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Hydrocarbons are organic compounds made of carbon and hydrogen, ranging from simple molecules like methane to polymers like polystyrene. They are flammable and used as fuels, solvents, and materials. Fossil fuels are natural mixtures of hydrocarbons, but their use causes pollution and contributes to climate change. Renewable energy sources are being developed as alternatives.
Hydrocarbons are organic chemical compounds made up entirely of carbon and hydrogen and range from simple molecules such as methane, to polymers such as polystyrene, which consists of thousands of atoms. The ability of carbon atoms to bond strongly to each other allows them to form an almost unlimited variety of chains, rings and other structures that form the backbone of organic molecules. Since each atom can form four bonds, these backbones include other elements, such as hydrogen. The compounds are flammable, as the two elements they contain combine easily with the oxygen in the air, releasing energy. Fossil fuels, such as oil and natural gas, are natural mixtures of hydrocarbons; even coal contains some, although it’s mostly just carbon.
Structure and naming conventions
The naming of hydrocarbons follows certain conventions, although in many cases the compounds may be better known by older names. In the modern system, the first part of the name represents the number of carbon atoms in the molecule: in ascending order, the first eight are prefixed meth-, eth-, prop-, but-, pent-, hex-, hept-, and oct -. Compounds in which the carbons are all joined by single bonds are collectively known as alkanes and have names ending in -ane. Therefore, the top eight alkanes are methane, ethane, propane, butane, pentane, hexane, heptane, and octane.
Carbon atoms can also form double or triple bonds with each other. Molecules that have double bonds are known as alkenes and have names ending in -ene, while those that have triple bonds are called alkynes and have names ending in -yne. Molecules that have only single bonds contain the maximum possible number of hydrogen atoms, and are therefore described as saturated. Where there are double or triple bonds, there are fewer places available for hydrogen, so these compounds are described as unsaturated.
To give a simple example, ethane has two carbon atoms joined by a single bond, leaving each capable of bonding to three hydrogen atoms, so its chemical formula is C2H6 and it is an alkane. There is a carbon-carbon double bond in ethene, so it can only have four hydrogens, making it an alkene with the formula C2H4. Ethyne has a triple bond, which gives it the formula C2H2 and makes it an alkyne. This compound is better known as acetylene.
Carbon atoms can also form rings. Ringed alkanes have names starting with cyclo-. Cyclohexane is therefore an alkane with six carbon atoms joined by single bonds to form a ring. A ring with alternating single and double bonds is also possible and is known as a benzene ring. Hydrocarbons containing a benzene ring are known as aromatics, because many of them have a pleasant smell.
Some hydrocarbon molecules have chains that branch. Butane, which normally consists of a single chain, can exist in a form where one carbon atom is bonded to two others, forming a branch. These alternative forms of a molecule are known as isomers. The branched isomer of butane is known as isobutane.
Production
Most hydrocarbon production comes from fossil fuels: coal, oil and natural gas, which are extracted from the ground in quantities of millions of tons per day. Crude oil is primarily a blend of many different alkanes and cycloalkanes, with some aromatic compounds. These can be separated from each other in petroleum refineries by distillation, due to their different boiling points. Another process that is used is known as “cracking”: catalysts are employed to break some of the larger molecules into smaller ones that are more useful as fuels.
Property
In general, the more complex a hydrocarbon, the higher its melting and boiling points. For example, the simplest types, such as methane, ethane and propane, with one, two and three carbon atoms, respectively, are gases. Many forms are liquids: examples are hexane and octane. Solid forms include paraffin wax – a mixture of molecules with between 20 and forty carbon atoms – and various polymers made up of chains of thousands of atoms, such as polyethylene.
The most important chemical properties of hydrocarbons are their flammability and their ability to form polymers. Those that are gases or liquids will react with the oxygen in the air, producing carbon dioxide (CO2) and water, and releasing energy in the form of light and heat. You need to supply energy to start the reaction, but once it’s started, it’s self-sustaining: These compounds will burn, as illustrated by lighting a gas stovetop with a match or spark. Solid shapes will also burn, but less easily. In some cases, not all of the carbon will form CO2; soot and smoke can be produced by some types when they burn in air, and with insufficient oxygen supply, any hydrocarbon can produce the toxic, odorless gas, carbon monoxide (CO).
it is used
The flammability of hydrocarbons makes them very useful as fuels and they are the primary energy source for today’s civilization. Worldwide, most of the electricity is generated by the combustion of these compounds and they are used to power virtually every mobile machine: cars, trucks, trains, planes and ships. They are also used in the manufacture of many other chemicals and materials. Most plastics, for example, are polymers of hydrocarbons. Other uses include solvents, lubricants, and aerosol can propellants.
Problems with fossil fuels
Hydrocarbons have been a very successful fuel source for the past two hundred years or so, but there are growing calls to scale back their use. Their combustion produces smoke and soot, causing serious pollution problems in some areas. It also produces large amounts of CO2. There is broad agreement among scientists that rising levels of this gas in the atmosphere are helping to trap heat, raising global temperatures and changing the Earth’s climate.
Also, fossil fuels won’t last forever. Burning fuel at the current rate, oil could run out in less than a century and coal in several centuries. All of this has led to calls for the development of renewable energy sources such as solar and wind energy and the construction of more nuclear power plants, which produce zero CO2 emissions. In 2007, the Nobel Peace Prize was awarded to former US Vice President Al Gore and the United Nations Intergovernmental Panel on Climate Change for their work in confirming and spreading the message that the burning of hydrocarbons is largely part responsible for global warming.
