Diazonium Salt & Diazotization - Chemistry Class 12 - NEET PDF Download

Introduction

Diazonium salts are reactive organic compounds formed by the diazotisation of primary aromatic amines using nitrous acid in an ice-cold acidic medium. They contain the functional group Ar–N₂⁺ and are stable only at low temperatures. Diazotisation is an important reaction in organic chemistry as diazonium salts act as key intermediates for the synthesis of various substituted aromatic compounds and azo dyes, as prescribed in NCERT Class XII Chemistry.

Diazonium Salt:
When a primary aromatic amine is treated with nitrous acid (generated in situ) in a cold aqueous acidic medium, an unstable ionic compound called a diazonium salt is formed. These salts are represented generally as Ar-N2+ X-, where Ar is an aryl group and X- is a counter ion (Cl-, BF4-, etc.). 

Structure

In a typical aryl diazonium salt the functional group is written as -N2+. The bonding may be represented as Ar-N≡N+ X-, but resonance forms are often used to show delocalisation of the positive charge between the two nitrogen atoms and the aryl ring. Aryl diazonium salts are stable at low temperature (0-5 °C) in aqueous solution when the counter ion is chloride or sulphate; they can be isolated as more stable salts (for example, tetrafluoroborate, BF4-) under appropriate conditions.

Preparation (Diazotisation)

Diazotisation is the process of converting a primary aromatic amine into its diazonium salt by reaction with nitrous acid at low temperature. Nitrous acid is not stored but generated in situ by mixing a nitrite (commonly NaNO2) with a mineral acid (commonly HCl) at 0-5 °C.

• General equation (aryl amine): ArNH2 + HNO2 + HCl → ArN2+Cl- + 2 H2O. This reaction is carried out strictly at 0–5 °C.
• Typical reagents and conditions: sodium nitrite (NaNO2) + dilute HCl; temperature maintained at 0-5 °C (ice-cold).
• Reason for using acid: protonates nitrous acid to form the nitrosonium ion (NO+), the active nitrosating electrophile.
• Only primary aromatic amines (ArNH2) give stable diazonium salts in aqueous medium; aliphatic diazonium salts are highly unstable and decompose immediately.

Mechanism of Diazotisation

The mechanism proceeds by generation of the electrophile followed by nitrosation of the amine and subsequent dehydration to give the diazonium ion. Key steps are:

• Formation of the nitrosonium ion: HNO2 + H+ → NO+ + H2O.
• Nitrosation of the amine: the lone pair on nitrogen of ArNH2 attacks NO+ to form an N-nitrosoamine (Ar-NH-NO) (sometimes written as Ar-N(H)-NO).
• Protonation and rearrangement: protonation promotes removal of water and leads to formation of the diazonium ion Ar-N2+.
• Association with the counter-ion: Ar-N2+ X- (for example, Cl-) gives the isolable diazonium salt.

Chemical Behaviour and Important Reactions

NOTE: These reactions are specifically mentioned in NCERT and are frequently tested in NEET. 

Aryl diazonium salts are versatile intermediates in aromatic chemistry because they easily lose N2 (a very stable gas) to produce reactive species that can be converted into many functional groups. Important transformations include:

• Hydrolysis (phenol formation): Warming an aryl diazonium salt with water gives the corresponding phenol: Ar-N2+Cl- + H2O → Ar-OH + N2 + HCl. This reaction is why diazotisation must be kept cold.
• Sandmeyer reactions: Replacement of the diazonium group by halogens or CN using copper(I) salts: Ar-N2+ + CuCl → Ar-Cl + N2, Ar-N2+ + CuBr → Ar-Br + N2, Ar-N2+ + CuCN → Ar-CN + N2.
• Iodination: Reaction with KI yields the aryl iodide: Ar-N2+ + I- → Ar-I + N2.
• Reduction: Treatment with hypophosphorous acid (H3PO2) or with other reducing agents reduces diazonium salt to the corresponding aromatic hydrocarbon: Ar-N2+ + H- → Ar-H + N2.
• Balz-Schiemann reaction (fluorination): Formation of aryl fluorides by converting the diazonium salt to its tetrafluoroborate (BF4-) salt and then heating: Ar-N2+ BF4- → Ar-F + BF3 + N2. This is a standard method to prepare aryl fluorides.
• Azo coupling: In cold, mildly alkaline medium, diazonium salts undergo electrophilic attack on activated aromatic rings (electron-rich rings such as phenols or anilines) to form azo compounds (Ar-N=N-Ar′), which are widely used as dyes and pigments.

Stability of Diazonium Salts

• Aryl diazonium salts are relatively stable in cold aqueous acidic solution (0-5 °C) but decompose on warming.
• Aliphatic diazonium salts are unstable and decompose readily with evolution of N2; they cannot be isolated in ordinary conditions.
• Counter-ion affects stability: tetrafluoroborate and tosylate salts can be isolated as crystalline solids and are more stable than chloride or sulphate salts.

Illustration:

Why ice cold condition have to be maintained in the diazotisation reaction of aniline?

Ans: Because benzene diazonium chloride is stable only at 0–5 °C and decomposes on warming to form phenol. Hence, diazotisation is carried out under ice-cold conditions. 

Additional explanation: The diazonium ion easily undergoes hydrolysis at higher temperatures to form phenol, and warming accelerates loss of N2 with competing side reactions. Maintaining ice-cold conditions (0-5 °C) preserves the diazonium ion long enough to carry out desired subsequent transformations such as Sandmeyer reactions or azo coupling. Also, cold conditions favour stepwise formation of the required nitrosonium electrophile and reduce undesired electrophilic substitution or decomposition pathways.

Examples

• Diazotisation of aniline: C₆H₅NH₂ + HNO₂ + HCl → C₆H₅N₂⁺Cl^- + 2 H₂O (performed at 0-5 °C).
• Preparation of chlorobenzene from aniline via Sandmeyer: C₆H₅N₂⁺Cl^- + CuCl → C₆H₅Cl + N₂.
• Formation of p-hydroxyazobenzene (azo coupling): diazonium salt of aniline (or its substituted derivative) couples with phenol in alkaline medium to form an azo dye (Ar-N=N-Ar′).

Applications

• Synthesis of substituted aromatic compounds (halides, nitriles, fluorides, phenols) from corresponding aromatic amines.
• Formation of azo dyes and pigments via azo coupling reactions.
• As intermediates in multi-step syntheses where selective replacement of an amino group is required.

Summary: Aryl diazonium salts (Ar-N2+ X-) are key intermediates formed by diazotisation of primary aromatic amines using nitrous acid at low temperature. They are versatile for introducing many functional groups on the aromatic ring or for making azo compounds, but they must be prepared and handled under ice-cold conditions because they decompose on warming, often producing phenols or other by-products. Only primary aromatic amines form stable diazonium salts in aqueous medium.