## 12.5 Phenylamine and Azo Compounds This section explores key reactions of phenylamine, focusing on its reactivity towards bromine and the preparation of diazonium salts. ### 1. Introduction to Phenylamine Phenylamine (also known as aniline) is an aromatic amine where an amino group ($-\mathrm{NH}_2$) is directly attached to a benzene ring. The lone pair of electrons on the nitrogen atom in the amino group can delocalize into the benzene ring, making the ring significantly activated towards electrophilic substitution reactions, particularly at the *ortho* and *para* positions. ### 2. Reactions of Phenylamine #### 2.1. Reaction of Phenylamine with Bromine Water (Electrophilic Substitution) Phenylamine is highly reactive towards electrophilic substitution. When bromine water is added to an aqueous solution of phenylamine at room temperature, several observations are made: * The reddish-brown colour of bromine water is **decolorized**. * A **white precipitate** is produced. The product formed is 2,4,6-tribromophenylamine. This reaction is a characteristic test for activated aromatic rings, as the amino group strongly activates the ring, leading to multiple substitutions even with a mild electrophile like bromine water. **Reaction Equation:** $$ \mathrm{C_6H_5NH_2(aq) + 3Br_2(aq) \rightarrow C_6H_2Br_3NH_2(s) + 3HBr(aq)} $$ **Structural Representation:** #### 2.2. Reaction of Phenylamine with Nitrous Acid (Preparation of Diazonium Salts) The reaction of primary aromatic amines (like phenylamine) with nitrous acid ($HNO_2$) in the presence of a strong acid (like $HCl$) at low temperatures is known as **diazotization**. This reaction forms highly versatile diazonium salts. ##### 2.2.1. Formation of Diazonium Salts * **Reagents:** Phenylamine, sodium nitrite ($NaNO_2$) or potassium nitrite ($KNO_2$), and hydrochloric acid ($HCl$). Nitrous acid ($HNO_2$) is generated *in situ* from $NaNO_2$ and $HCl$. $$ \mathrm{NaNO_2 + HCl \rightarrow HNO_2 + NaCl} $$ * **Conditions:** The reaction solution containing phenylamine in hydrochloric acid must be **cooled in ice (below $5^{\circ} \mathrm{C}$)**. The sodium or potassium nitrite solution is also ice-cooled and added slowly to the phenylamine solution. * **Product:** A solution containing phenyl diazonium chloride (also known as benzene diazonium chloride) is produced. * **Critical Temperature Control:** The temperature of the reaction mixture **should never go above $5^{\circ} \mathrm{C}$**. **Reaction Equation (Diazotization):** $$ \mathrm{C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O} $$ ##### 2.2.2. Stability and Decomposition of Diazonium Salts Diazonium salts are highly unstable compounds. * They tend to decompose readily **above $10^{\circ} \mathrm{C}$**. * Warming an aqueous solution of phenyl diazonium chloride (or benzene diazonium chloride) causes it to decompose, producing **phenol** and releasing nitrogen gas. **Decomposition Reaction (Hydrolysis):** $$ \mathrm{C_6H_5N_2^+Cl^-(aq) + H_2O(l) \xrightarrow{\text{warming}} C_6H_5OH(aq) + N_2(g) + HCl(aq)} $$ ### 3. Coupling Reactions and Azo Dyes Diazonium salts react with other aromatic compounds like phenols or amines to form **azo compounds**. These compounds contain the $-\mathrm{N=N}-$ (azo) group which links two aromatic rings. * **Conditions:** Reaction with phenol occurs in mildly alkaline conditions. * **Product:** 4-hydroxyazobenzene (an orange dye).