Structure of Amines: Alkyl and Aromatic Amines

The type of compounds that are derived from ammonia(NH₃) are known as amines. In Organic chemistry, they are basically classified as the functional groups of the organic nitrogen compounds that contain nitrogen atoms with a lone pair. A few of the naturally occurring amines include alkaloids that are found in certain plants such as histamine, catecholamine neurotransmitters such as dopamine, epinephrine, and norepinephrine. 

Key Terms: Aniline, amine, ammonia, carbon, amino acid, carbon bonding, dopamine, epinephrine, norepinephrine, atom, histamine, catecholamine, nitrogen

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Amines

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Amines are those compounds that are derived from ammonia(NH₃). They are classified as primary, secondary or tertiary according to the number of carbons bonded directly to the nitrogen atom. 

• Primary amines have one carbon binding to nitrogen.
• The secondary amines have two carbons bonded to nitrogen.
• The tertiary amines have three carbons bonded to nitrogen. 

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Types of Amines

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• Primary Amines 

When one hydrogen atom in ammonia is substituted by an alkyl or aromatic group then primary amines are formed. Amino acids and methylamine some examples of primary alkylamines.

• Secondary Amines

Amines that have two organic substitutes either alkyl or aryl ones, or both, and are bound to the nitrogen together with one hydrogen, are termed as secondary amines. Dimethylamine is a secondary amine.

• Tertiary Amines

Amines where the nitrogen consists of three organic substitutes are tertiary amines. Trimethylamine and EDTA are examples of tertiary amines.

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Structure of Amines

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• Alkyl Amines

Alkylamines consist of tetrahedral nitrogen centers where the C-N-C and C-N-H bond angle is 109 degrees. The distance between C-N bonds is smaller as compared to the C-C range. The amines can also display a chiral property wherein the center of the nitrogen atom holds for replacements which creates lone pairs.

The bond angle in the case of trimethylamine is 108 degrees which results the Pyramidal structure of trimethylamine-

Pyramidal structure of Trimethylamine

• Aromatic Amine

Due to the mixture of the lone pair with the aryl substituent, nitrogen almost has a planar structure in aromatic amines. The C-N range is shorter. In Aniline, the distance between C-N bonds is the same as the distance between C-C bonds.

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Basicity of Amines

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• Amines are very strong bases. The basicity of amines is varied by the molecule and the presence of the lone pair of nitrogen electrons.
• Usually, amines are the bases but 1^o and 2^o amines are the weak acids as the pKa value of ammonia is 34. Thus, amine acidity is measured by the pKa rather than its conjugate acid.
• The relevance of all the acid-base relationships lies in the need for differing intensity organic bases as reagents suited to the particular reaction requirements.
• Simple amine water solubility is larger due to the hydrogen bonding capacity that exists between the protons on the water molecules and the lone electron pairs.

Factors that affect the basicity of amines

1. The basicity of the amines is increased by the presence of an electron-donating group
2. The basicity of the amines is decreased by an electron-withdrawing group. For example- the Nitro group.

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Preparation of amines

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• Reduction of Nitro Compounds

The nitro compounds are reduced to amines by reduction with metals in an acidic medium or by passing them through the hydrogen gas in the presence of finely divided nickel, palladium, or platinum.

• Ammonolysis of alkyl halides

Alkyl benzyl halide undergoes a nucleophilic substitution reaction while reacting with an ethanolic solution of ammonia where the halogen atom is replaced by an amino group and this process of splitting of the C-X bond by an ammonia molecule is called ammonolysis.

• Reduction of Nitriles

On reduction, the nitriles with lithium aluminum hydride or catalytic hydrogenation are used to produce primary amines.

Reduction of Nitriles

• Reduction of Amides

On reduction, the amides with lithium aluminum hydride yield amines.

Reduction of Amides

• Gabriel phthalimide synthesis

Gabriel synthesis is used to prepare primary amines. Phthalimidewhen treated with ethanolic potassium hydroxide, forms potassium salt of phthalimide which when heated with alkyl halide followed by alkaline hydrolysis leads to the formation of the corresponding primary amine.

• Hoffmann bromamide degradation reaction

In the Hoffmann bromamide degradation reaction, the transfer of an alkyl or aryl group takes place from the carbonyl carbon of the amide to the nitrogen atom. The amine so formed, therefore, contains one carbon less than that present in the initial amine.

Hoffmann bromamide degradation reaction

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Uses of Amines 

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Various uses of amines include:

• Amines are extensively used in the manufacturing process of azo dyes.
• Amines are widely used in drugs.
• Amines are used in gas treatment during the removal of carbon dioxide from natural gas, etc.
• Amines are used as corrosion inhibitors.
• Amines are mainly used in the synthesis of many products.

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