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:
- 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.
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.
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.
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.
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.
Each carbon atom of the double bond accepts a bromine atom, causing the bromine solution to lose its colour
Edexcel GCSE Chemistry Notes
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