Amine 

1.0Introduction

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:

2.0Structure 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.

3.0Classification of Amines

Amines are classified according to the number of carbon atoms bonded directly to the nitrogen atom.

1. Primary Amines- When one of the hydrogen atoms of the ammonia molecule is replaced by an alkyl or aryl group. Eg: Methylamine CH₃NH₂, Aniline C₆H₅NH₂
2. Secondary Amines- Two organic substituents replace the hydrogen atoms of the ammonia molecule forming an amine. Eg: Dimethylamine (CH₃)₂NH, Diphenylamine  (C₆H₅)2NH
3. Tertiary Amines- When all 3 of the hydrogen atoms are replaced by an organic substituent, it could be an aryl or aromatic group. Eg: Trimethylamine $N(C{H}_3{)}_3$, Ethylenediaminetetraacetic acid (EDTA)
4. Cyclic Amines- These are secondary or tertiary amines in an aromatic ring structure. Eg: Piperidine $(C{H}_2{)}_{5}NH$, Aziridines $C_2{H}_{5}N$

4.0Physical Properties of Amines

The lower aliphatic amines are gaseous in nature with a fishy smell. Primary amines with three or four carbon atoms are liquids at room temperature whereas higher ones are solids.

Aniline and other arylamines are generally colourless, but they get coloured if stored outdoors due to atmospheric oxidation.

Lower aliphatic amines can form hydrogen bonds with water molecules hence they are soluble in water. An increase in the size of the hydrophobic alkyl part increases the molar mass of amines which results in a decrease in its solubility in water. Higher amines are insoluble in water.

Organic solvents like alcohol, benzene, and ether readily dissolve amines. Alcohols have higher polarity as compared to amines and hence they form stronger intermolecular hydrogen bonds.

Primary and secondary amines are often engaged in the intermolecular association as a result of hydrogen bonding between the nitrogen of one and hydrogen of the other molecule.

The intermolecular association is more prominent in the case of primary amines as compared to secondary amines due to the availability of two hydrogen atoms.

In tertiary amines, there is no intermolecular association due to the absence of free hydrogen atoms for bonding. The order of boiling point of amines is as follows: Primary > Secondary > Tertiary.

5.0Chemical Reactions of Amines

1. Reaction showing basic nature:

Amines being basic in nature react with acids to form salts.

Amine salts on treatment with a base like NaOH, regenerate the parent amine.

2. Acylation reaction :

Aliphatic and aromatic primary and secondary amines react with acid chlorides, anhydrides and esters by nucleophilic substitution reaction.

3. Hoffmann's carbylamine reaction or isocyanide test :

Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form isocyanides or carbylamines which are foul smelling substances.

Note: Dichlorocarbene is formed in this reaction.

4. Reaction with Benzenesulphonyl chloride (Hinsberg’s reagent) :

Note:  Hinsberg test is used to distinguish between primary, secondary and tertiary amines.

These days benzenesulphonyl chloride is replaced by p-toluenesulphonyl chloride (TsCl)

6.0Methods of Preparation of Amines

1. Reduction of nitro compounds : Nitro compounds are reduced to amines by passing hydrogen gas in the presence of finely divided nickel, palladium or platinum and also by reduction with metals in acidic medium.

2. Reduction of nitriles(cyanides) and isonitriles (isocyanides) : Nitriles on reduction with lithium aluminium hydride (LiAlH₄) or catalytic hydrogenation produce primary amines but isonitriles produce secondary amines.

3. Reduction of amides : Amides on reduction with lithium aluminium hydride (LiAlH₄) produce amines.

4. Gabriel phthalimide synthesis :  Gabriel synthesis is used for the preparation of 1° aliphatic amines. Phthalimide on treatment with ethanolic potassium hydroxide forms potassium salt of phthalimide which on reaction with alkyl halide followed by alkaline or acidic hydrolysis produces the corresponding primary amine.

7.0Uses 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.