{"id":380,"date":"2017-11-14T12:10:56","date_gmt":"2017-11-14T12:10:56","guid":{"rendered":"http:\/\/chemdictionary.org\/?p=380"},"modified":"2017-11-14T12:24:35","modified_gmt":"2017-11-14T12:24:35","slug":"nitrous-acid","status":"publish","type":"post","link":"https:\/\/chemdictionary.org\/nitrous-acid\/","title":{"rendered":"Nitrous acid"},"content":{"rendered":"

Definition of nitrous acid<\/h2>\n

Nitrous acid (HNO2<\/sub>) 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<\/sub>).<\/p>\n

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Structure<\/h2>\n

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 \u03c0 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.\u00a0Trans form predominates in room temperature. IR spectroscopy shows that trans isomer is more stable than cis by 2.3 kj mol-1<\/sup>.<\/p>\n

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Occurrence in atmosphere<\/h2>\n

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

Preparation<\/h2>\n

Nitrous acid is unstable and always prepared in situ. It can be made by reacting with sodium or potassium nitrite with hydrochloric acid.<\/p>\n

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Uses of nitrous acid<\/h2>\n

Identification of amines<\/h3>\n

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.<\/p>\n

With primary amines<\/h4>\n

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<\/sub>) with alcohol (-OH).<\/p>\n

CH3<\/sub>CH2\u00a0<\/sub>NH2\u00a0<\/sub>+ HNO2\u00a0<\/sub>\u00a0\u2192 CH3<\/sub>CH2\u00a0<\/sub>OH + N2\u00a0<\/sub>\u2191 + H2<\/sub>O<\/p>\n

With secondary amines<\/h4>\n

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.<\/p>\n

(CH3<\/sub>)2\u00a0<\/sub>NH + HNO2<\/sub>\u00a0\u2192 (CH3<\/sub>)2\u00a0<\/sub>N-N=O + H2<\/sub>O<\/p>\n

With tertiary amines<\/h4>\n

Colourless solution proves the presence of tertiary amines. Here an ion is formed by the reaction between tertiary amines and nitrous acid.<\/p>\n

Decomposition of nitrous acid<\/h2>\n

In warm or concentrated solutions, the nitrous acid produces nitric acid, water, and nitric oxide.<\/p>\n

3 HNO2<\/sub>\u00a0\u2192 HNO3<\/sub>\u00a0+ 2 NO + H2<\/sub>O<\/p>\n

Nitrous acid decomposes into\u00a0nitrogen dioxide,\u00a0nitric oxide, and water.<\/p>\n

2 HNO2<\/sub>\u00a0\u2192 NO2<\/sub>\u00a0+ NO + H2<\/sub>O<\/p>\n

Nitrogen dioxide reacts further with water to form nitric acid\u00a0and nitrous acid in aqueous solution.<\/p>\n

2 NO2<\/sub>\u00a0+ H2<\/sub>O \u2192 HNO3<\/sub>\u00a0+ HNO2<\/sub><\/p>\n

The oxidation of nitric oxide by air, gives nitric acid.<\/p>\n

2 HNO2<\/sub>\u00a0+ O2<\/sub>\u00a0\u2192 2 HNO3<\/sub><\/p>\n

Reactions<\/h2>\n

Nitrous acid acts both as oxidizing and reducing agent by losing or gaining electrons.<\/p>\n

Nitrous acid as an oxidizing agent<\/h3>\n

The oxidation number of nitrogen in nitrous acid is +3. When it reacts with H2<\/sub>S it produces nitric oxide (NO) where the oxidation number of nitrogen reduces to +2. Thus here nitrous acid acts as an oxidizing agent.<\/p>\n

2HNO2<\/sub> + H2<\/sub>S\u00a0\u2192 2NO + 2H2<\/sub>O + S<\/p>\n

Nitrous acid as a reducing agent<\/h3>\n

The oxidation number of nitrous acid increases from +3 to +5 when it reacts with I2<\/sub> or Br2<\/sub> in water to produce nitric acid.<\/p>\n

HNO2<\/sub> + Br2<\/sub> + H2<\/sub>O \u2192 HNO3<\/sub> + 2HBr<\/p>\n

HNO2<\/sub> + I2<\/sub> + H2<\/sub>O \u2192 HNO3<\/sub> + 2HI<\/p>\n

Summary<\/h2>\n