Alkenes are hydrocarbons with the functional group C=C.

The idea

Alkenes are a homologous series of unsaturated hydrocarbons whose molecules contain at least one carbon-carbon double bond, C=C. This double bond is the functional group: the reactive part that gives the series its characteristic chemistry. Because of the C=C double bond, alkene molecules contain fewer hydrogen atoms than the maximum possible, meaning they are not saturated with hydrogen. Ethene, C₂H₄, is the simplest alkene; propene, C₃H₆, and butene, C₄H₈, follow. The general formula CₙH₂ₙ applies to non-cyclic alkenes with one double bond.

What earns the marks

• Alkenes are hydrocarbons: their molecules contain carbon and hydrogen only.
• The functional group of an alkene is the carbon-carbon double bond, C=C.
• The C=C makes an alkene unsaturated, because the carbons are bonded to fewer hydrogen atoms than the maximum possible.
• Ethene, C₂H₄, is the simplest alkene and its displayed formula shows a double line between the two carbon atoms.
• The general formula of a non-cyclic alkene with one C=C is CₙH₂ₙ, so ethene is C₂H₄ and propene is C₃H₆.
• Alkanes are saturated and contain only single carbon-carbon bonds, so they are a different homologous series from alkenes.

Common mistakes and how to put them right

• Calling any hydrocarbon with a double bond an alkane.

  Put it right: The correct term is alkene, and the C=C must be identified.

• Writing the general formula as CₙH₂ₙ₊₂, which describes alkanes.

  Put it right: The correct alkene formula is CₙH₂ₙ.

• Drawing ethene with a single C–C bond.

  Put it right: The corrected displayed formula must show C=C.

Exam tips

• When asked to identify an alkene, locate the C=C and state that the compound is unsaturated.
• Use the general formula CₙH₂ₙ to deduce a molecular formula, then check that the structure contains one C=C.
• In displayed formulae, draw the double bond as two parallel lines between the carbon atoms and complete the outer shells with hydrogen atoms.