Phenylamine and azo compounds — practice questions

1. Calculate moles of nitrobenzene (reactant): Moles = Mass / $M_r$ = 12.30 g / 123.0 g mol⁻¹ = 0.100 mol
2. Determine theoretical moles of phenylamine (product): The reaction is $C_6H_5NO_2 \rightarrow C_6H_5NH_2$. The molar ratio is 1:1. Therefore, theoretical moles of phenylamine = 0.100 mol.
3. Calculate theoretical mass of phenylamine: Mass = Moles × $M_r$ = 0.100 mol × 93.0 g mol⁻¹ = 9.30 g
4. Calculate percentage yield: % Yield = (Actual Mass / Theoretical Mass) × 100 % Yield = (7.44 g / 9.30 g) × 100 = 80.0%

Answer: The percentage yield is 80.0%.

Step 1: Nitration of Benzene

• Reagents: Concentrated nitric acid ($HNO_3$) and concentrated sulfuric acid ($H_2SO_4$) (as catalyst).
• Conditions: Heat at 55°C.
• Product: Nitrobenzene ($C_6H_5NO_2$).

Step 2: Reduction of Nitrobenzene

• Reagents: Tin (Sn) and concentrated hydrochloric acid (HCl), followed by addition of excess NaOH(aq).
• Conditions: Heat under reflux.
• Product: Phenylamine ($C_6H_5NH_2$).

Step 3: Diazotisation of Phenylamine

• Reagents: Sodium nitrite ($NaNO_2$) and dilute hydrochloric acid (HCl).
• Conditions: Maintain temperature below 10°C (e.g., in an ice bath).
• Product: Benzenediazonium chloride ($C_6H_5N_2^+Cl^-$).

Step 4: Coupling Reaction

• Reagents: Phenol ($C_6H_5OH$) dissolved in aqueous sodium hydroxide (alkaline conditions).
• Conditions: Maintain temperature below 10°C.
• Product: 4-hydroxyphenylazobenzene (orange precipitate).