6.2.9/6.2.10 - Chemical Reactions of the Alkenes

• combustion of alkenes

  burn in air and oxygen to form water and carbon dioxide in a more smoky flame that alkanes because it contains a higher percentage of carbon
• what happens during electrophilic addition reactions of alkenes?

  double bond has lots of electrons so is attacked by electrophiles so opens up and atoms are added to carbon atoms
• electrophiles

  positively charged species that accept electrons - have a vacant orbital
• nucleophiles

  negatively charged species with a lone pair of electrons
• carboncations

  a reactive species which contains a lone pair of electrons
• conditions for addition reaction of hydrogen

  nickel catalyst

  temperature of 150 degrees
• role of the nickel catalyst in addition of hydrogen

  provides surface area for reactants to bind to (adsorption) so bonds weaken so reaction happens and products are released (deadsorption)

  heterogenous catalyst
• how is margarine made?

  by hydrogenating unsaturated vegetable oil (liquid containing cis-C=C bonds) so changing to C-C bonds which turns the oily liquid into soft, unsaturated solids like margarine
• problem with margarine

  saturated fats contribute to heart disease
• partial hydrogenation to make margarine

  when fats are partially saturated so less saturated fats are present

  but trans-fats are still present which lead to high blood cholesterol
• how are trans fats removed from margarine?

  hydrogenate a proportion of vegetable oil completely to remove all C=C bonds hence all trans-fats then blend it with untreated oil to make a spread of the correct texture with no trans-fats
• what does the addition of hydrogen to alkenes make?

  an alkane
• what does the addition of halogens to alkenes form?

  dihalogenoalkanes - halogens added across the double bond
• trend in addition reaction of halogens

  as you go down group 7 the reaction becomes slower as you go down group 7 because reactivity of halogens decreases
• what happens when you test for alkenes?

  bromine water decolourises the solution because bromine is added across the double bond forming colourless dibromoalkane

  electrophilic addition reaction
• what does the addition of halogen halides to alkenes form?


  halogenoalkanes
• temperature for addition reaction of hydrogen halides

  room temperature
• conditions for addition of steam to alkenes

  300 degrees

  60-70 atm

  acid catalyst - phosphoric acid
• product of addition of steam to alkenes

  alcohol (reversible reaction)
• conditions for oxidation by potassium manganate (VII)

  acidic conditions

  room temperature
• what happens during the oxidation of alkenes by cold potassium manganate (VII) under acidic conditions?

  oxidises alkene, converting it to a diol (2 -OH groups)

  purple MnO4^- ions are reduced to pale pink Mn^2+ ions
• observation from oxidation of alkenes by potassium manganate (VII)

  purple solution decolourises