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

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