halogenoalkane

• halogenoalkanes can be primary, secondary or tertiary
• halogenoalkanes can undergo substitution or elimination
• substitution is swapping halogen for another atom or group of atoms
• nucleophile is electron pair donator

  OH-, NH3, CN-
• Nucleophilic substitution

  1. arrow from nucleophile to carbocation
  2. arrow from bond to halogen
  3. nucleophile replaces halogen
• weaker c-x bonds are easier to break making the substitution reaction faster
• nucleophilic substitution with cyanide ion requires

  reagent: KCN dissolved in ethanol

  conditions: heat under reflux
• nucleophilic substitution with ammonia requires

  reagent: NH3 dissolved in ethanol

  condition: heat under pressure
• nucleophilic substitution of ammonia steps

  1. arrow from lone pair to carbocation
  2. arrow to halogen
  3. ammonia replaces halogen group and nitrogen has +
  4. another NH3 has arrow going to hydrogen on ammonia already bonded
  5. arrow going from N-H to N+
  6. compound ends in NH2 with NH4 X
• more substitution reactions can occur between the halogenoalkane and the amines formed leading to a lower yield of the amine

  excess ammonia prevents this
• elimination: removal of small molecule (often water) from organic molecule
• elimination with alcoholic hydroxide ions

  reagent: potassium or sodium hydroxide

  condition: in ethanol, heat
• aqueous solvent = substitution

  alcoholic solvent = elimination
• elimination steps

  1. arrow from OH to hydrogen next to halogen
  2. arrow from C-H to C-C (double bond forms)
  3. arrow from C-X to X
  4. produces an alkene, X, and water
• chloroalkanes and chlorofluoroalkanes can be used as solvents
• halogenoalkane can be used as refrigerant, pesticide and aerosol propellant
• ozone layer in upper atmosphere filters out suns UV radiation
• ozone in lower atmosphere is a pollutant and contributes towards the formation of smog
• man-made CFCs caused a hole to form in the ozone layer
• chlorine radicals are formed in the upper atmosphere when energy from UV radiation causes C-Cl bonds in CFCs to break
• chlorine free radical atoms catalyse decomposition of the ozone because they are regenerated- provide an alternative route with lower activation energy
• ozone equation

  Cl. + O3 --> ClO. + O2

  ClO. + O3 --> 2O2 + Cl.

  Overall equation 2 O3 --> 3 O2
• Regenerated Cl radical means one Cl radical can destroy thousands of ozone molecules
• hydrofluorocarbons are now used for refrigerators and air conditioning

  they are safer because they do not have C-Cl bond
• C-F is stronger than C-Cl and is not affected by UV radiation