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Nitrous acid

Definition of nitrous acid

Nitrous acid (HNO2) is an unstable weak acid available in cold acidic solution or in the form of its salts or esters. Nitrous acid decomposes to form nitric oxide (NO) and nitric acid (HNO3).

By Calvero (Own work) [Public domain], via Wikimedia Commons

Structure

Nitrous acid is a planar molecule where two oxygen atoms attached with one nitrogen atom by one with single bond and one with double bond. The p orbitals of two oxygens and nitrogen in O-N-O bond are in a same plane and the π bonded electrons are delocalized between these orbitals. As a result, both of these bonds have double bond characteristic and the rotation is restricted around the O-N-O bond. Thus geometrical isomers are possible. The single bonded oxygen bonds with hydrogen in such a way that can make cis or trans isomer in gas phase. Trans form predominates in room temperature. IR spectroscopy shows that trans isomer is more stable than cis by 2.3 kj mol-1.

By Ben Mills (Own work) [Public domain], via Wikimedia CommonsBy Ben Mills (Own work) [Public domain], via Wikimedia Commons

By Ben Mills (Own work) [Public domain], via Wikimedia Commons

Occurrence in atmosphere

Nitrous acid is produced naturally in atmosphere by a heterogeneous reaction of nitric oxide (NO) and water, that controls ozone in the lower atmosphere .

Preparation

Nitrous acid is unstable and always prepared in situ. It can be made by reacting with sodium or potassium nitrite with hydrochloric acid.

Uses of nitrous acid

Identification of amines

Previously nitrous acid was widely used for the identification of different types of amines. Firstly amines are acidified with hydrochloric acid (HCl), then sodium or potassium nitrite is added to the solution. The nitrous acid is formed in situ to react with amine for the identification.

With primary amines

The observation of colourless and odourless nitrogen gas proves the presence of primary amine. Here lots of different organic compounds are produced with the formation of equimolar nitrogen gas. Measuring the moles of nitrogen gas formed we can get the number of primary amines present in the solution. The product includes an alcohol by replacing the functional group amine (-NH2) with alcohol (-OH).

CH3CHNH+ HNO → CH3CHOH + N↑ + H2O

With secondary amines

No gas is observed, instead yellow oil of nitrosamine proves the presence of secondary amines. This compound is carcinogens, thus this type of reaction is avoided now a days.

(CH3)NH + HNO2 → (CH3)N-N=O + H2O

With tertiary amines

Colourless solution proves the presence of tertiary amines. Here an ion is formed by the reaction between tertiary amines and nitrous acid.

Decomposition of nitrous acid

In warm or concentrated solutions, the nitrous acid produces nitric acid, water, and nitric oxide.

3 HNO2 → HNO3 + 2 NO + H2O

Nitrous acid decomposes into nitrogen dioxide, nitric oxide, and water.

2 HNO2 → NO2 + NO + H2O

Nitrogen dioxide reacts further with water to form nitric acid and nitrous acid in aqueous solution.

2 NO2 + H2O → HNO3 + HNO2

The oxidation of nitric oxide by air, gives nitric acid.

2 HNO2 + O2 → 2 HNO3

Reactions

Nitrous acid acts both as oxidizing and reducing agent by losing or gaining electrons.

Nitrous acid as an oxidizing agent

The oxidation number of nitrogen in nitrous acid is +3. When it reacts with H2S it produces nitric oxide (NO) where the oxidation number of nitrogen reduces to +2. Thus here nitrous acid acts as an oxidizing agent.

2HNO2 + H2S → 2NO + 2H2O + S

Nitrous acid as a reducing agent

The oxidation number of nitrous acid increases from +3 to +5 when it reacts with I2 or Br2 in water to produce nitric acid.

HNO2 + Br2 + H2O → HNO3 + 2HBr

HNO2 + I2 + H2O → HNO3 + 2HI

Summary

  • Nitrous acid (HNO2) is an unstable weak acid available in cold acidic solution or in the form of its salts or esters.
  • Nitrous acid decomposes to form nitric oxide (NO) and nitric acid (HNO3).
  • Nitrous acid is a planar molecule where two oxygen atoms attached with one nitrogen atom by one with single bond and one with double bond.
  • Nitrous acid can be made by reacting with sodium or potassium nitrite with hydrochloric acid.
  • Previously nitrous acid was widely used for the identification of different types of amines.
  • Nitrous acid decomposes into nitrogen dioxide, nitric oxide, and water.
  • Nitrous acid acts both as oxidizing and reducing agent by losing or gaining electrons.
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