Alkane (saturated hydrocarbon)

Structure and properties

Carbon atoms in alkanes are sp3-hybridized and form four sigma bonds, either C–C or C–H. This tetrahedral geometry gives individual carbon centers approximately 109.5° bond angles and allows free rotation about single bonds (single bonds). Because alkanes contain only single bonds they are distinguished from unsaturated hydrocarbons such as alkenes and alkynes, which contain double or triple bonds. The sigma bond character of C–C and C–H contacts explains the relatively low chemical reactivity of alkanes under many conditions; however, they do undergo reactions such as combustion and radical halogenation (sigma bonds are cleaved under appropriate energy input).

Common members and trends

Early, commonly cited alkanes include methane, ethane, propane and butane. As carbon count increases the molar mass rises, which typically raises melting and boiling points and decreases volatility. Branching lowers surface area and intermolecular contact, often reducing boiling points relative to straight-chain isomers. Long-chain alkanes are found in waxes and paraffinic materials; medium-length alkanes are used as fuels and solvents.

Origin, occurrence and production

Most naturally occurring alkanes are produced from fossil organic matter that, under heat and pressure over geological timescales, yielded the complex mixtures in crude oil and the gaseous hydrocarbons of petroleum and natural gas. Alkanes are separated and refined by fractional distillation and chemical processing (cracking, reforming, hydroprocessing) to produce fuels, lubricants and chemical feedstocks.

Uses and environmental notes

Alkanes are foundational to modern industry: short-chain gases (methane, propane, butane) are fuel and heating sources; liquid alkanes serve as gasoline and diesel components or as solvents; longer chains provide lubricants, paraffin waxes and materials for further chemical conversion. Their combustion releases energy but also emits carbon dioxide and other pollutants; methane in the atmosphere acts as a potent greenhouse gas until oxidized, so both use and leakage have environmental significance.

Reactivity, derivatives and distinctions

Alkanes are chemically less reactive than unsaturated hydrocarbons but participate in important transformations. Typical reactions include combustion, free-radical halogenation, cracking (thermal or catalytic bond cleavage), and catalytic isomerization to give branched isomers. A fragment derived from an alkane by removal of one hydrogen is called an alkyl group (often abbreviated R). Cyclic saturated hydrocarbons (cycloalkanes) are closely related but follow different molecular formulas and properties. For further reading, consult introductory resources on hydrocarbons and organic functional groups via links provided below.

• General information on chemical compounds
• Carbon: element and bonding
• Hydrogen: basic properties
• Methane and simple alkanes
• Single covalent bonds in organic molecules
• Alkenes and unsaturated hydrocarbons
• Sigma bonds and bond types
• Petroleum and crude oil
• Natural gas and gaseous hydrocarbons