Reactions of Halogenoalkanes Edexcel AS Unit 2

Halogenoalkanes are compounds with one or more Hydrogen atom is an alkane has been replaced by halogen atoms and has only single bonds in the molecule.

They can be Primary, Secondary, or Tertiary. Tertiary halogenoalkanes are the most reactive of the three.

Halogenoalkanes have London forces which are stronger than the corresponding alkanes, and they have permanent dipole-dipole forces.

They are insoluble in water.

Chloroalkanes.

- Add Phosporus Pentachloride to the alcohol.

- Add concentrated  Sulphuric Acid to a mixture of propan-2-ol and potassium chloride. The sulphuric acid protonates the alcohol and releases hydrogen chloride.

Bromoalkanes.

- Add 50% Sulphuric Acid, slowly, with cooling to the alcohol. Add to Solid Potassium Bromide and heat. Then distill off the bromoalkane.

Iodoalkanes.

- Warm alcohol with a mixture of damp red phosphorus and iodine. the iodine reacts with the phosphorus to produce phosphorus triiodide, which reacts with the alcohol to form the iodoalkane.

Reagent: Sodium (or Potassium) Hydroxide.

Conditions: Heat under reflux in aqeous solution.

Product: Alcohol.

Mechanism:  Nucleophilic Substitution.

Reagent:concentrated Potassium Hydroxide.

Conditions: Heat under reflux in ethanol solution.

Product: Alkene.

Mechanism: Elimination.

Reagent: Excess Ammonia.

Conditions: Concentrated solution of ammonia at room temperature, or heated in a sealed tube.

Procuct: Amine.

Mechanism: Nucleophilic Susbtitution.

Reagents: Water containing dissolved Silver Nitrate.

Condtitions: Water, ethanol and silver nitrate.

Products: Alcohol and AgX precipitate.

Mechanism: Nucleophilic Substitution.