A hydrocarbon is a chemical compound composed exclusively of carbon and hydrogen atoms. Hydrocarbons are among the simplest organic compounds and form the structural basis of many larger organic molecules. Their diversity arises because carbon can form four covalent bonds and can bond repeatedly to other carbon atoms, producing chains, branches, rings and combinations of single and multiple bonds. Major classes include alkanes, alkenes, alkynes and aromatic hydrocarbons.
Alkanes contain only carbon carbon and carbon hydrogen single bonds, alkenes contain at least one carbon carbon double bond, and alkynes contain at least one carbon carbon triple bond. Aromatic hydrocarbons contain aromatic ring systems with delocalised electrons, with benzene being the classic example. Hydrocarbons can be gases, liquids or solids depending on molecular size, structure and intermolecular forces. Methane is a small gaseous hydrocarbon, while long chain hydrocarbons occur in waxes and heavy petroleum fractions. Hydrocarbons are fundamental feedstocks for fuels, polymers, solvents and many other chemical products, although their combustion and extraction have major environmental consequences.
Hydrocarbon structures can be classified as saturated or unsaturated. Saturated hydrocarbons, particularly alkanes, contain the maximum number of hydrogen atoms consistent with their carbon framework and have only single carbon carbon bonds. Unsaturated hydrocarbons contain double or triple bonds and therefore have fewer hydrogen atoms relative to the corresponding saturated structure. Alkenes and alkynes are examples of unsaturated hydrocarbons. Hydrocarbons can also be acyclic, meaning they contain open chains, or cyclic, meaning that carbon atoms form one or more rings.
Acyclic hydrocarbons may be straight chain or branched. Cyclic compounds include cycloalkanes and cyclic unsaturated hydrocarbons. Aromatic hydrocarbons form a distinct class characterised by cyclic delocalisation and aromatic electronic stability rather than simply by the presence of a ring. Structural isomerism occurs when compounds have the same molecular formula but different arrangements of atoms. For example, butane and 2 methylpropane have the same molecular formula but different carbon skeletons. This structural diversity strongly affects boiling point, melting point, reactivity and combustion behaviour.
Hydrocarbons occur naturally in geological deposits and are major constituents of crude oil and natural gas. Natural gas commonly contains methane as its principal component, along with smaller amounts of ethane, propane, butane and other substances. Crude oil is a complex mixture containing hydrocarbons across a broad range of molecular sizes and structures, together with compounds containing sulfur, nitrogen, oxygen and trace metals.
Petroleum refining separates crude oil into fractions based partly on boiling ranges and then transforms hydrocarbons through processes such as cracking, reforming, isomerisation and alkylation. Cracking breaks larger hydrocarbons into smaller molecules, producing useful fuels and petrochemical feedstocks. Reforming can change molecular structures and increase the octane characteristics of gasoline components. Hydrocarbons are also produced synthetically from coal, biomass and other feedstocks. Their widespread use arises from their high energy content, chemical versatility and established infrastructure, but their combustion releases carbon dioxide and can produce pollutants such as nitrogen oxides, carbon monoxide and particulate matter depending on conditions.
The physical properties of hydrocarbons depend strongly on molecular size and structure. Small hydrocarbons such as methane, ethane and propane are gases under ordinary conditions, while intermediate hydrocarbons are commonly liquids and very large hydrocarbons can be waxy solids. As molecular size increases, dispersion forces generally become stronger, increasing boiling points. Branching often lowers boiling point relative to a straight chain compound with the same molecular formula because branching changes molecular surface area and packing.
Hydrocarbons are generally nonpolar or only weakly polar because carbon and hydrogen have relatively similar electronegativities. Consequently, they have poor solubility in water, which is strongly polar, but they dissolve more readily in nonpolar or weakly polar solvents. Density is often lower than that of water for many liquid hydrocarbons, although exact values depend on composition. Their volatility, viscosity and melting behaviour depend on molecular structure and intermolecular interactions. These physical characteristics are important in fuel formulation, petroleum separation, environmental transport and industrial processing.
Hydrocarbons undergo several important types of chemical reaction. Alkanes commonly undergo combustion and substitution reactions under appropriate conditions. Alkenes and alkynes readily undergo addition reactions because their multiple bonds contain pi bonds that can react with electrophiles and other reagents. Aromatic hydrocarbons can undergo electrophilic aromatic substitution while preserving the aromatic ring system.
Combustion of hydrocarbons in sufficient oxygen ideally produces carbon dioxide and water and releases substantial energy because the products have strong bonds and lower chemical free energy under appropriate conditions. Incomplete combustion can produce carbon monoxide, soot and other products. Hydrocarbons can also undergo oxidation, halogenation, cracking and rearrangement depending on structure and conditions. Their reaction chemistry is central to petrochemical manufacturing. For example, ethene can be polymerised to polyethylene, while alkenes can be converted into alcohols and other functionalised compounds. The relative simplicity of hydrocarbons makes them important starting materials for synthesising much more chemically diverse organic molecules.
Hydrocarbons are especially important as energy sources because oxidation of carbon hydrogen bonds ultimately releases energy when products such as carbon dioxide and water form. Methane, propane, petrol components, diesel range hydrocarbons, kerosene and other petroleum derived mixtures are widely used as fuels. The energy released depends on composition and combustion efficiency. However, hydrocarbons are not simply interchangeable fuels. Molecular size, branching, aromaticity and impurities influence ignition characteristics, volatility, energy density, emissions and suitability for particular engines.
Octane rating is important for spark ignition fuels and measures resistance to knocking under defined test conditions, while cetane characteristics are relevant to compression ignition fuels. Aviation fuels require carefully controlled freezing, volatility and combustion properties. Hydrocarbon fuels have enabled large scale transport and industrial development but their use is associated with greenhouse gas emissions, air pollution and ecological impacts from extraction, refining and accidental releases. These considerations have encouraged research into electrification, low carbon fuels, renewable feedstocks and more efficient chemical processes.
Hydrocarbons are also essential raw materials rather than merely fuels. Ethene, propene, butenes and aromatic hydrocarbons are major petrochemical feedstocks used to manufacture polymers, solvents, detergents, coatings, pharmaceuticals and countless other products. Ethene is used to make polyethylene and numerous derivatives, while propene is used to make polypropylene and other chemicals. Benzene, toluene and xylenes are important aromatic feedstocks for plastics, resins and chemical intermediates. Hydrocarbon chemistry also provides starting materials for synthetic routes in organic chemistry.
Chemists can introduce oxygen, nitrogen, sulfur, halogens and other elements into hydrocarbon frameworks to create molecules with very different properties. This process is known as functionalisation. Hydrocarbon skeletons therefore serve as molecular frameworks from which more complex substances can be constructed. Modern chemical research also investigates catalytic conversion of hydrocarbons into higher value products and methods for using renewable carbon sources as alternatives to fossil derived hydrocarbons.
Hydrocarbons are significant in environmental chemistry because their behaviour after release depends on volatility, solubility, molecular size and chemical reactivity. Small hydrocarbons can evaporate into the atmosphere, where sunlight and atmospheric oxidants can transform them into secondary pollutants. Volatile organic compounds can contribute to photochemical ozone formation when they react in the presence of nitrogen oxides and sunlight. Larger petroleum hydrocarbons can persist in soil or sediments and may be difficult to remove.
Oil spills can affect aquatic ecosystems through physical coating, toxicity and oxygen depletion. Some aromatic hydrocarbons, particularly certain polycyclic aromatic hydrocarbons, can be persistent and carcinogenic. Environmental analytical chemistry therefore distinguishes individual hydrocarbons and groups of related compounds rather than treating all hydrocarbons as equally hazardous. Hydrocarbons are also part of natural geological processes and biological carbon cycles, so their environmental significance depends on source, concentration, chemical identity and exposure route. Their chemistry consequently links energy use, industrial manufacturing, atmospheric science, pollution and the global carbon cycle.
