3.3 alkenes

• • double carbon bonds contain one sigma bond and one pi bond
  • the pi bond is exposed and have high electron density, electrophiles attack it because they like electrons
• electrophilic addition

  -when an electrophile attacks a double carbon pi bond and its an addition as two molecules react to form one
• electrophilic addition ( alkene + halogen )

  • reagent - bromine
  • type of reagent - electrophile Br
  • product - dihaloalkane
  • condition - room temperature
• numbers need to be added to the alkene name when positional isomers can occur
• electrophilic addition of Br2

  • s the Br2 molecule approaches the alkene, the Pi bond repels the electron pair in the Br-Br bond, this induces a dipole
  • Br2 becomes polar and electrophilic (Br+)
• the intermediate formed when theres a positive charge on the carbon atom is called a carbocation
• electrophillic addition (hydrogenhalide)

  • alkene + hydrogen halide produces a haloalkane
  • this reaction leads to two products if the alkene is unsymetrical
• • HBr is a polar molecule becuase Br is more electronegative than H
  • H+ is attracted to the electron rich pi bond
• in electrophilic addition to alkenes the major product is formed via the more stable carbocation
• exam anwser (stable carbocation)

  • draw out both carbocations, identify if its primrary , secondary or tertiary
  • state which is more stable and why
  • if both carbocations are secondary then both will be equally stable and there will be a 50/50 split
• electrophilic addition of sulfuric acid

  (stage 1)

  • alkene + sulfuric acid = alkyl hydrogensulfate
  • condition : room temp
  • reagent : concentrated sulfuric acid

  (stage 2)

  • alkyl hydrogensulfate + water = alcohol + sulfuric acid
  • hydrolysis reaction
  • condition : warm mixture

  overall reaction : sulfuric acid acts as a catalyst as it keeps getting regenerated
• industrial hydration of alkenes to alcohols

  • industrially produced alcohols occur in one step, alkenes are reacted with water in the presence of an acid catalyst

  essential conditions:

  • high temperatures (300-600)
  • high pressures (70atm)
  • catalyst : concentrates H3PO4

  high pressures means it cant be done in a laboratory , but there are no waste products so have a high atom economy
• test for alkenes

  • using bromine water to test for double bonds , bromine water decolourises if theres a positive result
• polychloroethene

  • waterproof , electrical insulator and doesnt react with acids
  • in its pure form its a rigid plastic due to strong intermollecular bonding
  • when plasticiser is added , intermollecular forces weaken and the chains move more easily and its more flexible
• Numbers need to be added to the name when positional isomers can occur
• E-Z isomerism occurs when

  1. there's a double bond so there's restricted rotation
  2. there are two different atoms/groups at both ends of the double carbon bond
• what is E-Z isomerism?

  stereoisomers have the same structural formula but different spatial arrangment
• when an unsymmetrical alkene reacts with HBr or sulphuric acid, two products can be formed

  • Br bonds with the carbon with the least hydrogen
  • the more stable carbocation forms the major product
• If the alkene is unsymmetrical, addition of hydrogen bromide can lead to twoisomeric products.
• the carbocation intermediate is more stable because the positive carbon had a methyl group on either side which releases electrons, lowering the charge on the ion making it more stable
• how is biofuel purified?

  fractional distiallation
• draw the reaction of Br2 with ethene


  electrophilic addition

  
• draw the mechanism of Hbr with ethene


  electrophilic addition

  
• draw the mechanism of the reactions with propene and H2SO4

  electrophilic addition

  
• draw the structure of H2SO4 in exams and the real structue


  H2SO4

  
• what is the arrangement of bonds and bond angle

  planar and 120 degrees