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6.2.2 Alkenes

Alkenes Structure & Formula

  • Alkenes are unsaturated hydrocarbons
  • All alkenes contain a double carbon bond, which is shown as two lines between two of the carbon atoms i.e. C=C
  • All alkenes contain a double carbon bond, which is the functional group and is what allows alkenes to react in ways that alkanes cannot
  • The names and structure of the first four alkenes are shown below:

Table of alkenes

Displayed formula Name Molecular formula
ethene ethene C2H4 
propene propene C3H6 
but-1-ene but-1-ene C4H8 
pent-1-ene pent-1-ene C5H10 

The first four members of the alkene homologous series

  • 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
  • This makes them much more reactive than alkanes

Double Bond Opening, IGCSE & GCSE Chemistry revision notesA carbon-carbon double can break and form a single bond, allowing more atoms to attach to the carbon atoms

Exam Tip

The numbers in butene, pentene and hexene refer to the carbon atom in which the C=C begins, counting from the left. E.g. pent-2-ene, C5H10 has the C=C between the 2nd and 3rd carbon atoms. In pent-3-ene the C=C bond is between the 3rd and 4th carbon atoms from the left.

Reactions of Alkenes

Combustion of Alkenes

  • These compounds undergo complete and incomplete combustion but because of the higher carbon to hydrogen ratio they tend to undergo incomplete combustion, producing a smoky flame in air.
  • Complete combustion occurs when there is excess oxygen so water and carbon dioxide form e.g:

C4H8           +          6O2       →      4CO2       +         4H2O

butene      +      oxygen    →  carbon dioxide   +  water

  • Incomplete combustion occurs when there is insufficient oxygen to burn so a mixture of products can form, e.g:

C4H8     +          4O2          →        4CO           +         4H2O

butene      +      oxygen    →  carbon monoxide   +  water

  • In addition to carbon monoxide, carbon in the form of soot can be produced:

C4H8     +          2O2          →        4C           +         4H2O

butene      +      oxygen    →  carbon   +     water

  • This is more likely to occur in higher alkenes with larger number of carbons
  • This is seen as smoky yellow flames when the alkenes burn

Addition Reactions

  • The chemistry of the alkenes is determined by the C=C functional group
  • Since all members of the alkene homologous series contain the same functional group then they all react similarly
  • Alkenes mainly undergo addition reactions in which atoms of a simple molecule add across the C=C double bond
  • The carbon-carbon double bond opens up, forming a single bond between the carbons allowing for two more atoms to bond, one on each carbonAlkene - General Equation Additon, IGCSE & GCSE Chemistry revision notesDiagram showing the general equation for the addition reaction of alkenes

Hydrogenation

  • Alkenes undergo addition reactions with hydrogen in which an alkane is formed
  • These are hydrogenation reactions and occur at 150ºC using a nickel catalyst
  • Hydrogenation reactions are used to manufacture margarine from vegetable oils
    • Vegetable oils are polyunsaturated molecules which are partially hydrogenated to increase the Mr and turn the oils into solid fats

Addition- Hydrogen to Ethene, IGCSE & GCSE Chemistry revision notesHydrogen atoms add across the C=C in the hydrogenation of ethene to produce ethane

Halogenation

  • The halogens also participate in addition reactions with alkenes
  • 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
  • The reaction occurs readily at room temperature

Bromine-Addition-to-Ethene, IGCSE & GCSE Chemistry revision notesBromine atoms add across the C=C in the addition reaction of ethene and bromine

Bromine Water Test

  • 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 Alkenes, IGCSE & GCSE Chemistry revision notes

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 bromine atoms add across the C=C bond, hence the solution no longer contains free bromine so it loses its colour

Exam Tip

Since members of the same homologous series react similarly, we can deduce the reactions of other compounds of the same series based on observations of how just one member of the series reacts.

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