Hydrocarbon

Hydrocarbons form a fundamental class of organic compounds composed exclusively of hydrogen and carbon atoms. They represent the simplest structures in organic chemistry and serve as the building blocks of numerous natural and synthetic substances. Occurring in a wide range of physical states—from gases such as methane and propane to liquids like benzene and hexane, and solids such as paraffin wax—they underpin many industrial, environmental and biological processes. Their significance extends from being major components of fossil fuels to forming essential raw materials for organic synthesis and polymer production.

General Characteristics of Hydrocarbons

Hydrocarbons are part of the group 14 hydrides, displaying properties that vary according to their structural type. Most are colourless and hydrophobic, exhibiting faint odours often likened to gasoline or naphtha. Their molecular diversity allows them to exist not only as simple linear chains but also as branched structures, cyclic compounds, aromatic rings and large macromolecules such as polystyrene and polyethylene.
In industrial and environmental contexts, the term hydrocarbon frequently refers to naturally occurring fossil resources including petroleum, natural gas and coal. These substances provide the world’s dominant energy supply through combustion, which releases heat, steam and carbon dioxide. Methane, for instance, constitutes the primary component of natural gas. Increasingly, the environmental impact of hydrocarbons has come under scrutiny, as carbon dioxide from fossil fuel combustion and methane from natural gas extraction and agriculture represent major contributors to anthropogenic greenhouse gases.

Classification of Hydrocarbons

The International Union of Pure and Applied Chemistry (IUPAC) classifies hydrocarbons into broad categories based on the nature of their carbon–carbon bonds.
Saturated Hydrocarbons (Alkanes and Cycloalkanes)Saturated hydrocarbons contain only single covalent bonds between carbon atoms and are fully saturated with hydrogen. Their general molecular formula for open-chain alkanes is CₙH₂ₙ₊₂, while cyclic variants follow CₙH₂ₙ, where n denotes the number of carbon atoms. Cycloalkanes feature exactly one ring structure and share similar chemical behaviour with their linear counterparts.
These hydrocarbons form the backbone of petrol and other fuel fractions. Their inertness makes them stable under normal conditions, although they can undergo substitution reactions when exposed to halogens. Chlorination, for example, converts methane to chloroform in a free-radical chain process. The presence of branching leads to the formation of structural isomers, while some branched alkanes display chirality, a feature important in biological molecules such as chlorophyll and tocopherol.
Unsaturated Hydrocarbons (Alkenes and Alkynes)Unsaturated hydrocarbons possess one or more double or triple bonds. Alkenes contain carbon–carbon double bonds and follow the general formula CₙH₂ₙ for non-cyclic species. Those with triple bonds are termed alkynes and conform to CₙH₂ₙ₋₂. Their characteristic reactivity arises from the presence of π-bonds, which readily participate in addition reactions involving hydrogen, halogens, water and other reagents.
Aromatic Hydrocarbons (Arenes)Aromatic hydrocarbons include at least one aromatic ring, the most familiar being benzene. Aromatic compounds possess delocalised electrons that confer exceptional stability. They are significant industrially, contributing to emissions from gasoline-powered vehicles and forming the basis of numerous chemical feedstocks.
The term aliphatic denotes non-aromatic hydrocarbons and encompasses alkanes, alkenes and alkynes. Saturated aliphatic hydrocarbons are sometimes called paraffins, while aliphatic compounds with double bonds may be referred to as olefins.

Industrial Applications and Natural Occurrence

The primary utilisation of hydrocarbons lies in their role as energy sources. They are processed from crude oil through fractional distillation and other refining methods to produce fuels such as petrol, jet fuel, naphtha and specialised solvents. Lighter hydrocarbons (C₆–C₁₀) dominate gasoline formulations, whereas heavier fractions yield lubricants, waxes and tars. High-viscosity residues are used in roofing materials, bitumen for road surfaces and wood preservatives such as creosote.
Hydrocarbons also form precursors to large-scale chemical intermediates. Ethane and propane are converted to ethylene and propylene, essential in polymer manufacture. Benzene consumption continues to rise globally due to its importance in producing plastics, resins and synthetic fibres.
In nature, hydrocarbons are found far beyond geological deposits. Certain insects, including the Brazilian stingless bee Schwarziana quadripunctata, use cuticular hydrocarbon mixtures to distinguish kin from non-kin, with variations influenced by age, sex and social role. Some plants, such as Euphorbia lathyris, produce hydrocarbons that may serve as renewable biofuel sources. Microorganisms capable of degrading hydrocarbons play important roles in mitigating pollution, especially in contaminated soils.

Chemical Reactions of Hydrocarbons

Hydrocarbons exhibit a range of chemical behaviours influenced by their degree of saturation and molecular structure.
CrackingCracking involves breaking long-chain saturated hydrocarbons into smaller molecules, particularly alkenes and alkynes, using high temperatures (around 500 °C) and heterogeneous catalysts. This process underpins the production of lighter, more valuable hydrocarbons needed for fuel and chemical synthesis.
Oxidation and CombustionHydrocarbons undergo oxidation reactions at elevated temperatures. Combustion, their most important reaction, occurs in the presence of excess oxygen, producing carbon dioxide, water and heat. In limited oxygen supply, incomplete combustion yields carbon particles (soot) and water vapour. The combustion of methane illustrates the ideal reaction:
CH₄ + 2O₂ → CO₂ + 2H₂O
For alkanes containing n carbon atoms, the balanced equation becomes:
CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → nCO₂ + (n + 1)H₂O
Partial oxidation, which forms compounds such as maleic acid from butane or phenol and acetone from cumene, is a carefully controlled industrial process. Autoxidation begins with hydroperoxide formation and is central to the drying of oils and the rancidification of fats.
HalogenationSaturated hydrocarbons react with halogens—especially chlorine and fluorine—via free-radical mechanisms. Sequential chlorination can yield fully substituted products such as carbon tetrachloride and hexachloroethane.
Substitution and AdditionAromatic hydrocarbons undergo substitution reactions while preserving the integrity of their aromatic ring. The large-scale production of ethylbenzene from benzene and ethene provides the precursor to styrene, which is polymerised to form polystyrene.
Unsaturated hydrocarbons participate readily in addition reactions. Reagents add across double or triple bonds, enabling hydrogenation, halogenation and hydration. Polymerisation is a central industrial application in which alkenes form plastics such as polyethylene and polybutylene. Alkynes like acetylene can polymerise to produce polyacetylene.
MetathesisSome hydrocarbons undergo metathesis, in which substituents attached to carbon–carbon bonds are exchanged. Depending on the nature of the bond involved, the process is classified either as alkane metathesis or as alkene metathesis, the latter holding particular significance in modern synthetic chemistry.