Chemistry Properties of Diazonium Salts

A Diazonium salt is an organic compound with a molecular structure R−N2+X− produced by removing a hydrogen atom from an organic compound. Diazonium salts are produced through the reaction of primary amines with nitrous acid (diazotization); their most notable feature is their instability. The aliphatic diazonium salts are transient intermediates, quickly dissolving into nitrogen and carbonium ions. Some aromatic diazonium salts are stable enough to separate, although they react fast, losing nitrogen or forming azo compounds.

Diazonium salts have the following properties:

• They are ionic and water soluble.
• Benzenediazonium chloride is water-soluble. However, it only reacts when warmed.
• They are colourless crystalline solids.
• Benzenediazonium fluoroborate is insoluble in water. It is pretty stable at room temperature.

Preparation of Diazonium Salts

Laboratory preparation

One of the most frequent ways to make diazonium salt is to react with nitrous acid and aromatic amines. An aqueous solution of sodium nitrite and aniline, dissolved in hydrochloric acid (HCl), produces diazonium chloride at 0–5 °C. There are three steps in this procedure. The first step of the reaction between aniline and HCl forms an aniline hydrochloride salt. The reaction of NaNO2 and mineral acid produces HNO2, which reacts with aniline hydrochloride to form diazonium chloride at 0–5 °C. HNO2 is unstable. As a result, the reaction between NaNO2 and mineral acid produces nitrous acid in the reacting vessel.

NaNO2+C6H5NH2 +2HCl (Temperature0–5°C)−−−−−−−−−−−→C6H7N2Cl +NaCl+2H2O

When one drop of the resulting solution turns the starch iodide paper blue, stop adding the NaNO2 solution. Nitrous acid is a toxic gas. Hence, it is usually made by reacting sodium nitrate with a mineral acid during the reaction itself. The benzene diazonium chloride solution is pale yellow.

Method of preparation

When aniline and nitrous acid react, it forms the diazonium salt. First, we add an aniline solution to a bottom flask containing HCI. We add a sodium nitrite solution to the flask after the benzene diazonium chloride has formed. When NaNO2 reacts with a mineral acid, it creates a toxic gas. As a result, it is critical to keep the reaction temperature below 50 °C to avoid the diazonium group decomposing and releasing N2 as soon as it forms.

Despite being a crystalline compound, we cannot crystallise diazonium salt and separate it from the reaction mixture. It is impossible because it causes an explosion. Hence, we use it as a solution to the reaction mixture.

Reactivity

Substrate displacement reactions with diazonium salts can generate nitrogen as a byproduct. A coupling reaction is another approach to creating a diazo compound. Diazonium salts are high-energy compounds that can be heated or mechanically decomposed. The reaction enthalpy of the diazo functional group ranges from -160 to -180 kJ/mol, and we may increase its stability by producing double salts like zinc chloride. It allows the isolation of some diazonium salts in solid form. Additionally, complexation with crown ethers reduces or suppresses thermal decomposition. Substances that promote decomposition and reduce the onset temperature of decomposition include transition metal impurities, nitrous acid, bases, and direct sunlight.

Scientists studied diverse methods to remove diazonium compounds, but none is universal. We can add an aqueous hypophosphorous acid (H3PO2) solution sometimes with a catalyst. We should not handle more than 0.75 mmol of diazonium salts at a time. Other salts or substances such as crystalline complexes may significantly stabilise the isolated diazonium salts. Moreover, we should not heat diazonium salts over 40 °C or scratch the dried powder with metal spatulas.

Other Properties

In organic synthesis, diazonium salts are a frequent intermediate. They are seldom separated as final products due to their undesirable physical features. We can synthesise many aromatic derivatives and azo compounds using them as intermediates. Some common displacement reactions to make aromatic diazonium salts include Sandmeyer/Schiemann/Gattermann processes, which create aryl halides; Griess reactions, which produce phenols; Gomberg-Bachmann reactions, which have diaryl compounds; Gutermann reactions, which provide aryl nitro compounds, and the Meerwein reactions, which produce aromatic alkenes.

Conclusion

Diazonium salts are organic compounds having the chemical formula R – +N N, where R is an atomic group formed when we remove a hydrogen atom from an organic molecule. According to tabular linear free energy relationship constants, the diazonium group (N2+) is one of the most strong electron-withdrawing substituents. A diazonium salt is an organic compound with a nitrogen-nitrogen triple bond and a generic alkyl or aryl side group.

The term ‘salt’ comes from the molecule’s diazo (dinitrogen) component that exists as an ionic salt, with a chloride ion acting like a typical counter-ion for the positively charged nitrogen atom. Organic compounds have the formula RN2+X–, where X is an anion and R is an alkyl or aryl group. Alkane diazonium species and acidic protons on diazonio-substituted phenols and benzoic acids have lower pKa values than their unsubstituted counterparts.