A hydrocarbon is a chemical compound containing only carbon and hydrogen. Hydrocarbons range from small gaseous molecules to liquids, waxes, and large molecular structures. They are fundamental subjects of organic chemistry and major constituents of petroleum and natural gas. Their properties depend on the arrangement of their carbon skeletons and the kinds of bonds joining their atoms. Compounds containing additional elements, such as oxygen or nitrogen, are not hydrocarbons, even when much of their structure consists of carbon and hydrogen. (old.goldbook.iupac.org)
Structure and classification
Carbon can form four covalent bonds and connect to other carbon atoms in chains, branches, and rings. This capacity produces considerable structural diversity from just two elements. In conventional bonding descriptions, carbon–carbon single, double, and triple bonds differ in geometry and electron distribution. These differences help determine a hydrocarbon’s chemical behavior. (openstax.org)
The principal families are:
- Alkanes: Acyclic saturated hydrocarbons containing only single bonds, with general formula CₙH₂ₙ₊₂. Examples include methane, CH₄, ethane, C₂H₆, and propane, C₃H₈. “Saturated” indicates that the carbon framework carries the maximum number of hydrogen atoms compatible with its single-bond structure. (old.goldbook.iupac.org)
- Cycloalkanes: Saturated hydrocarbons containing rings. Those with exactly one ring have formula CₙH₂ₙ, as in cyclohexane, C₆H₁₂. Additional rings further reduce the hydrogen count relative to an acyclic alkane. (openstax.org)
- Alkenes: Unsaturated hydrocarbons containing carbon–carbon double bonds. Acyclic compounds with exactly one double bond have formula CₙH₂ₙ; ethene, C₂H₄, is the simplest example. (openstax.org)
- Alkynes: Hydrocarbons containing carbon–carbon triple bonds. Acyclic compounds with exactly one triple bond have formula CₙH₂ₙ₋₂; ethyne, C₂H₂, is also called acetylene. (openstax.org)
- Aromatic hydrocarbons: Compounds containing aromatic ring systems, exemplified by benzene, C₆H₆. Their classification rests on aromaticity, an electronic property associated with cyclic electron delocalization, rather than on smell. Benzene has six π electrons distributed around its planar carbon ring. (openstax.org)
A hydrocarbon may contain several rings, multiple double or triple bonds, or both aromatic and nonaromatic portions. Consequently, the simple formulas above apply only to the specified structural families. (openstax.org)
Isomerism and naming
Different hydrocarbons can share a molecular formula while having different structures, a phenomenon called isomerism. Butane and 2-methylpropane both have formula C₄H₁₀, but one has an unbranched carbon chain and the other a branched skeleton. Such constitutional isomers can differ in physical properties. Restricted rotation about double bonds also permits geometric isomerism when each double-bonded carbon bears two different substituents. (openstax.org)
Systematic names encode structural information. For simple branched alkanes, naming begins with selection of the longest continuous carbon chain, followed by numbering and identification of attached alkyl groups. The endings -ane, -ene, and -yne distinguish the principal single-, double-, and triple-bond families. Numbers identify the positions of branches and multiple bonds; cyclo- indicates a ring. Common names, including ethylene for ethene, remain widely used. (openstax.org)
Physical properties
Hydrocarbons are generally nonpolar and interact largely through dispersion forces, part of the broader category of van der Waals forces. They usually have low solubility in water but dissolve more readily in compatible nonpolar liquids. Many liquid hydrocarbons therefore serve as solvents. Within a comparable alkane series, boiling points generally increase with molecular size as intermolecular attractions become stronger. (openstax.org)
Molecular shape also matters: branching generally lowers an alkane’s boiling point relative to its unbranched isomer. Melting points depend strongly on molecular symmetry and crystal packing, so their trends are less uniform. At ordinary room conditions, the smallest alkanes are gases, intermediate members are liquids, and sufficiently long-chain members are solids. These distinctions help explain the behavior of gaseous fuels, gasoline-range liquids, and paraffin waxes. (openstax.org)
Chemical reactions
Hydrocarbon combustion releases energy. Complete combustion with sufficient oxygen produces carbon dioxide and water; for methane:
CH₄ + 2O₂ → CO₂ + 2H₂O.
Incomplete combustion can instead generate carbon monoxide and carbonaceous particles. Alkanes are otherwise relatively unreactive under many ordinary conditions, although they undergo reactions such as halogen substitution when appropriately activated. (openstax.org)
Alkenes and alkynes undergo addition reactions at their multiple bonds. Catalytic hydrogenation adds hydrogen to unsaturated bonds, while other additions can introduce halogens or oxygen-containing groups. Catalysis controls many industrial transformations. Aromatic compounds commonly undergo substitution reactions that preserve their aromatic ring system, rather than the straightforward addition reactions characteristic of ordinary alkenes. (openstax.org)
Alkenes can also undergo polymerization, joining many small molecules into a polymer. Polyethylene, polypropylene, and polystyrene are hydrocarbon polymers because their molecular structures contain only carbon and hydrogen. Other plastics contain additional elements and do not belong to this category. (openstax.org)
Occurrence and industrial processing
Petroleum and natural gas are major commercial sources of hydrocarbons. Petroleum is a complex mixture, whereas methane is the principal component of natural gas. Petroleum formation involves the burial and geological transformation of ancient organic material over long periods. Hydrocarbons also occur in living organisms and can be synthesized chemically. (eia.gov)
Refineries use distillation to separate crude oil into fractions according to boiling range. These fractions are mixtures, not individual pure compounds. Cracking converts heavier hydrocarbon molecules into smaller ones, while reforming and other processes change molecular structures to obtain desired fuel components. Hydrocarbons supply transportation and heating fuels, lubricants, and feedstocks for chemical manufacturing. (eia.gov)
Environmental significance
Burning hydrocarbon fossil fuels transfers carbon from geological stores into the atmosphere as carbon dioxide. Methane released without combustion is itself a greenhouse gas. Many other emitted hydrocarbons participate in atmospheric reactions contributing to ground-level ozone formation. This behavior varies by compound: the United States Environmental Protection Agency excludes methane and ethane from its regulatory definition of volatile organic compounds because of their negligible photochemical reactivity for ozone formation. (eia.gov)