Unit 4: Halogens and Halogenoalkanes

• What is a nucleophile?

  A nucleophile is an atom or molecule that donates a pair of electrons to form a chemical bond with an electron deficient atom
• What is the link between polar bonds and nucleophiles?

  The difference in electronegativity between two atoms in a compound leads to a permanent dipole between them. Therefore nucleophiles will attack the partially positive atom
• What are some examples of nucleophiles?

  OH- (lone pair on oxygen)

  NH3 (lone pair on nitrogen)

  H2O (lone pair on oxygen)

  CN- (lone pair on carbon)
• What is a reagent?

  A substance that comes out of a bottle and is a liquid, containing the nucleophile.

  ie in NaOH,

  • NaOH is the reagent
  • OH- is the nucleophile
• How many bonds does a cyanide ion have?

  Three
• During a halogenoalkane reaction with ammonia, what conditions need to be met?


  Excess ammonia
• What happens during the reaction of a halogenoalkane with ammonia?

  1. Halogen is substituted with the nucleophile (NH3) which forms four bonds, becoming unstable and positively charged.
  2. The excess ammonia ion (NH3) reacts with the hydrogen on the ammonia-halogenoalkane compound.
  3. Lone pairs are transferred from the nitrogen on the excess ammonia to the Nitrogen in the compound, breaking the bond.
  4. A product of a neutral ammonium salt is formed ie NH4Br
• Conditions for thermal cracking

  • High temperatures 700-1200K
  • High pressure 7000 KPa
  • Produces more alkenes for chemical feedstock
• Catalytic cracking conditions

  • Zeolite catalyst
  • Temperatures below 720K
  • Pressure >1 atm
  • Products- alkenes, branched alkanes, cycloalkanes and aromatic compounds
  • better fuels
• What are structural isomers?

  Compounds with the same molecular formula but a different structural formula
• What are chain/ branched isomers?


  When the carbon chain is arranged differently to the straight chain isomer
• What is a positional isomer?

  The functional group is arranged in a different position
• What is a functional group isomer?

  A compound with the same molecular formula but a different structural formula and functional group
• What determines the rate of nucleophilic substitution?

  The bond enthalpy of the C-X bond (X is the halogen). The smaller the bond enthalpy, the faster the rate of reaction
• What is the positive inductive effect?


  When nucleophilic substitution is happening, temporarily, the carbon that loses the halogen becomes positively charged.

  • The more R groups surrounding the carbon, the quicker the rate of reaction.
• What is an elimination reaction?

  A reaction where an atom/ atoms are removed from a reactant
• What is the nature of hydroxide ions?

  They are proton acceptors therefore they can act as a base depending on the conditions
• What are the products of an elimination reaction?

  Alkene and a leaving group.
• Reacting a haloalkane with a strong base gives amixture of products.

  • An elimination product- an alkene
  • A substitution product- alcohol
• When does an elimination reaction happen?

  ONLY in the prescence of a strong base
• When does a substiution reaction occur?

  • in primary compounds
  • at lower temperatures
  • with a weak base/ in aqueous conditions
• When does an elimination reaction occur?

  • in a tertiary compound
  • at high temperatures
  • in the prescence of a strong base
  • ethanoic conditions
• What is the polarity of haloalkanes like?

  As you go down group 7, polarity decreases due to a smaller difference in electronegativity
• Are halogenoalkanes soluble in water?

  No. They have no hydrogen bonds and no ion-dipole interaction
• Why do salts dissolve in water?

  Due to positive and negative ion interaction and dipole interaction
• Why do halogenoalkanes have higher boiling points?

  They have permanent dipole-dipole forces compared to other compounds with weak vanderwaals forces