Nitrous acid is a monobasic weak acid which is known only in solution in the form of nitrite salts used in making diazides from amines. This occurs by nucleophilic attack of the amine on the nitrite reprotonating by the surrounding solvent and double elimination of water.
The reaction between nitrous acid and amines were used in 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 unstable and prepared always in situ and written as HONO to represent its structure. It is usually formed by reacting a solution which consists of potassium or sodium nitrate with hydrochloric acid.
Nitrous acid is a weak acid and we get the following chemical reaction.
The equilibrium position lies towards 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.
Primary Amines and Nitrous Acid:
The major observation is an outburst of odorless and colorless 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 an 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.
Tertiary Amines and Nitrous Acid:
The result is different in this case. Here, nothing special happens and you are left with a colorless solution. What occurred is the formation of an ion by reaction with an acid present. For example, with trimethylamine, you would get a trimethylammonium ion, (CH3)3NH+.
Also, Check ⇒ Structure & Properties of Nitrous acid