Alkenes

• Alkenes

  Molecules with one or more C=C double bonds
• Alkenes

  • General formula is CnH2n
  • C atoms in a C=C double bonds have three bonds in total, which means that the shape around C=C double bonds is planar and the bond angle 120°
• σ bond


  Same as a regular single covalent bond
• π bond


  The added bond that turns a single bond into a double bond
• C=C double bonds are stronger than C-C single bonds
• Sigma bonds are stronger than pi bonds
• Pi bonds are more reactive than sigma bonds
• Alkenes are more reactive than alkanes
• There is no rotation around a C=C double bond
• Geometrical isomers of 1,2-dichloroethene

  • Isomer 1
  • Isomer 2
• Restricted rotation or no rotation around C=C bond prevents isomer 1 from changing into isomer 2
• Electrophile


  An electron-pair acceptor
• Electrophiles that react with alkenes

  • Hydrogen halides (e.g. HCl, HBr)
  • Halogens (e.g. Cl2, Br2)
  • Sulphuric acid (H2SO4)
• Electrophilic addition with hydrogen halides

  1. Reagent: HCl, HBr

  2. Conditions: None

  3. Type of product: Haloalkane
• Carbocation

  Reactive intermediate in electrophilic substitution, a positive molecular ion with the charged atom being carbon
• Tertiary carbocations are more stable than primary carbocations due to the positive inductive effect of the alkyl groups
• Asymmetrical alkenes reacting with HBr

  1. bromopropane is the major product because the secondary carbocation is more stable than the primary carbocation
• Electrophilic addition with halogens

  1. Reagent: Cl2, Br2

  2. Conditions: None

  3. Type of product: Haloalkane
• Bromine reacts with alkenes because the C=C bond is electron rich and induces a dipole in the Br2
• Chemical test for alkenes

  • Reagent: Br2
  • Observation when alkene present: Orange to colourless
  • Observation when alkene not present: No visible change
• Electrophilic addition with sulphuric acid

  1. Reagent: H2SO4

  2. Conditions: None

  3. Type of product: Alkyl hydrogen sulphate
• Alkyl hydrogen sulphates


  Name the carbon chain as an alkyl chain (methyl, ethyl etc.) and end the name in 'hydrogensulphate'
• Making alcohols from alkenes in a two-step process

  1. Step 1: alkene + H2SO4 → alkyl hydrogen sulphate

  2. Step 2: alkyl hydrogen sulphate + H2O → alcohol + H2SO4
• Direct hydration of alkenes

  1. Reagent: H2O

  2. Conditions: Conc. H3PO4 catalyst

  3. Type of product: Alcohol
• Addition polymers


  Long chains made from lots of alkene monomers
• Monomers and repeating units of addition polymers

  • Monomer: (CH3)2C=CHCH3, Repeating unit: (CH3)2CHCH2
  • Monomer: CH2=CHCH3, Repeating unit: CH2CHCH3
  • Monomer: CH2=CH2, Repeating unit: CH2CH2