon atom in an aromatic ring is a special case and can have only one hydrogen atom attached to it.

Molecular graph

A "molecular graph" of a hydrocarbon is a simple representation of the carbon skeleton of the molecule under consideration. Each line in the molecular graph represents a covalent bond that connects two carbon atoms. Thus, although the graph shows no symbols for carbon atoms, a carbon atom is (by convention) located at each end of each line. The symbols for hydrogen atoms are not shown either, but each carbon atom holds as many hydrogen atoms as it needs to form four covalent bonds.

Uses of hydrocarbons

They are significant sources of fuel and raw materials as a feedstock for the production of organic chemicals.

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 vapors can be harmful if inhaled.

Hydrocarbons as fuel

Most hydrocarbons are combustible. When burnt, they produce carbon dioxide, water, and a lot of heat. We rely on this energy to heat our homes (and other buildings) and to generate electricity. When heating a home, for example, oil or natural gas is burnt and the energy released is used to heat water or air. The hot water or air is then circulated around the building. 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 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 United States and other countries. By some, hydrocarbons in the atmosphere are considered to be dangerous air pollutants.

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