Alkanes and Alkenes

Alkanes and Alkenes

Understanding Alkanes

  • Alkanes are a class of hydrocarbons, compounds consisting of only hydrogen and carbon atoms.
  • They are characterised by single covalent bonds between carbon atoms and are therefore termed as saturated hydrocarbons.
  • The simplest alkane is methane (CH4), followed by ethane (C2H6), propane (C2H8), and so on.
  • The general formula for alkanes is CnH2n+2, where n represents the number of carbon atoms.

Properties of Alkanes

  • Alkanes are generally non-polar molecules, meaning they are mostly insoluble in water but soluble in organic solvents.
  • They have low reactivity because the carbon-carbon and carbon-hydrogen bonds in alkanes are relatively stable.

Combustion of Alkanes

  • Alkanes are important as fuels. When burnt in oxygen, they undergo complete combustion to form carbon dioxide and water.
  • However, with a limited oxygen supply, alkanes undergo incomplete combustion, resulting in poisonous carbon monoxide, or even producing soot (carbon).

Understanding Alkenes

  • Alkenes are also hydrocarbons, but unlike Alkanes, they contain at least one carbon-carbon double bond. This makes them unsaturated hydrocarbons.
  • The simplest alkene is ethene (C2H4), followed by propene (C3H6), and so on.
  • The general formula for alkenes is CnH2n.

Properties of Alkenes

  • Alkenes are more reactive than alkanes due to the presence of a double bond which can easily break and form new bonds.
  • Like alkanes, alkenes are also non-polar and insoluble in water, but soluble in organic solvents.

Reactions of Alkenes

  • Alkenes can participate in addition reactions, where the carbon-carbon double bond breaks to allow other atoms or molecules to add to the carbons.
  • For instance, alkenes react with bromine water (a test for unsaturation) to produce a colourless dibromo compound.
  • Alkenes also burn in air, but combustion is usually less clean than for alkanes, giving sooty flames.