Alkanes: The Simplest Hydrocarbons

What makes an alkane

Alkanes are the simplest organic compounds: hydrocarbons built only from carbon and hydrogen, with every carbon–carbon bond a single bond. They are called saturated because each carbon carries the maximum number of hydrogen atoms it can hold.

The family follows the general formula CnH2n+2: each additional carbon brings two more hydrogens, plus the two terminal hydrogens. Methane is CH4, ethane is C2H6, propane is C3H8, and so on.

Every carbon in an alkane is sp3 hybridized and tetrahedral, bonded to four other atoms. The bonds are strong, nonpolar sigma bonds — which explains both the stability and the low reactivity of the family.

Naming the first ten alkanes

The first ten straight-chain alkanes form the vocabulary of organic naming:

• Methane CH4, ethane C2H6, propane C3H8, butane C4H10, pentane C5H12
• Hexane C6H14, heptane C7H16, octane C8H18, nonane C9H20, decane C10H22

The prefixes meth-, eth-, prop-, but- are historical; from pent- upward they are the Greek numbers. Memorizing these ten roots pays off constantly, because every systematic organic name is built on them.

Branched alkanes keep the same formulas but different structures — butane and 2-methylpropane are both C4H10. These structural isomers are covered in the isomerism article.

Structure and shape

Alkane carbon skeletons are not straight rods: each tetrahedral carbon kinks the chain at about 109.5°, producing the familiar zigzag shape in structural drawings.

Single bonds rotate freely, so alkane chains constantly twist into different conformations — like a flexible rope. This flexibility lets long chains coil and pack in various ways, affecting properties such as melting point.

In the gas phase the molecules tumble independently; as liquids they slide past one another held only by weak London dispersion forces. There is no hydrogen bonding and no permanent dipole anywhere in a pure alkane.

Physical properties and trends

Physical properties of alkanes follow beautifully regular trends. Boiling point rises with chain length because longer molecules have more surface area and therefore stronger London dispersion forces: methane boils at –162°C (a gas), hexane at 69°C, and decane at 174°C (liquids).

Branching lowers the boiling point. The compact, branched 2-methylpropane boils at –11.7°C while straight-chain butane boils at –0.5°C — less surface area means weaker intermolecular attractions.

Alkanes are nonpolar, so they do not mix with water but dissolve readily in each other and in other nonpolar solvents. They are also less dense than water, which is why oil spills float.

Combustion

The most important reaction of alkanes is complete combustion: burning in plentiful oxygen to give carbon dioxide and water. For methane:

CH4 + 2O2 → CO2 + 2H2O

Every alkane follows the same pattern, and balancing these equations is standard practice: balance carbon first, then hydrogen, then oxygen. For ethane: 2C2H6 + 7O2 → 4CO2 + 6H2O.

With limited oxygen, incomplete combustion produces carbon monoxide and soot (carbon particles) alongside CO2 and water. This is why poorly ventilated flames are dangerous and smoky — and why efficient burners matter for both safety and the environment.

Why alkanes are unreactive

Alkanes are famously unreactive — petroleum can be stored for years without change — and the reason is structural. Their C–C and C–H bonds are strong sigma bonds with no exposed pi electrons, no polarized sites, and no lone pairs for reagents to attack.

Most reagents simply find nothing to react with. Acids, bases, and oxidizers that readily attack other functional groups leave alkanes untouched under ordinary conditions.

The exceptions prove the rule: alkanes react only under vigorous conditions — combustion at high temperature, or halogenation in ultraviolet light, where energetic radicals force the issue. That single radical pathway, free-radical substitution, is the subject of its own article.

Common mistakes

Real-world applications

• Methane (natural gas) and propane heat homes and fuel cooking worldwide.

• Petrol and diesel are mixtures of alkanes and related hydrocarbons refined from crude oil.

• Paraffin wax — long-chain alkanes — is used in candles and coatings.

• Alkanes are the starting materials for making many other organic compounds through substitution reactions.