Alkenes

• Electrophile is an electron pair acceptor
• Structural isomer is compounds with the same molecular formula but a different structural formula
• Stereoisomer is compounds with the same structural formula but a different arrangement of atom in a space
• E/Z isomerism is an isomerism that occurs as a result of restricted rotation around the planar C=C double bond
• Heterolytic fission is the breaking of a bond with two different species formed (two oppositely charge ions)
• Curly arrows show the movement of a pair of electrons
• Alkenes have the general formula CnH2n
• The shape around each carbon atom in the C=C bond is trigonal planar and the bond angle is 120
• Z isomers is the same side
• E isomers is opposite sides
• For E/Z isomerism to occur there must be 2 different atoms or groups attached to each carbon
• How to assign the E/Z isomerism:

  1. Assign a priority to each atom attached to directly each carbon using the atomic number
  2. If the highest priority are on the same side its a Z isomer
  3. If the highest priority are on opposite sides its a E isomer
• Alkenes are more reactive than alkanes because:

  • The pi bond is weaker than the sigma bond
  • due to the high electron density of the C=C bond, alkenes are more susceptible to attack electrophiles
• Addition reactions create a saturated molecule
• Observations during addition reaction of alkenes with halogens:

  Bromine water decolourises
• Hydrogen sulphates can be hydrolysed with warm water to form an alcohol.

  Sulfuric acid behaves as a catalyst as it is used up in step 1 but then regenerated
• Conditions of hydration reactions of alkenes with water (steam)

  phosphoric acid catalyst, 300 degrees, 60 atm
• Conditions of addition reactions of alkenes with hydrogen

  150 degrees and nickel catalyst
• The major product of an electrophilic addition is the molecule which is formed via a secondary carbocation, which is more stable than the primary

  The secondary carbocation has more alkyl groups, which are electron releasing, so the positive charge on the carbon is reduced (positive inductive effect)
• Addition polymerisation is when an alkene undergoes an addition reaction to itself
• Properties of polymers with no branching:

  • Has only Van Der Waals, which are weak intermolecular forces
  • However they can pack closely together
  • This makes the attraction stronger and makes the polymer strong and rigid
• Properties of branched polymers:

  • Has only Van Der Waals, which are weak intermolecular forces
  • They cannot pack closely together
  • This makes the attraction weaker
  • Making the polymer flexible and giving it a lower melting point
• Properties of polymers with Electronegative side chains:

  • has permanent dipole- dipole forces, which are stronger than VDW
  • This makes the polymer hard and rigid but brittle
• Plasticisers are small molecules which are added to polymers to get between neighbouring polymer chains to push them further apart. This weakens the intermolecular forces, so the material is more flexible and can slide past each other more easily