Reactions of Amines

Amines are regarded as derivatives of ammonia in which one, two or all three hydrogen atoms are replaced by alkyl or aryl groups. They constitute an important class of organic compounds. They occur widely throughout both plants and animals. They are found among proteins, vitamins, alkaloids, hormones, etc. Synthetic examples include polymers, drugs, dyestuffs, etc.

Classification of Amines

The amines are classified as primary, secondary, or tertiary according to one, two or three hydrogen atoms of ammonia molecule are replaced by alkyl or aryl groups in ammonia molecule. If one hydrogen atom of ammonia is replaced by alkyl or aryl group, we get RNH2 or ArNH2, a primary amine. If two hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get secondary amine. If three hydrogen atoms of ammonia are replaced by an alkyl or aryl group, we get tertiary amine. 

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Reaction of Amines

1. Reaction with Acids - Amines being basic react with acids to form salts.

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The salts of amines are ionic compounds and are soluble in water. On treatment with aqueous hydroxide, amines are regenerated.

2. Reaction of Amines With Metal Ions - Like ammonia, amines combine with metal ions such as Ag and Cu ions to form complex compounds. The lone pair of electrons in ammonia is used to form a co-ordinate bond of the amine with metal ions. 

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3. Alkylation - Primary and secondary amines react with alkyl halides to form tertiary amines. The primary or secondary amine acts as a nucleophile and performs nucleophilic substitution at an alkyl halide. On removal of HX, secondary or a tertiary amine is regenerated respectively. The secondary amine is a more powerful nucleophile and again reacts similarly with another alkyl halide forming tertiary amine. At each stage of the reaction, an equivalent amount of a strong acid is formed. This can protonate the amine making lone pair on nitrogen not available for nucleophilic attack and therefore, stop the reaction before completion. Therefore, for the neutralisation of the acid and for liberating the nucleophile, a base such as carbonate is added. Finally, the tertiary amine reacts with alkyl halides to form quaternary ammonium salts.

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4. Acylation - Aliphatic and aromatic primary and secondary amines (which contain replaceable hydrogen atoms) react with an acid chloride, acid anhydride and esters to form substituted amides. The process of introducing an acyl group into a molecule is called acylation. 

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5. Carbonyl Amine Reaction - This reaction is also known as “Schiff’s base formation reaction”. In this reaction, amines react with aldehydes and ketones. Both aliphatic and aromatic primary amines react with aldehydes and ketones to form imines also called Schiff’s bases or anils. 

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6. Carbylamine Reaction - Aliphatic and aromatic primary amines when warmed with chloroform and an alcoholic solution of potassium hydroxide, form isocyanide or carbylamine which have a very unpleasant or foul smell. This reaction cannot be called a Carbon Amine Reaction.

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Secondary and tertiary amines do not give this test. Therefore, this test can be used to distinguish between primary amines from secondary and tertiary amines.

7. Reaction with Aryl Sulphonyl Chloride - Benzene sulphonyl chloride which is also known as “Hinsberg’s reagent” reacts with primary and secondary amines to form sulphonamides.

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Since different amines react differently with benzene sulphonyl chloride, this reaction can be used for the distinction between primary, secondary, and tertiary amines and also for the separation of their mixtures.

8. Coupling Reaction in Amines - Aromatic amines react with diazonium salts to form azo compounds in an acidic medium called dyes. The reaction is known as coupling or diazo reaction.

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9. Amine Reaction with Water - Nitrogen of amine group donates its lone pair to proton and form positive ion on reacting with water. 

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Did You Know?

• Aliphatic amines of low molecular weight are used as solvents.

• Amines are used as intermediates in drug manufacture and as reagents in organic synthesis.

• Aromatic amines are used for the manufacture of polymers, dyes and as intermediates for additives in the rubber industry.