3.3.4 Alkenes

• Alkenes are unsaturated hydrocarbons and have at least one C=C double covalent bond.
• Alkenes are relatively reactive due to high electron density in the C=C bond.
• Electrophiles are electron pair acceptors.
• Electrophiles are positively charged ions.
• Electrophiles are polar molecules with a delta+ region.
• Electrophiles attack the electron dense C=C bond.
• A carbocation is an organic ion which contains a carbon atom that is positively charged.
• A carbocation is an organic ion which contains a carbon atom that is positively charged.
• Alkenes will burn in air to form CO2 and H20 .
• State the mechanism of electrophilic addition.

  1. Electrophile is attracted to the double bond.
  2. Electrophiles are positively charged and accept a pair of electrons from the electron dense pi bond.
  3. A positive ion called a carbocation is formed.
  4. A nucleophilic substitution reaction now occurs and the negatively charged ion forms a bond with the carbocation.
• When ethene reacts with HBr, the only product is bromoethane. However when the C=C isn't in the middle more than one product can be formed.
• The positive inductive effect states that alkyl groups have a tendency to release electrons.
• The more alkyl groups attached to a carbocation, the more stable the carbocation is.
• A tertiary carbocation (3 Alkyl) is more stable than a secondary carbocation (2 Alkyl) is more stable than a primary carbocation (1 Alkyl).
• A tertiary carbocation (3 Alkyl) is more stable than a secondary carbocation (2 Alkyl) is more stable than a primary carbocation (1 Alkyl).
• A tertiary carbocation (3 Alkyl) is more stable than a secondary carbocation (2 Alkyl) is more stable than a primary carbocation (1 Alkyl).
• The product will tend to be the most stable carbocation.
• Addition polymers are formed when many alkene monomers open up their double bonds and join together.
• The chains in addition polymers are held together by van Der Waals forces, the longer and closer together the polymer chains the stronger the van Der Waals forces are.
• Stronger van der Waals forces mean that the addition polymer is more stronger and rigid.
• Addition polymers are made of a repeating unit.
• A plasticiser can be used with addition polymers to make them more flexible.
• Rigid PVC is used for window frames and drain pipes.
• Plasticised PVC is used for cable insulation, clothing and tiles.
• Outline the mechanism of an electrophilic addition reaction between ethene and hydrogen bromide.

  Mechanism:

  
• Outline the mechanism of an electrophilic addition reaction between ethene and Bromine.

  Mechanism:

  
• Outline the mechanism of an electrophilic addition reaction between ethene and sulfuric acid.

  Mechanism: