A Brief Note On All Reactions Of Nitrous Acid

The Nitrous Acid

Nitrous acid is a chemical compound with a molecular weight of 47.014 grams that is naturally present in solid form but is very unstable and hence must always be formed in the form of diluted solutions. It is depicted as HNO2 chemically. It is a weak acid, with a boiling point of 158 degrees Celsius and a melting point of 271 degrees Celsius. It is also an extraordinarily strong oxidiser, hence flammable. By nature, this acid is corrosive, and when heated, the fumes are acutely toxic, making it an environmentally hazardous acid. 

Clinically used in the treatment of cyanide poisoning, mixed along with sodium thiosulfate. The sodium nitrite formed here acts as a vasodilator. 

Nitrous acid is produced by the reaction between water and Nitric oxide.

             H2O + NO → HNO2

Another chemical reaction that can form Nitrous Acid is between sodium nitrite and mineral acids such as HCl, HBr, etc.

             NaNO2 + HCl  →HNO2 + NaCl

There are a number of reactions that are performed by HNO2

Let us discuss these reactions a little bit:

The major driving force in the Liebermann reagent that is used as a spot-test for alkaloids is the nitrous acid.

• Decomposition Reactions

Nitrous acid in its gaseous form can perform decomposition reactions in order to form nitric oxide, nitrogen dioxide and water. 

                           2HNO2→ NO2 + NO + H2O

Nitrogen dioxide in an aqueous solution breaks into nitrous acid and nitric acid.

                                 2 NO2 + H2O →  HNO2 + HNO3

When HNO2 is present in a warm or concentrated solution, then it results in the production of nitric acid, water, and nitric oxide.

                                3 HNO2  → HNO3 + 2 NO + H2O

In the presence of air, the nitrous acid can get re-oxidised into nitric acid.

                                2HNO2 + O2 → 2 HNO3

• Reduction Reactions

Nitric oxide (NO) is formed by the reaction between I– and Fe2+ ions:

    2 HNO2 + 2 KI + 2 H2SO4 →  I2 + 2 NO + 2 H2O + 2 K2SO4

    2 HNO2 + 2 FeSO4 + 2 H2SO4  → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4 

In presence of Sn2+ ions, N2O is achieved: 

     2 HNO2 + 6 HCl + 2 SnCl2→ 2 SnCl2 + N2O + 3H2O + 2KCl

NH2OH formation takes place, when HNO2 reacts with SO2

      2 HNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2NH2OH

In the alkali solution, if there is the presence of Zn, then NH3 is formed.

          5 H2O + KNO2 + 3 Zn → NH3 + KOH + 2 NH2OH

The reactions between N2H5+ result in the formation of HN3, followed by the release of N2 gas.

       HNO2 + [N2H5]+   → HN3 + H2O + H3O+

        HNO2 + HN3 → N2O + N2 + H2O

An extraordinary feature that is possessed by HNO2 is that it can easily kinetically control a reaction and also keeps it thermodynamically stable, which is not seen in the reaction with nitric acid.

• HNO2 also is used in the production of diazonium salts.

                  HNO2 + ArNH2  + H → ArN2+ + 2 H2O

Ar- Aryl group

• Nitrous acid rapidly reacts with aliphatic alcohols and produces alkyl nitrites, the vasodilators. 
• Carcinogens known as nitrosamines are formed as a result of the reaction between nitrous acid and secondary amines.

Among all these, nitric oxide is a component of immense importance. It was discovered by Carl Wilhelm Scheele, a Swedish-German chemist. He also introduced the essential component of life, Oxygen. It is a nonflammable molecule that is an inert gas naturally. There are a number of uses of NO that can be seen. It is used in anaesthetics as it is non-opioids. It can also be seen in the household items cleaners like dishwashers, detergent, etc. Another use is as a fragrance ingredient. It is a major oxidiser used in organic chemistry. 

In humans, it is an important signalling molecule, acting as a powerful vasodilator and vasoconstrictor. It is given to newborn babies suffering from hypoxia. These are some uses of NO; there are many more important functions performed by NO. 

Conclusion

Nitrous acid, despite being a harmful compound, can benefit humankind in a lot of ways, especially as it forms nitric oxide. Nitric oxide, in turn, plays an essential role in the human body as it maintains the dilation and constriction of the blood vessels. Also, nitrous acid has pharmaceutical importance as it is a component in cyanide treatment drugs.  I hope everything I tried to explain here was clear enough to the viewers.