Chapter 12 – Amines

Amines can primarily be classified into 3 types depending upon the number of hydrogens that have been replaced by alkyl or aryl groups; they are explained below-  

1. Primary Amines- If R or Ar replace one hydrogen atom of ammonia, we get RNH2 or AR, a primary amine. 
2. Secondary Amines- If two hydrogen atoms of ammonia or one hydrogen atom of R-NH2 are replaced by another alkyl or aryl group, we will get R-NHR’, a secondary amine. 
3. Tertiary Amines- When a replacement of another hydrogen atom by alkyl or aryl group leads occurs, a tertiary amine is obtained.

Certain physical properties of amines are as follows- 

1. The lower aliphatic amines are gases with a fishy odour to them. Primary amines with three or more carbon atoms are liquid, and the higher ones remain solid. 
2. Lower aliphatic amines are soluble in water, which is because they can form hydrogen bonds with water molecules. 
3. Primary and secondary amines are also engaged in intermolecular interaction, which occurs due to hydrogen bonding between nitrogen and hydrogen of another molecule. However, this association and interaction is more likely to take place in primary amines than secondary amines because there are two hydrogen atoms available for the process of bond formation in it.

They mostly occur among proteins, vitamins, alkaloids, and hormones. Some examples of this are ephedrine and adrenaline. Some synthetic examples are polymers, drugs, and dyestuff; novocain is also another synthetic compound that has amino in it. Other than that, quaternary ammonium salts are also used as surfactants, and Diazonium salts also play an important role in preparing certain aromatic compounds, including dyes.

Amines can be prepared using the following way- 

1. Reduction of nitro compounds– In this method, nitro compounds are reduced to amines by passing hydrogen gas in the presence of a specific and finely divided nickel, palladium, and platinum. Nitroalkanes can also be reduced to the corresponding alkanamines. 

                      H2/Pd                

            NO2 ———-> NH2

                     Ethanol

                     Sn+ HCl     

            NO2 ———-> NH2

                     Or Fe+ HCl

In this case, reduction with iron scrap and hydrochloric acid is preferred due to FeCI2 getting hydrolysed to release hydrochloric acid during the reaction. 

1. Ammonolysis of alkyl halides- An alkyl or benzyl halide on reaction with an ethanolic ammonia solution goes under a nucleophilic substitution reaction in which a halogen atom gets replaced by an amino (-NH2) group. This process is known as ammonolysis. 
2. Reduction of nitriles- Nitriles on reduction with lithium aluminum hydride or catalytic hydrogenation produce the primary amines. This reaction is mostly used to prepare amines, which contain one more carbon atom that they started with. 
3. Reduction of amides- The amides on reduction with lithium hydride also yields amines. 
4. Gabriel phthalimide synthesis- Gabriel synthesis is a process that is used for the preparation of primary amines. Phthalimide treatment with ethanolic potassium hydroxide forms potassium salt, which is then heated with an alkyl halide and then followed by alkaline hydrolysis, which forms the corresponding primary amine. However, aromatic amines cannot be prepared using this method. 
5. Hoffman bromamide degradation reaction- This method was developed by Hoffman to prepare primary amines by treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide. In such a degradation reaction, the migration of an alkyl or aryl group occurs from the carbonyl carbon of the amide to the nitrogen atom. However, the amine formed through this contains one carbon less than in the present amide.