Alkanes

What an alkane is

• Alkanes are the simplest family of organic compounds: hydrocarbons (carbon and hydrogen only) in which every carbon–carbon bond is a single bond
• A molecule with only single bonds is described as saturated — every carbon atom carries as many hydrogens as it possibly can, so the molecule cannot take on any more atoms
• Each alkane molecule consists of a chain of carbon atoms with hydrogen atoms filling all the spare bonds
• Alkanes form a homologous series — they share a single functional group (the absence of any double or triple bonds), a common general formula, and a steady gradation of physical properties as the chain grows

General formula

• The general formula of the alkane series is:

C_nH_2n+2

• where n is the number of carbon atoms in the molecule (n = 1, 2, 3, …)
• Plug in n to get the molecular formula:
  • n = 1 → CH₄ → methane
  • n = 2 → C₂H₆ → ethane
  • n = 3 → C₃H₈ → propane
• The +2 accounts for the extra hydrogen at each end of the chain that has no neighbouring carbon to bond to

The first five alkanes

The first five members of the series are the ones that show up most often in exam questions:

The names follow a fixed pattern: the stem counts the carbons (meth-, eth-, prop-, but-, pent-) and the -ane ending signals the alkane family. Both pieces are needed in exam answers.

Trends down the homologous series

• As the chain lengthens (methane → pentane → hexane → …):
  • Boiling point rises steadily — bigger molecules have stronger intermolecular forces
  • Volatility falls — the heavier alkanes do not evaporate as readily
  • Viscosity rises — longer-chain liquid alkanes are stickier
  • Flammability falls — heavier alkanes are harder to ignite, although they all still burn

Chemistry of the alkanes

• Alkanes are described as generally unreactive — their molecules have only strong C–C and C–H single bonds that do not break readily under everyday conditions
• The three reactions of alkanes worth memorising are:
  • Combustion in oxygen (the basis of their use as fuels — see topic 22 section 2)
  • Cracking when heated with a catalyst (the basis of converting long-chain into short-chain — see topic 22 section 4)
  • Substitution by halogens in the presence of ultraviolet light — covered next in section 2