Alkenes are hydrocarbons with a double carbon-carbon bond.

The idea

Alkenes are a homologous series of unsaturated hydrocarbons. Their defining feature is a double covalent bond between two carbon atoms, written as C=C. Ethene has the formula C₂H₄, with two carbon atoms joined by a double bond and two hydrogen atoms bonded to each carbon. The double bond means alkenes have two fewer hydrogen atoms than the corresponding alkane, so ethene is C₂H₄ while ethane is C₂H₆. The double bond makes alkenes reactive, so they undergo addition reactions, decolourise bromine water and can join together in addition polymerisation. General formula is CₙH₂ₙ for the chain members studied.

What earns the marks

• Alkenes contain only carbon and hydrogen, so they are hydrocarbons.
• The defining feature is a double covalent bond between two carbon atoms, shown as C=C.
• Ethene is C₂H₄ and has a C=C double bond with two hydrogen atoms on each carbon.
• Alkenes are unsaturated because they have fewer hydrogen atoms than the corresponding alkane.
• The double bond makes alkenes reactive, so they undergo addition reactions and addition polymerisation.
• The general formula of the alkene homologous series is CₙH₂ₙ.

Common mistakes and how to put them right

• Drawing ethene with a single C-C bond.

  Put it right: Draw C=C with two shared pairs of electrons.

• Saying alkenes are saturated.

  Put it right: Alkenes are unsaturated because of the C=C double bond.

• Using the alkane general formula CₙH₂ₙ₊₂ for alkenes.

  Put it right: Use CₙH₂ₙ for alkenes.

Exam tips

• Show the double bond clearly in displayed and structural formulae.
• Use the word unsaturated and link it to the C=C bond.
• Check hydrogen counts: ethene C₂H₄, propene C₃H₆, butene C₄H₈.