Alkenes: Formulae & Structures

Specification Point 9.12C:
  • Recall the formulae of molecules of the alkenes, ethene,propene, butene, and draw the structures of these molecules, showing all covalent bonds (but-1-ene and but-2-ene only)
  • Alkenes are a homologous series of hydrocarbon compounds with at least one double bond between two of the carbon atoms on the chain.
  • The general formula for alkenes is:

CnH2n

  • E.g. an alkene with 8 carbons has 8 x 2 = 16 hydrogen atoms.
  • All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C.

Alkene formulae & covalent bond table, Edexcel GCSE Chemistry

Unsaturation in Hydrocarbons

Specification Point 9.13C:
  • Explain why the alkenes are unsaturated hydrocarbons, describing that their molecules contain the functional group C=C
  • All alkenes contain a double carbon bond, which is the alkene functional group and is what allows alkenes to react in ways that alkanes cannot.
  • Compounds that have a C=C double bond are also called unsaturated compounds.
  • That means they can make more bonds with other atoms by opening up the C=C bond and allowing incoming atoms to form another single bond with each carbon atom of the functional group.
  • Each of these carbon atoms now forms 4 single bonds instead of 1 double and 2 single bonds.

Double Bond Opening, Edexcel GCSE Chemistry

Diagram showing the C=C double bond in an alkene opening to form a C-C single bond which can form more bonds

Bromination of Ethene

Specification Point 9.14C:
  • Recall the addition reaction of ethene with bromine, showing the structures of reactants and products, and extend this to other alkenes
  • Alkenes undergo addition reactions in which atoms of a simple molecule add across the C=C double bond.
  • The reaction between bromine and ethene is an example of an addition reaction
  • The same process works for any halogen and any alkene in which the halogen atoms always add to the carbon atoms involved in the C=C double bond.

Addition of Chlorine to Ethene, Edexcel GCE Chemistry

Chlorine atoms add across the C=C in the addition reaction of ethene and chlorine

Bromine Water Test

Specification Point 9.15C:
  • Explain how bromine water is used to distinguish between alkanes and alkenes
  • Alkanes and alkenes have different molecular structures. 
  • All alkanes are saturated and alkenes are unsaturated.
  • The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. 
  • This allows us to tell alkenes apart from alkanes using a simple chemical test called the bromine water test.

Bromine Test for Alkenes, Edexcel GCSE Chemistry

Diagram showing the result of the test using bromine water with alkanes and alkenes

  • Bromine water is an orange coloured solution.
  • When bromine water is added to an alkane, it will remain as an orange solution as alkanes do not have double carbon bonds (C=C) so the bromine remains in solution.
  • But when bromine water is added to an alkene, the alkene will decolourise the bromine water as alkenes do have double carbon bonds.
  • The bromine atoms add across the C=C double bond, hence the solution no longer contains bromine which is what gives it the orange colour.

Addition of Bromine to Ethene, Edexcel GCSE Chemistry

Each carbon atom of the double bond accepts a bromine atom, causing the bromine solution to lose its colour

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.