Nucleophilic Properties (Oxford AQA International A Level Chemistry)

Nucleophilic Substitution with Amines

• The lone pair on the nitrogen atom in amines makes them good nucleophiles, just like ammonia

• When ammonia reacts with a halogenoalkane, a nucleophilic substitution reaction takes place forming a primary amine

• For example chloromethane reacts with ammonia in two steps to make methylamine and ammonium chloride

CH₃Cl + NH₃ → [CH₃NH₃]⁺Cl ^-

[CH₃NH₃]⁺Cl ^- + NH₃ → CH₃NH₂ + NH₄⁺Cl ^-

• The methylamine is also a good nucleophile so can undergo further substitution with chloromethane to make dimethylamine

CH₃Cl   + CH₃NH₂ → [(CH₃)₂NH₂]⁺Cl ^-  

[(CH₃)₂NH₂]⁺Cl ^- + NH₃ → (CH₃)₂NH + NH₄⁺Cl ^-

• The dimethylamine can further substitute giving a tertiary amine, trimethylamine

CH₃Cl + (CH₃)₂NH → [(CH₃)₃NH]⁺Cl ^-  

[(CH₃)₃NH]⁺Cl ^- + NH₃ → (CH₃)₃N + NH₄⁺Cl ^-

• The final substitution occurs when the tertiary amine reacts with the chloromethane to make a quaternary ammonium salt

CH₃Cl + (CH₃)₃N → [(CH₃)₄N]⁺Cl ^-  

• Since all these multiple substitutions occur, it is not a very efficient way to synthesise amines

• If you want to just produce the primary amine then a large excess of ammonia is used to ensure it is the dominant nucleophile in the reaction vessel

Mechanism of nucleophilic substitution

Quaternary ammonium salts

• Quaternary ammonium salts are chemical analogues to ammonium chloride, NH₄Cl, in which the hydrogens have been replaced by four alkyl groups

• They are very useful in the manufacture of cleaning products such as conditioners for washing hair and in fabric softeners

• In this role they act as cationic surfactants

Cationic surfactants

• Surfactants are chemicals that lower the surface tension between immiscible liquids and allow wetting to take place

• They are cationic due to the positive charge on the nitrogen atom

• The hydrocarbon groups are hydrophobic and the amine group is hydrophilic

• This allows the molecules to cluster on the surface of water with their hydrophilic charged ends in the water and their hydrophobic tails on the surface, hence the term surfactant

• When hair or clothing is wet it can pick up negative charges that facilitate the build up of static electricity when dry

• The conditioner or softener is attracted to the wet surfaces due to the positive charge on the surfactant

• This forms a coating on the hair or clothing, giving a smooth coating and preventing the build up of static