Alkenes

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  • Alkenes
    • Stereoisomerism
      • Have the same structural formula but different arrangement of atoms
      • EZ Isomers
        • Two different groups attached to either end
        • Can't rotate because of the double bond
          • Alkanes can rotate because of single bond
        • E - priority groups at opposite sides
        • Z - Priority groups at the same side
          • Priority group is  atoms with bigger Ar
    • Electrophilic addition reactions
      • Reaction of hydrogen bromide with Alkene
        • Works the same with hydrogen chloride
        • Bromine is more electronegative than hydrogen
          • Makes the molecule polar
          • H delta+ is attracted to the electron rich pi bond
            • Lead to a major and a minor product when the alkene is unsymmetrical
              • Markownikoff's rule
                • Bromine added to the carbon with the fewest hydrogens attached to it
              • Both products are haloalkanes
              • Major product formed by the more stable carbocation
                • More stable carbocation = electrons from methyl groups reduce the ions charge
        • At room temperature
      • Reaction of sulphuric acid with alkenes
        • Two stages
          • Electrophilic addition and then hydrolysis
            • Electrophilic addition produces alkyl hydrosulphate
            • Hydrolysis makes alcohol
              • Add water to split molecules
        • Room temperature
        • Sulphuric acid can be drawn in two different ways
          • H2SO4 in equations
          • H-OSO2OH in the mechanism
          • Role of sulphuric acid is a catalyst as it is regenerated
        • Can have major and minor products
      • Reaction of bromine with alkenes
        • At room temperature
        • The electron pair in the  Br-Br bond is repelled inducing a dipole
          • Becomes polar and electrophilic
          • Electron from C=C transfers to Br
            • C has a positive charge after
              • Carbocation
                • :Br- attracted to C+
                  • Product is a dihaloalkane
            • Makes :Br-
              • :Br- attracted to C+
                • Product is a dihaloalkane
      • Double bonds in alkenes have areas of high electron density
        • Attracts electrophiles
      • Addition reaction: two molecules become one product
    • Industrial hydration
      • Alkenes are converted to alcohols in one step rather than two
        • Uses water in the presence of an acid catalyst
          • Process is called hydration
        • Needs high pressures so can't be done in a labratory
        • Has a high atom economy so it is cheaper to carry out overall
    • Testing for Alkenes
      • Bromine de-colourises in the presence of a double bond
        • Brown to clear/transparent
    • Addition Polymers
      • Poly(alkenes) are unreactive
        • Because of strong C-C, C-H bonds
      • Repeated unit doesn't have a double bond
      • Poly(ethene)
        • Used to make plastic bags/buckets/bottles
        • Flexible, easily moulded, waterproof, chemical proof and low dencity plastic
      • Poly(Propane)
        • Stiffer polymer used in utensils and containers and the fibres in robe and carpets
      • Formed from alkenes in a process called addition polymerisation
    • Unsaturated hydrocarbons
      • General formula CnH2n
      • C=C
      • Numbers are added to the names to show the position of the double bond

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