34.2 phenylamine and azo compounds

Describe the preparation of phenylamine

Nitration of benzene

Reduction of nitrobenzene with hot Sn/ concentrated HCl

Deprotonation: add NaOH

List the 2 ways that phenylamine can react

Reaction of phenylamine with Br2 at room temperature

Reaction of phenylamine with H2O2 or NaNO2

Explain how phenylamine reacts with Br2

• Electrophilic substitution

• reacts at RTP

• Aqueous Br2

• Lone pair of electrons in nitrogen atom in phenylamine amine donate electron density into benzene ring

• Delocalisation of electrons causes increased electron density in the benzene ring

• Therefore benzene ring becomes activated and more readily attacked by electrophiles.

• Incoming electrophiles directed to 2,4,6 positions

Explain the reaction of phenylamine with HNO2

The amine group of phenylamine reacts with nitrous acid at temperature below 10 to form diazonium salts

Since nitric acid is unstable it has to be made in the test tube by reacting sodium nitrite and dilute acid.

Upon further heating with water, diazonium salts form phenol.

Describe and explain the relative basicities of aqueous ammonia, ethylamine and phenylamine

1. (Strongest) Ethylamine

2. Ammonia

3. (Weakest) Phenylamine

• In ethylamine, the electron-donating alkyl group donates electron density to the N atom causing its lone pair to become more available to form a dative covalent bond with a proton.

• Ammonia lacks an electron-donating group

  • Hence, it is less basic than ethylamine

  • However it is more basic than phenylamine as the lone pair on the nitrogen is not delocalised

• In phenylamine, the lone pair of electrons overlap with conjugated system on benzene ring and become delocalised.

  • Therefore, lone pair of electrons become less readily available to form bond with proton.

How are azo compounds formed?

Coupling reaction of benzenediazonium chloride salt with alkaline phenol

• Diazonium ion acts as an electrophile and substitutes into the benzene ring of phenol, at the 4th position

• Alkaline conditions are required to deprotonate the organic product and form the azo compound

Why are azo compounds so stable?

Delocalised electrons in the pi bonding systems of 2 benzene rings are extended through N=N which acts as a bridge between the 2 rings

What is the azo group?

R1-N=N-R2

How are azo dyes made?

1. Formation of nitrous acid

2. Diazotisation: reaction between nitrous acid and phenylamine to form diazonium ion

3. Coupling reaction: The diazonium ion acts as an electrophile and substitutes into the benzene ring of the phenol, at the 4th position

   1. Alkaline conditions are required to deprotonate the organic product and form the azo compound

   2. Forming the azo compound: The azo compound is formed by the electrophilic substitution reaction of benzenediazonium chloride and phenol

      • 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

How can other azo dyes be made?

Via a similar route

the yellow dye can be formed from the coupling reaction between benzenediazonium chloride and C₆H₅N(CH₃)₂ instead of phenol (C₆H₅OH)

Draw the coupling reaction between benzenediazonium choride and phenol