Substitution Reactions of Halogenoalkanes (CIE AS Chemistry)

• 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

 Explaining the polarity of a carbon-halogen bond

Due to large differences in electronegativity between the carbon and halogen atom, the C-X bond is polar

Reaction with NaOH

• The reaction of a halogenoalkane with aqueous alkali results in the formation of an alcohol
• The halogen is replaced by the OH^-
• The aqueous hydroxide (OH^- ion) behaves as a nucleophile by donating a pair of electrons to the carbon atom bonded to the halogen
• For example, bromoethane reacts with aqueous alkali when heated to form ethanol
  • Hence, this reaction is a nucleophilic substitution
  • The halogen is replaced by a nucleophile, :OH^– 

CH₃CH₂Br + :OH^– → CH₃CH₂OH + :Br^–

Reaction with KCN

• 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
  • For example, bromoethane reacts with ethanolic potassium cyanide when heated under reflux to form propanenitrile
  • The halogen is replaced by a nucleophile, :CN^– 

CH₃CH₂Br + :CN^– → CH₃CH₂CN + :Br^–

 

• 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

Reaction with NH₃

• The nucleophile in this reaction is the ammonia, NH₃ molecule
• An ethanolic solution of excess ammonia (NH₃ in ethanol) is heated under pressure with the halogenoalkane
• For example, bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine
  • The product is a primary amine
  • The halogen is replaced by an amine group NH₂

CH₃CH₂Br + NH₃ → CH₃CH₂NH₂ + HBr 

• It is very important that the ammonia is in excess as the product of the nucleophilic substitution reaction, the ethylamine, can act as a nucleophile and attack another bromoethane to form the secondary amine, diethylamine

Reaction with aqueous silver nitrate

• Halogenoalkanes can be broken down under reflux by water to form alcohols
• The breakdown of a substance by water is also called hydrolysis

• For example, bromoethane reacts with aqueous silver nitrate solution to form ethanol
• This reaction is classified as a nucleophilic substitution reaction with water molecules in aqueous silver nitrate solution acting as nucleophiles, replacing the halogen in the halogenoalkane

C₂H₅Br + H₂O  C₂H₅OH + HBr

Nucleophilic substitution with OH^–

In nucleophilic substitution with OH^–, the bond that forms and the bond that breaks must both involve the carbon atom that is bonded to the leaving group

• This reaction is similar to the nucleophilic substitution reaction of halogenoalkanes with aqueous alkali, however, hydrolysis with water is much slower than with the OH^- ion in alkalis
  • 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

Comparing water and the hydroxide ion as nucleophiles

 

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