properties of alkenes

• pi bonds are formed by electrons in the adjacent p-orbitals overlapping above and below the carbon atoms

  -> this bond can only be made after a sigma bond has been formed

  • the pi bond holds the atoms in position by restricting rotation around the double bond so alkene molecules have a flat shape in the region of the double bonds - they are described as planar
• the pi bond is the reactive part of the molecule because of the high electron density around it
• alkene double bond shape and bond angle are trigonal planar and 120 degrees
• alkenes are more reactive than alkanes due to high electron density of the double bond + the fact the pi bond is slightly easier to break
• intermolecular forces of alkenes are only alkenes due to non-polar nature of the double bond
• alkenes are insoluble in water because they have a double bond between carbon atoms which are non-polar bonds
• types of isomers that can be formed using alkenes are :

  • E/Z isomers due to restricted rotation
  • Cis-trans isomers if two of the same substituents are attached to each carbon
• most stable type of carbocation intermediate are alkyl groups as they have a positive inductive effect, so the most stable carbocation is the one bonded to the most other carbon atoms

  e.g. a tertiary carbocation
• major products will be formed from tertiary carbocations (or the most stable available
• Carbocation

  An electrically charged species that contains a positively charged carbon atom. Can be primary, secondary, or tertiary.
• Stability


  The tendency of a carbocation to persist without undergoing further reaction. A more stable carbocation is less reactive and persists for a longer time.
• Inductive effect

  A phenomenon in which the electrons in a bond are attracted towards a more electronegative atom. In carbocations, the positive charge on the carbon atom leads to a positive inductive effect, which stabilizes the carbocation.
• Alkyl groups

  Have the most positive inductive effect among carbocations, since they have a greater number of carbon atoms bonded to the positively charged carbon. This makes them more stable.
• Tertiary carbocation

  A carbocation with three other carbon atoms bonded to the positively charged carbon. It is more stable than secondary or primary carbocations due to the positive inductive effect.