September 13, 2025 | UR Gate
Preparation of Diazonium Salt (Diazotization) and its Reactions
diazonium salt preparation and reactions in the practical organic chemistry lab, phenyl diazonium chloride, hydroxyazo benzene, P- (Phenylazo) Phenyl, Result.
The reaction of primary aromatic and aliphatic amines with nitrous acid yields diazonium salts.
Reactions of this type yield various products, and the resulting materials depend on the type of amine, whether it is primary, secondary, tertiary, aliphatic, or aromatic.
Diazonium salts decompose even at low temperatures, so they are used immediately after preparation. The diazonium salt of a primary aromatic amine is relatively stable, and this salt converts to phenol upon heating. Diazonium salts of primary alkyl amines are unstable and decompose immediately after formation, yielding alcohols, olefins, or alkyl halides.
Methods of Diazonium Salt Reaction:
1. Substitution by Nitrogen
Note:
Nitrogen refers to N2 gas.
Diazonium salts (ArN₂⁺) are highly reactive compounds, widely used in organic chemistry to synthesize a variety of new substances. As illustrated in the image, the diazonium group can be readily substituted by other functional groups.
The first two reactions demonstrate substitution by halogens. When a diazonium salt reacts with halide salts (like NaX or NaCl) under appropriate conditions (often involving heating), the diazonium group (N₂⁺) departs as stable nitrogen gas (N₂). This reaction effectively allows for the introduction of a halogen atom (X or Cl) in place of the diazonium group, yielding an aryl halide (ArX or ArCl).
The third reaction showcases substitution by a hydroxyl group (OH). When a diazonium salt reacts with water (H₂O) in an acidic medium (H⁺), the diazonium group decomposes, releasing nitrogen gas (N₂) as before. Subsequently, a water molecule attacks the carbon atom that lost the diazonium group. This attack, followed by the loss of a proton, ultimately leads to the formation of a hydroxyl group attached to the aromatic ring, resulting in a phenol (ArOH). These three reactions provide efficient routes for introducing halogens and hydroxyl groups onto aromatic rings.
2. Coupling
Beyond simple substitution reactions, diazonium salts (ArN₂⁺) are pivotal in forming new carbon-nitrogen bonds through coupling reactions. The general principle involves a diazonium salt reacting with an activated aromatic compound. This activated compound typically contains an electron-donating group (represented as 'G' in the diagram), such as an amine group (-NR₂, -NHR, -NH₂) or a hydroxyl group (-OH).
In this electrophilic aromatic substitution-like process, the diazonium salt acts as an electrophile, attacking the electron-rich aromatic ring of the coupling partner. The result is the formation of a new bond between a nitrogen atom from the diazonium salt and the aromatic ring of the coupling partner. This creates an azo compound, characterized by the -N=N- linkage between two aromatic systems. These azo compounds are often intensely colored, making them valuable as dyes.
A specific example is shown where phenyldiazonium chloride (where Ar is phenyl and X is Cl) couples with phenol (where G is -OH). This reaction yields p-hydroxyazo benzene, also known as p-(phenylazo) phenyl. This colored compound is a classic example of an azo dye formed through this coupling mechanism.
In essence, diazonium coupling is a powerful method for linking two aromatic rings via a nitrogen bridge, utilizing diazonium salts to create vibrant azo compounds.
Procedure:
- In a suitable round-bottom flask, add 150 mL of concentrated hydrochloric acid to 10 mL of water and 2.5 mL of aniline.
- Cool the flask to 0-5°C while adding 20 mL of cold water (some aniline hydrochloride PhNH3HCl will crystallize, but this does not affect the reaction).
- Add a solution of sodium nitrite (NaNO2), prepared by dissolving 4g of NaNO2 in 35 mL of cold water (H2O), in portions while stirring and maintaining the temperature between 0-5°C.
- After completing the addition of the solution, test the reaction mixture to confirm the presence of excess nitrous acid. This is done by taking a drop of the solution and placing it on starch/potassium iodide paper. If no excess nitrous acid is present, add more sodium nitrite. Once an excess is confirmed (indicated by the disappearance of the blue color on the paper), the solution can be used for diazonium salt reactions.
Note:
The first reaction shows the formation of nitrous acid (HONO) from sodium nitrite and an acid, which is essential for the diazotization process. The second reaction illustrates how excess nitrous acid reacts with potassium iodide and an acid to produce iodine (I₂), which then forms a distinct blue complex with starch, serving as a visual indicator for the presence of excess nitrous acid.
