Aromatic Amines (Edexcel International A Level Chemistry): Revision Note

Aromatic Amine - Formation

• Phenylamine is an organic compound consisting of a benzene ring and an amine (NH₂) functional group

  • Phenylamine is sometimes known as aminobenzene or aniline

• Nitrobenzene, C₆H₅NO₂, can be reduced to phenylamine, C₆H₅NH₂, according to the following two-stage reaction:

The two-stage reduction reaction of nitrobenzene to phenylamine

Stage 1 - Reduction of nitrobenzene

• Nitrobenzene, C₆H₅NO₂, is reacted with tin, Sn, and concentrated hydrochloric acid, HCl

  • The combination of tin and hydrochloric acid acts as a reducing agent

• The reaction mixture is heated under reflux in a boiling water bath

• The nitrobenzene has been reduced, in the presence of acid, by gaining electrons from tin

  • The tin is oxidised to a mixture of tin(II), Sn²⁺, and tin(IV), Sn⁴⁺ 

• The reduction reaction of nitrobenzene does not form phenylamine directly

• Due to the acidic conditions, the reduction reaction forms the phenylammonium ion, C₆H₅NH₃⁺  

Stage 2 - Formation of phenylamine

• Excess sodium hydroxide solution, NaOH (aq), is added to the phenylammonium ions, C₆H₅NH₃⁺

  • This causes them to deprotonate and form the desired phenylamine product 

  • A mixture of tin compounds, including tin(II) hydroxide and tin(IV) hydroxide is also formed from reactions between the sodium hydroxide solution and the tin ions in stage 1

Stage 3 - Purification of phenylamine

• The crude phenylamine product undergoes steam distillation to produce a cloudy distillate

• Sodium chloride is added to the distillate before the mixture is added to a separating funnel

• Ether is added to the separating funnel resulting in an aqueous layer at the bottom and an organic layer, containing the phenylamine, on the top

  • The sodium chloride aids separation by increasing the polarity of the aqueous layer causing the phenylamine to "salt out" into the organic layer

• The aqueous layer is discarded and the organic layer is distilled

  • The ether will boil off easily and is discarded 

  • The phenylamine fraction that boils off at 180 - 185 ^oC is retained and can have its boiling point tested to check the purity of the product 

Aromatic Amine - Reactions

• Azo (or diazonium) compounds are organic compounds that have an R₁-N=N-R₂ group

• They are often used as dyes and are formed in a coupling reaction between the diazonium ion and an alkaline solution of phenol

Azo compounds are characterized by the presence of an R₁-N=N-R₂ group

Coupling of benzenediazonium chloride with phenol in NaOH

• Azo compounds can be formed from the coupling reaction of a benzenediazonium chloride salt with alkaline phenol

• Making an azo dye is a multi-step process

Formation of Azo Compounds Table

Reaction mechanism of the formation of azo compounds

• The delocalised electrons in the π bonding systems of the two benzene rings are extended through the -N=N- which acts as a bridge between the two rings

• As a result of the delocalisation of electrons throughout the compound, azo compounds are very stable