Classification of Amines: Primary, Secondary and Tertiary Amines

Amines, the derivatives of ammonia, are classified as primary (1°), secondary (2°) and tertiary (3°) based on the number of hydrogen atoms replaced by alkyl or aryl groups in ammonia molecule. If one alkyl group has replaced one hydrogen atom of NH₃, it is a primary amine. In the same way, if two hydrogens are replaced, it is a secondary amine and if three hydrogens are replaced, it is tertiary amine.

Key Terms: Nomenclature, amine, electrons, Alkyl Group, hydrogen, tertiary amine, primary amine, atoms, secondary amine, aryl groups, molecule

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Define Amines

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Amines are derivatives of Ammonia (NH₃). Amines are obtained by the replacement of one or more Hydrogen atoms by the Alkyl or Aryl group. Amines are an important part of organic compounds and have many uses and are naturally present in Vitamins, Proteins, Hormones, and Alkaloids. Amines are also used in the making of various medical drugs. Some examples of Amines are Methyl Amine (CH₃NH₂), Aniline (C₆H₅NH₂), Ethyl Amine (C₂H₅NH₂), etc.

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Structure of Amines

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Ammonia is slightly distorted tetrahedral in shape. There is a lone pair of electrons that occupies one of the tetrahedral positions. These Nitrogen atoms of amines are sp₃ hybridized, with bulky lone pair compressing the H-N-H bond which is less than 109.5^o.

Structure of Amines

The C-N bond is slightly stronger in aromatic amines, due to the partial double-bonded character that arises due to the delocalization of lone pair of N with the benzene ring.

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Classification of Amines

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On the basis of the number of Hydrogen atoms replaced in NH₃ molecules, Amines are classified as Primary (1^o Amine), Secondary (2^o Amine), and Tertiary (3^o Amine). 

• Primary (1^o Amine): In 1^o Amine a single hydrogen atom of NH₃ is replaced by Aryl or Alkyl group. 

For Example CH₃NH₂ (Methyl Amine). 

• Secondary (2^o Amine): Two hydrogen atoms of NH₃ are replaced by Aryl or Alkyl Group in Secondary Amines. 

For Example CH₃NHCH₃ (Dimethyl Amine)

• Tertiary (3^o Amine): In 3^o Amine three hydrogen atoms of NH₃ are replaced by Aryl or Alkyl group. For example: 

Classification of Amines

Primary and secondary amines are involved in an intermolecular association. This is due to the hydrogen bonding that exists between the nitrogen of one molecule and hydrogen of another. 

Compared to secondary amines, this intermolecular association is more in primary amines. This is because, for the hydrogen bond formation in it, there are two hydrogen atoms available

Due to the absence of hydrogen atoms available for hydrogen bond formation, the tertiary amines do not have an intermolecular association.

The order of boiling points of Isomeric amines is as following:

Primary > Secondary > Tertiary 

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Nomenclature of Amines

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• Aliphatic amines are named by adding a prefix ‘mine’ to the alkyl group. 

For example: 

Aliphatic amines

• Secondary or Tertiary amines are named by adding the prefix ‘di’ or ‘tri’ before the name of an alkyl group. 

For example:

Secondary or Tertiary amines

• Aromatic Amines are named after parent member- aniline.

For example:

Aromatic Amines

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