What Are Hydrocarbons?

Hydrocarbons are organic chemical compounds composed exclusively of carbon and hydrogen atoms. They form the molecular basis of fossil fuels, petrochemical feedstocks, and many synthetic materials, making them among the most economically significant class of chemical compounds in industrial civilization. The enormous variety of hydrocarbon structures arises from carbon's ability to form four covalent bonds, allowing chains, branched chains, and ring structures of widely varying length and complexity.

Hydrocarbon chemistry sits at the intersection of organic chemistry, thermodynamics, and chemical engineering. The properties of a given hydrocarbon, including its boiling point, heat of combustion, reactivity, and phase at ambient conditions, follow predictably from its molecular structure and the number of carbon atoms it contains. This structure-property relationship underpins both the separation processes used in petroleum refining and the synthetic routes used in the production of pharmaceuticals, plastics, and specialty chemicals.

Saturated Hydrocarbons

Saturated hydrocarbons, also called alkanes or paraffins, contain only single carbon-carbon (C-C) and carbon-hydrogen (C-H) bonds. Each carbon atom is bonded to the maximum number of hydrogen atoms, leaving no unsaturation. Methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10) are the simplest alkanes and are the primary components of natural gas and liquefied petroleum gas. Longer-chain alkanes, from pentane (C5) through hexadecane (C16) and beyond, constitute gasoline, diesel, and lubricating oil fractions. Cycloalkanes are saturated ring structures; cyclohexane appears prominently in petrochemical processing streams. The Chemistry LibreTexts chapter on alkanes covers systematic nomenclature, physical properties as a function of chain length, and the combustion reactions that make alkanes valuable as fuels.

Unsaturated Hydrocarbons

Unsaturated hydrocarbons contain one or more carbon-carbon multiple bonds, which reduce the hydrogen count relative to the equivalent alkane and introduce sites of chemical reactivity. Alkenes (olefins) carry at least one carbon-carbon double bond (C=C); ethylene (C2H4) and propylene (C3H6) are the most produced organic chemicals globally and serve as the monomers for polyethylene and polypropylene respectively. Alkynes contain at least one triple bond (C-C triple bond); acetylene (C2H2) is used in welding and as a synthetic precursor. The Energy Education overview of hydrocarbons describes how increasing degrees of unsaturation lower the boiling point relative to the equivalent-carbon-number alkane and raise reactivity toward addition reactions. Dienes, compounds with two double bonds, are important in the production of synthetic rubber.

Aromatic Hydrocarbons

Aromatic hydrocarbons contain one or more benzene rings, six-carbon cyclic structures with delocalized pi electrons that confer exceptional stability and distinctive chemical behavior compared to open-chain compounds of similar molecular weight. Benzene (C6H6), toluene (C7H8), and the xylenes (C8H10 isomers) form the BTX fraction of petroleum refinery streams and are feedstocks for a wide range of polymers, solvents, and dyes. Polycyclic aromatic hydrocarbons (PAHs), which contain fused benzene rings, include naphthalene, anthracene, and compounds with known carcinogenic properties, making their measurement in environmental and occupational settings a public health concern. The LibreTexts chapter on alkanes, cycloalkanes, alkenes, alkynes, and aromatics presents the electronic structure of benzene and the electrophilic aromatic substitution reactions that dominate its chemistry.

Applications

Hydrocarbons and their derivatives are used across a wide range of industrial and commercial domains, including:

• Transportation fuels including gasoline, diesel, jet fuel, and liquefied natural gas
• Petrochemical feedstocks for polyethylene, polypropylene, PVC, and synthetic rubber production
• Heating and power generation through combustion of natural gas and fuel oil
• Lubricating oils and greases for machinery and engine applications
• Solvents in coatings, adhesives, and industrial cleaning processes