Nitrous acid is a weak, monobasic acid that is known to exist mainly in a solution form containing nitrite salts. This acid is said to be weak and unstable. As for the composition, nitrous acid is a mixture of nitric acid and nitrous gas, as well as a hint of oxygen gas. Sometimes, this acid is also found in the gas phase.
| Overview and Physical Properties | |
|---|---|
|
Chemical formula
|
HNO2 |
| Molar mass | 47.013 g/mol |
| Appearance | Pale blue solution |
| Density | Approx. 1 g/ml |
| Melting point | Only known in solution or as gas |
| Acidity (pKa) | 3.15 |
| Conjugate base | Nitrite |
Nitrous Acid Chemical Properties
- Nitrous acid is extremely volatile and usually releases thick fumes during reactions.
- It has a boiling point of 82oC.
- In a gaseous state, heat has no effect on nitrous acid.
- When it is mixed with H2O, we will notice an effervescence taking place, and this will further lead to the evolution of nitrous gas.
Structure
In the gas phase, nitrous acid has a planar molecule and can either adopt a “syn” or an “anti” form. At room temperatures, the “anti” form is more predominant, and IR measurements tend to show that it is more stable by around 2.3 kJ/mol.
Nitrous Acid Preparation
Nitrous acid is unstable and prepared always in situ and written as HONO to represent its structure. It is usually formed by reacting a solution that consists of potassium or sodium nitrate with (mineral) hydrochloric acid. The reaction between nitrous acid and amines was used in the past as a way of distinguishing primary, secondary and tertiary amines. However, a product with secondary amine is a strong carcinogen, and hence, this reaction is not carried out at this level.
Nitrous acid is a weak acid, and we get the following chemical reaction:
The equilibrium position lies towards the right because nitrous acid is a weak acid. In every reaction, the amine is acidified with hydrochloric acid, adding a solution of sodium nitrite. The nitrite and the acid form nitrous acid, which reacts with the amine.
Alternatively, nitrous acid is also produced by dissolving dinitrogen trioxide in water.
- N2O3 + H2O → 2 HNO2
Primary Amines and Nitrous Acid
The major observation is an outburst of odourless and colourless gas, and nitrogen is given off. Unfortunately, no single equation exists that can be framed for this reaction since we had lots of various organic products. For example, you obtain alcohol where the group -NH2 is replaced by OH. Let’s take 1-aminopropane for example.
CH3CH2CH2NH2 + HNO2 -> CH3CH2CH2OH + H2O + N2
The propan 1 ol will be a single product among many that involve propan 2 ol, 1 chloro propane, propene and others.
The nitrogen is given off in quantities as described in the equation. By measuring the amount of nitrogen formed, this reaction can be used to work out the amount of amine in the solution.
Secondary Amines and Nitrous Acid
In this case, there is no gas produced. In fact, you get a yellow oil known as nitrosamine. These compounds are strong carcinogens; hence, you need to avoid them.
Example:
Tertiary Amines and Nitrous Acid
The result is different in this case. Here, nothing special happens, and you are left with a colourless solution. There is a formation of an ion by reaction with an acid present. For example, with trimethylamine, you would get a trimethylammonium ion (CH3)3NH+.
Reactions of Nitrous Acid
Decomposition
Gaseous nitrous acid can decompose into nitrogen dioxide, nitric oxide and water.
- 2 HNO2 → NO2 + NO + H2O
Reduction
With Zn in an alkali solution, NH3 is formed.
- 5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2
With I− and Fe2+ ions, NO is formed.
- 2 KNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4
- 2 KNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4
With Sn2+ ions, N2O is formed.
- 2 KNO2 + 6 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O + 2 KCl
With SO2 gas, NH2OH is formed.
- 2 KNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH
Oxidation
When oxidation is carried by nitrous acid, it has kinetic control over thermodynamic control. This can be seen when dilute nitrous acid oxidises I− to I2. However, dilute nitric acid cannot oxidise.
- I2 + 2 e− ⇌ 2 I− Eo = +0.54 V
- NO−3 + 3 H+ + 2 e− ⇌ HNO2 + H2O Eo = +0.93 V
- HNO2 + H+ + e− ⇌ NO + H2O Eo = +0.98 V
Uses of Nitrous Acid – HNO2
- Nitrous acid is the main chemphore in the Liebermann reagent, and it is used to spot-test for alkaloids.
- Used in the preparation of diazonium salts from amines. The resulting diazonium salts are reagents in azo coupling reactions to produce azo dyes.
- The salts are also used in organic syntheses, such as the Sandmeyer reaction.
- Used to remove the toxic nature of the potentially explosive compound sodium azide.
- Nitrous acid is also used as an oxidiser in liquid fuel rockets.
- Nitrous acid can be reacted with aliphatic alcohols to produce alkyl nitrites.
- It can react with two α-hydrogen atoms in ketones to create oximes, and this compound can be further oxidised to a carboxylic acid or reduced to form amines.
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