Alkane

Alkanes are a class of acyclic saturated hydrocarbons consisting solely of carbon and hydrogen atoms connected by single covalent bonds. They form one of the fundamental families in organic chemistry and represent the simplest type of hydrocarbon. Each carbon atom in an alkane is sp³-hybridised, forming four sigma bonds either to hydrogen or other carbon atoms. Because they contain no double or triple bonds, alkanes are considered fully saturated compounds.

Structure and General Formula

The IUPAC definition of an alkane applies to any acyclic, branched or unbranched saturated hydrocarbon with the general formula:
CnH2n+2\mathrm{C_nH_{2n+2}}Cn​H2n+2​
This includes straight-chain molecules such as methane, ethane, and propane, as well as branched structures such as isobutane or neopentane. Some sources loosely extend the term to include saturated ring systems (cycloalkanes), but these are usually classified separately as their molecular formula is CₙH₂ₙ.
The carbon skeleton—sometimes called the carbon backbone—may range from a single carbon atom (methane) to extremely long chains such as higher alkanes used in waxes. Molecules with more than about 17 carbons typically appear as waxy solids at standard temperature and pressure.
Alkanes form a homologous series, increasing in molecular mass by increments of a methylene unit (–CH₂–), which has a mass of approximately 14.03 u.

Structural Diversity and Isomerism

Although alkanes have simple bonding, they can vary greatly in structure due to structural isomerism. Alkanes with more than three carbon atoms can be arranged as:

• Linear (normal) chains
• Branched chains
• Cyclic forms, classified as cycloalkanes (not alkanes in the strict sense)

The number of constitutional isomers increases sharply with the number of carbons. For example:

• C₄H₁₀ → 2 isomers (butane, isobutane)
• C₆H₁₄ → 5 isomers
• C₈H₁₈ → 18 isomers
• C₃₂H₆₆ → over 27 billion isomers
• C₆₀H₁₂₂ → extremely large numbers, many unstable

Branched alkanes are generally more thermodynamically stable than their linear analogues. The heavily branched alkane 2,2,3,3-tetramethylbutane is markedly more stable than its linear isomer octane.
Chirality may also arise. For instance, 3-methylhexane possesses a stereogenic carbon atom and thus exists as a pair of enantiomers.

Biological Occurrence and Activity

Most alkanes have limited direct biological activity. Methane is produced by methanogenic microorganisms, and certain insects and plants employ long-chain alkanes as pheromones or protective waxes. Generally, however, alkanes serve more as structural components or energy sources rather than active metabolites.

Industrial Sources

The chief commercial sources of alkanes are:

• Petroleum (crude oil)
• Natural gas

These natural mixtures contain a broad range of chain lengths and structures, which are separated and refined for fuels, lubricants, waxes, and industrial feedstocks.

Alkyl Groups

An alkyl group is a molecular fragment derived from an alkane by removal of one hydrogen atom, leaving one free valence. Representatives of alkanes in larger organic structures are often abbreviated R–, while Alk– may be used when specifically referring to an alkyl group instead of other hydrocarbon groups.

Nomenclature

The systematic naming of alkanes follows IUPAC guidelines. Straight-chain (unbranched) alkanes are named using the Greek numerical prefix corresponding to the number of carbons, followed by the suffix -ane. The first eight members are:

1. Methane (CH₄)
2. Ethane (C₂H₆)
3. Propane (C₃H₈)
4. Butane (C₄H₁₀)
5. Pentane (C₅H₁₂)
6. Hexane (C₆H₁₄)
7. Heptane (C₇H₁₆)
8. Octane (C₈H₁₈)

The prefixes derive partly from older names based on related compounds (e.g., methanol, ether, propionic acid, butyric acid). Higher alkanes simply append -ane to the numerical prefix, adjusting for vowel elision (e.g., hexane, octane, decane).
Non-linear alkanes are named by identifying the longest continuous chain, then numbering it to assign substituents (alkyl groups) the lowest possible locants. Examples include:

• 2-methylbutane (isopentane)
• 2,2-dimethylpropane (neopentane)

In petroleum chemistry, straight-chain alkanes are sometimes called n-paraffins, and branched alkanes isoparaffins.

Physical and Chemical Properties

Key characteristics of alkanes include:

• Nonpolarity and low reactivity
• Low melting and boiling points, increasing with molecular size
• Combustibility, producing carbon dioxide and water
• Conformational flexibility, allowing rotation around C–C single bonds

At room temperature:

• Methane to butane → gases
• Pentane to hexadecane → liquids
• Higher alkanes → waxy solids