Testing for Alkenes and Alkanes

Testing for Alkenes and Alkanes

1. Combustion

   • Combust like any hydrocarbon
   • Are rarely used as fumes
   • C4H8 + 6O2 → 4CO2 + 4H2O

2. Addition

   • Alkene + Halogen -> Haloalkane(s)
     • No substitution
     • One product
     • Halgen added directly to Alkene
       • Because of double bond
         • Bond 'opens up' leaving single covalent bond linking the 2 carbons
           • The spare 2 electrons can form single bonds with halogen
     • eg.
       • propene + bromine -> 1,2dibromopropane
         • CH3CH=CH2Br2 -> CH2BrCHBrCH3

3. Combustion Reaction

   • Alkenes
     • Likely to incompletely combust
       • Therefore produce carbon monoxide
   • Alkanes
     • Likely to completely combust
       • Therefore don't produce carbon monoxide
   • Burn the substance
     • If produces carbon dioxide and watert
       • Alkane
     • If produces carbon monoxide, carbon and water
       • The carbon is released as soot
       • Alkene

4. Haloalkane Reaction with Bromine

   • Alkene/Alkane + Halogen -> Haloalkane
   • 1. Mix the unknown liquid with orange bromine water and shake
     • 2. If liquid has been decolourised a dibroalkane has been produced
       • Therefore it is an alkene
       • Wirte 'colourless' not 'clear' or 'see-through'
     • If stays orange the substance is an alkane
     • If the unknown substance is a gas::
       • Bubble the chemical through the bromine water

5. A haloalkane reaction with bromine is a better test to do than the combustion reaction test.

   • This is because sometimes the combustion reactions are not reliable
     • Alkens are likely not certain to incompletely combust

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