3.4.2 Nucleophilic Substitution

• A nucleophile is an electron-rich species that can donate a pair of electrons
  • ‘Nucleophile’ means ‘nucleus / positive charge loving’ as nucleophiles are attracted to positively charged species
  • Nucleophilic refers to reactions that involve a nucleophile

Formation of alcohols

• The nucleophile in this reaction is the hydroxide ion, OH^- 
• An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used to form an alcohol
• This reaction is very slow at room temperature, so the reaction mixture is warmed
• This is an example of a hydrolysis reaction and the product is an alcohol
  • The rate of this reaction depends on the type of halogen in the halogenoalkane
  • The stronger the C-X bond, the slower the rate of the reaction
  • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
  • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

The halogen is replaced by the nucleophile, OH^- 

• This reaction could also be done with water as the nucleophile, but it is very slow
  • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
  • In water, the oxygen atom only carries a partial 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 the aqueous alkali

Reaction with water 

• The water molecule is a weak nucleophile, but it will eventually substitute for the halogen 
• This occurs much more slowly compared to when warm aqueous sodium hydroxide is used
• An alcohol is produced
  • RX + H₂O → ROH + H⁺ + X^-
  • CH₃CH₂Br + H₂O → CH₃CH₂OH + H⁺ + Br^-
• If silver nitrate solution in ethanol is added to the solution, the silver ions will react with the halide ions as soon as they form, giving a silver halide precipitate
  • Ag⁺ (aq) + X^- (aq) → AgX (s) 

Formation of nitriles

• The nucleophile in this reaction is the cyanide ion, CN^- 
• An ethanolic solution of potassium cyanide (KCN in ethanol) is heated under reflux with the halogenoalkane
• The product is a nitrile
  • E.g. bromoethane is heated under reflux with ethanolic potassium cyanide to form propanenitrile

The halogen is replaced by a cyanide group, CN ^-

• The nucleophilic substitution of halogenoalkanes with KCN extends the carbon chain by adding an extra carbon atom 
• 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 molecule, NH₃ 
• An ethanolic solution of excess ammonia (NH₃ in ethanol) is heated under pressure with a primary halogenoalkane
  • An excess of ammonia is used because the product is more reactive than ammonia so further substitution reactions could occur
• The product is a primary amine
  • E.g. bromoethane reacts with excess ethanolic ammonia when heated under pressure to form ethylamine

The halogen is replaced by an amine group, NH₂

Formation of alkenes 

• The halogenoalkanes are heated under reflux with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X^- ion and leaving an alkene as an organic product
  • E.g. bromoethane is heated under reflux with ethanolic sodium hydroxide to form ethene

Production of an alkene from a halogenoalkane by reacting it with ethanolic sodium hydroxide and heating it

 

Hydrogen bromide is eliminated to form ethene