Classification of Amines

You have heard of Ammonia. Haven’t you? But, it is not the only nitrogen product that is popular. There are so many other products of ammonia that are derived by altering the hydrogen atoms. We give them names like Primary amine or Secondary amine etc. In this chapter, we will look at the classification of amines in detail. We will also have a brief look at the nomenclature of the classification of amines.

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What are Amines?

Amines are organic compounds derived from ammonia (NH₃). They are derived by replacing one or more hydrogen atoms with an alkyl or aryl group. An amine is a functional group that contains a lone pair on a nitrogen atom. Amines are structurally similar to ammonia in that nitrogen can link up to three hydrogen atoms. They are also known as alkylamines and arylamines because they include an alkyl or an aryl group. For example:

CH3NH2	C2H5NH2	C6H5NH2	C6H5CH2NH2 / C7H9N
Methyl Amine	Ethyl Amine	Aniline	Benzylamine

Structure of Amine

The form of an amine molecule is a somewhat flattened triangular pyramid, with the nitrogen atom at the top. Above the nitrogen atom is an unshared electron pair. Nitrogen is trivalent with a lone pair because it possesses 5 valence electrons. According to VSEPR theory, nitrogen in amines is sp³ hybridized and has a pyramidal rather than tetrahedral form due to the existence of a lone pair.

Classification of Amines

On the basis of the number of hydrogen atoms replaced in NH₃ molecule, amines are categorized into three types:

1° or the Primary Amines

In 1° amine, one hydrogen atom of NH₃ is replaced by an alkyl or aryl group. For example:

• CH₃NH₂ – Methyl Amine

2° or the Secondary Amines

In 2° amine, two hydrogen atoms of NH₃ are replaced by alkyl or aryl groups. For example:

• CH₃NHCH₃  – Dimethyl Amine

Browse more Topics under Amines

• Chemical Reactions of Amines
• Diazonium Salts
• Nomenclature of Amines
• Physical Properties of Amines
• Preparation of Amines

3° or Tertiary Amines

In tertiary amine, all the three hydrogen atoms of NH₃ are replaced by alkyl or aryl groups. For example:

Nomenclature of Amines

The earlier and most generally used method for naming amines is to name each group linked to the nitrogen atom before adding the ending -amine.

• In the common naming system, we name aliphatic amines by prefixing an alkyl group to a mine, i.e. alkylamine. For example:

• Secondary and tertiary amines, having two or more similar groups are named by adding the prefix ‘di’ or ‘tri’ before the name of the alkyl group. For example:

• Aromatic amines are named as derivatives of the parent member, aniline (C₆H₅NH₂). The -NH₂ group is directly linked to the benzene ring in arylamines. The suffix ‘e’ of arene is replaced with ‘amine’ when identifying arylamines according to the IUPAC standard. C₆H₅-NH₂ is hence known as benzenamine in the IUPAC system. For example:

Preparation of Amines

Let us look at a few methods by which amines are prepared:

1. Reduction of Nitro compounds

By passing hydrogen gas through finely divided nickel, palladium, or platinum, nitro compounds are converted to amines. Reduction using iron scrap and hydrochloric acid is preferable because the FeCl₂ produced during the reaction is hydrolyzed, releasing hydrochloric acid.

2. Reduction of Nitriles

Primary amines are formed when nitriles are reduced with lithium aluminium hydride (LiAlH₄) or catalytically hydrogenated. This reaction is utilized for amine series ascent, or the synthesis of amines with one more carbon atom than the beginning amine.

3. Reduction of Amides

Amines are formed when amides are reduced with lithium aluminium hydride.

4. Gabriel phthalimide synthesis

Gabriel synthesis is a method for producing primary amines. We get potassium salts of phthalimide in this procedure by treating phthalimide with ethanolic potassium hydroxide. When this is heated with alkyl halide and then alkaline hydrolyzed, primary amine is formed.

5. Hoffmann bromamide degradation reaction

Hoffmann discovered a method for producing primary amines by reacting an amide with bromine in an aqueous or ethanolic sodium hydroxide solution. An alkyl or aryl group migrates from the amide’s carbonyl carbon to the nitrogen atom during this degradation reaction. The amine thus generated contains one less carbon than the amide.

Uses of Amines

• Direct applications of amines and their salts include corrosion inhibitors in boilers and lubricating oils (morpholine), antioxidants for rubber and roofing asphalt (diarylamines), stabilizers for cellulose nitrate explosives (diphenylamine), gamma radiation protection (diarylamines), developers in photography (aromatic diamines), and so on.
• Some polyfunctional amines, such as ephedrine and epinephrine (adrenaline), are useful medicines and anaesthetics.
• It is utilized in water purification, pharmaceutical production, and the manufacture of insecticides and herbicides.
• It participates in the synthesis of amino acids, which are the building blocks of proteins in living organisms. Amines also produce a wide range of vitamins.
• Serotonin is an important amine that serves as a major neurotransmitter. It regulates hunger and is essential for the speed with which the brain processes in general.
• Amines are used to make pain relievers such as Morphine and Demerol, which are also known as analgesics.

Physical Properties of Amines

• Lower aliphatic amines are gases that have a fishy odour.
• Primary amines with 3 or more carbon atoms are liquid, while those with four or more are solid.
• Lower aliphatic amines are water soluble due to their ability to establish hydrogen bonds with water molecules. Higher amines are essentially water insoluble.
• Because of hydrogen bonding between the nitrogen of one molecule and the hydrogen of another, primary and secondary amines form intermolecular associations. Because there are two hydrogen atoms available for hydrogen bond formation in primary amines, this intermolecular connection is stronger than in secondary amines.
• Tertiary amines lack intermolecular interaction because there is no hydrogen atom available for hydrogen bond formation.
• The following are the boiling temperatures of isomeric amines: Primary > Secondary > Tertiary
• The majority of aliphatic amines are not extremely toxic, and many are non-harmful natural components of foods and medications

Solved Examples for You

Question: Write a detailed note on the nomenclature of amines.

Answer: In organic chemistry, the names of the compounds that are globally accepted are given according to the guidelines given by IUPAC for the nomenclature of organic compounds. The naming of aliphatic amines is done by prefixing the alkyl group to amines and thus the names of aliphatic amines are of the form of an alkylamine.

For example, CH₃NH₂ is named methylamine (alkyl part + amine =methylamine). Prefixes such as di and tri are appended before the names of the alkyl group when two or more identical groups are present. If more than one amino group is present in the amine then the parent chain and the position of amino groups are identified by numbering the carbon atoms in the parent chain.

The numbering is done in such a way that the carbon atom bearing the –NH₂  groups get the lowest numbers.  Prefixes along with the numbers are then used to denote the number of amino groups and their position in the molecule. For example, H₂N-CH₂-CH₂-NH₂  is named ethane 1, 2-diamine. If –NH₂ group is attached to a benzene ring then it is called arylamines.

One of the simplest examples of arylamine is C₆H₅NH₂. It is commonly known as aniline which is also an accepted IUPAC name. When we name arylamines according to the guidelines given by IUPAC then ‘e’ of the arene is replaced by an amine, for example, C₆H₅-NH₂ is named benzenamine.