Alkenes

• Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds (C=C)

  • Their general formula is CnH2n
• Unsaturated compound has molecules in which one or more carbon and carbon bonds are not single
• The presence of the double bond, 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
• Alkenes are very reactive because of the presence of the C=C double bond which undergoes addition reactions
• The amount of longer chain hydrocarbons produced is far greater than needed

  • These long chain hydrocarbon molecules are further processed to produce other products
• Catalytic cracking - converting longer-chain molecules into short-chain and making more useful hydrocarbons

  • Shorter chain alkanes, alkenes and hydrogen are produced from the cracking of longer chain alkanes
  • Alkenes can be used to make polymers and the hydrogen used to make ammonia
  • Kerosene and diesel oil are often cracked to produce petrol, other alkenes and hydrogen
• Catalytic cracking diagram

  A) heat

  
• Cracking involves heating the hydrocarbon molecules to around 600 – 700C to vaporise them

  • The vapours then pass over a hot powdered catalyst of alumina or silica
  • Breaks covalent bonds in the molecules as they come into contact with the surface of the catalyst, causing thermal decomposition reactions
  • The molecules are broken up in a random way which produces a mixture of smaller alkanes and alkenes
  • Hydrogen and a higher proportion of alkenes are formed at higher temperatures and higher pressure
• 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 using bromine water
• Bromine water test - tests for unsaturation

  • Bromine water is an orange coloured solution of bromine
  • When bromine water is shaken with an alkane, it will still be orange as alkanes don't have double carbon bonds (C=C) so the bromine remains in solution
  • When bromine water is shaken with an alkene, the alkene will decolourise the bromine water as alkenes have double carbon bonds (C=C) and the bromine atoms add across the C=C double bond
  • This reaction between alkenes and bromine is called an addition reaction
• Bromine Water Test

  A) alkane

  B) alkene

  
• Addition reaction - a reaction in which a simple molecule adds across the cabon-carbon double bond of an alkene

  • The C=C double bond becomes a C-C single bond and other atoms hoin onto the 2 C atoms
  • Only 1 product is formed

  
• Alkenes also burn but are rarely burnes as they are too valuable and tend to undergo incomplete combustion with smoky flames
• When describing what happens to bromine water in an alkene ensure you say colourless, and not clear
• Changing alkenes to alkanes requires 150C and a nickel catalyst

  • it’s an addition reaction with H2
  • hydrogenation reactions
• Changing alkenes to alcohols requires steam (H2O), 300C, concentrated phosphoric acid catalyst, and 60atm

  • hydration reaction
  • its an addition reaction with H2O
• Alkenes with more than 2 C=C can have isomers that are differently named depending on the position of the C=C

  • But-1-ene would have the C=C at the start of the cabon chain but But-2-ene would have the C=C in the middle of the carbon chain
• Double bonds always break in addition reactions
• In alkene reactions with halogens, there are no requirements
• Catalytic cracking or thermical cracking - alkenes are obtained from alkanes by breaking large chain hydrocarnons into smaller more useful hydrocarbons

  • catalytic - hight temperatures + catalystic
  • temperature - high temperature and pressure
• Cracking diagram

  A) hydrocarbon

  B) heat

  C) catalyst

  
• Ethene is an unsaturated hydrocarbon that is formed by cracking alkanes
• Hydrogen can be produced by cracking e.g. C2H6 → C2 + H6
• • Cracking experiment

  A) Al2O3 catalyst

  B) mineral wool with hydrocarbon

  C) heat

  D) water

  E) ethene

  F) heat

  G) water goes into

  H) breaks

  
• Define unsaturated hydrocarbon

  A compound that contains only carbon and hydrogen with at least one double bond
• How are alcohols formed from alkenes?

  The hydration reaction requires high temperature, high pressure of 60atm, and phosphoric acid catalyst.
• How do you draw a polymer of an alkene?

  1. Start with the carbon=carbon double bond
  2. Add the atoms / groups of atoms above and below the C=C
  3. Replace the C=C with -C-C-
• How to form alcohols from alkenes?


  A hydration reaction which is an addition reaction with H2O. The conditions required are steam (H2O), 300C, concentrated phosphoric acid catalyst, and 60atm
• What are the uses of alkenes?

  Making plastics, alcohols, polymers, solvents
• What is the empirical formula for alkenes?

  CH2
• What catalyst is needed for alkenes to react with hydrogen?

  Nickel