Aromatic Amines (Edexcel International A Level Chemistry)

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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, downloadable IB Chemistry revision notes

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

 Nitrogen Compounds - Azo Compounds, downloadable AS & A Level Chemistry revision notes

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

Step  Description Reaction Conditions
1. Formation of nitrous acid  Nitrous acid is very unstable so has to be made in a test tube using sodium nitride (NaNO2) and hydrochloric acid N/A
2. Diazotization The reaction between nitrous acid and phenylamine to form the diazonium ion is called diazotization

The reaction mixture must be kept below 10oC using ice as otherwise the diazonium ion will thermally decompose to benzene and nitrogen (N2)

Dilute acid (such as HCl) 

3. Coupling reaction The diazonium ion acts as an electrophile and substitutes into the benzene ring of the phenol at the 4th position Alkaline conditions are required to deprotonate the organic product and form the azo compound 

Nitrogen Compounds - Formation Azo Compounds (1), downloadable AS & A Level Chemistry revision notes Nitrogen Compounds - Formation Azo Compounds (2), downloadable AS & A Level Chemistry revision notes

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

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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.