• Halogenoalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
  • The halogen-carbon bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge
• A nucleophilic substitution reaction is one in which a nucleophile attacks a carbon atom which carries a partial positive charge
  • An atom that has a partial negative charge is replaced by the nucleophile
• In an elimination reaction of halogenoalkanes, a halogen halide is eliminated to form an alkene

Hydrolysis

• Halogenoalkanes can be broken down under reflux by water to form an alcohol
  • The breakdown of a substance by water is also called hydrolysis
• The reaction is classified as a nucleophilic substitution reaction as a water molecule in the silver nitrate solution acts as a nucleophile and replaces the halogen
  • Eg. bromoethane reacts with aqueous silver nitrate solution to form ethanol

The halogen is replaced by an hydroxyl group, OH^–

• This reaction is similar to the nucleophilic substitution reaction of halogenoalkanes with aqueous alkali, however the hydrolysis with water is much slower than with the OH^– ion
  • The hydroxide ion is a better nucleophile than water as it carries a full formal negative charge
  • In water, the oxygen atom only carries a partial negative charge

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than with aqueous alkali

• The halogenoalkanes have different rates of hydrolysis, so this reaction can be used as a test to identify halogens in a halogenoalkane by measuring how long it takes for the test tubes containing the halogenoalkane and aqueous silver nitrate solutions to become opaque

Formation of nitriles

• The nucleophile in this reaction is the cyanide, CN^– ion
• Ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
• The product is a nitrile
  • Eg. bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile

The halogen is replaced by a cyanide group, CN

• The nucleophilic substitution of halogenoalkanes with KCN adds an extra carbon atom to the carbon chain
• This reaction can therefore be used by chemists to make a compound with one more carbon atom than the best available organic starting material

Formation of primary amines by reaction with ammonia

• The nucleophile in this reaction is the ammonia, NH₃ molecule
• Ethanolic solution of excess ammonia (NH₃ in ethanol) is heated under pressure with the halogenoalkane
• The product is a primary amine
  • Eg. bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine

The halogen is replaced by an amine group,  NH₃

Elimination reaction

• The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming a X^– ion and leaving an alkene as organic product
  • Eg. bromoethane reacts with ethanolic sodium hydroxide when heated to form ethene

Hydrogen bromide is eliminated to form ethene