Reactions of Amines (Diazotization, Carbylamine Reaction)

Amines are organic compounds containing a lone pair of electrons attached to a nitrogen atom. They can be considered derivatives of ammonia NH 3 where one or more of the hydrogen atoms are replaced by alkyl or aryl groups. Amines are classified as primary ( RNH 2 ), secondary ( R 2 NH ), or tertiary ( R 3 N ), depending on how many hydrogen atoms are replaced.

Diazotization

Diazotization is a chemical reaction used to convert primary aromatic amines into diazonium salts. These diazonium salts are very useful intermediates in organic synthesis. The general reaction for diazotization is as follows:

RNH 2 + HCl + NaNO 2 → RN≡N⁺ Cl⁻ + H 2 O

where RNH 2 is a primary aromatic amine, HCl is hydrochloric acid, and NaNO 2 is sodium nitrite.

Mechanism of diazotization

The reaction proceeds via the formation of nitrous acid ( HNO 2 ) from sodium nitrite and hydrochloric acid. The primary amine then reacts with the nitrous acid to form the diazonium salt.

The steps are as follows:

1. Formation of Nitrous Acid:

   NaNO 2 + HCl → HNO 2 + NaCl

2. Reaction with aromatic amines:

   RNH 2 + HNO 2 + HCl → RN≡N⁺ Cl⁻ + 2H 2 O

Applications of diazonium salts

Diazonium salts play an important role in the synthesis of azo dyes and can also be used to prepare various aryl derivatives by substitution reactions by replacing the diazo group with other substituents like halogen, –OH, etc.

Carbylamine reaction

The carbylamine reaction is a test for primary amines. It involves the formation of isocyanides (also called carbylamines) from primary amines, chloroform CHCl 3 and a base. The general reaction is as follows:

RNH 2 + CHCl 3 + KOH → RNC + KCl + 2H 2 O

where RNH 2 is a primary amine, CHCl 3 is chloroform, and KOH is potassium hydroxide. The product RNC is an isocyanide, which usually has a very unpleasant odor.

Mechanism of the carbylamine reaction

The reaction produces dichlorocarbene :CCl 2 , which reacts with the primary amine:

1. Chloroform reacts with potassium hydroxide to form dichlorocarbene:

   CHCl 3 + KOH → :CCl 2 + KCl + H 2 O

2. Dichlorocarbene reacts with amine:

   RNH 2 + :CCl 2 → RNC + 2HCl

Importance of the carbylamine reaction

This reaction is a useful test for identifying primary amines, since secondary and tertiary amines do not form isocyanides under the same reaction conditions. The characteristic odor of isocyanides serves as an indicator for the presence of primary amines.

Conclusion

The reactions of amines, particularly the diazotization and carbylamine reactions, are important in the context of organic synthesis. Diazotization helps to form important intermediates such as diazonium salts, which can then be converted into a wide range of useful aromatic compounds. The carbylamine reaction serves uniquely as a diagnostic tool to identify primary amines due to the characteristic odor of its product, the isocyanide. Understanding these reactions allows chemists to manipulate and alter amines for a variety of applications in the laboratory and industry.

Visual Example

Below is the reaction flow chart of diazotization:

_RNH2 + _NaNO2 + HCl Rn≡N⁺ Cl⁻ + H ₂ O

Below is a reaction flow chart of the carbylamine reaction:

₂ + CHCl ₃ + KOH RNC + KCl + 2H ₂ O

By studying these reactions in detail, students gain a better understanding of the chemical behavior of amines, paving the way for a better understanding of organic synthesis and chemical transformations in future studies of chemistry.

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