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Chapter 12 – Amines

The following Topics and Sub-Topics are covered in this chapter and are available on MSVgo:

Introduction to Amines

Amines are an important class of organic compounds. They can be defined as ammonia derivatives, which are an important class of organic compounds and are obtained by replacing one or more hydrogen atoms of ammonia molecules. These notes will help you revise and understand key concepts of amines.

As mentioned above, amines are derived from ammonia by replacing one, two, or all three hydrogen atoms with alkyl and aryl groups. 

Like ammonia, nitrogen atoms of amines are also trivalent, one of them carrying an unshared pair of electrons. Thus, the nitrogen orbitals in amines are sp3 hybridised and are pyramidal. Every single one of these sp3 hybridised orbitals of nitrogen overlap with orbitals of either nitrogen or hydrogen, depending on the composition of the amines present at that moment. 

The fourth orbital of nitrogen in all the amines consists of an unshared pair of electrons; due to this occurrence, the angle of C-N-E where E is C or H becomes less than 109.5 and is equivalent to 108.

Amines can primarily be classified into 3 types depending upon the number of hydrogens that have been replaced by alkyl or aryl groups; they are explained below-  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

The above discussion must have helped the readers to understand more about the concept of amines. However, to learn more about the nomenclature, basics, and chemical reactions related to amines, head to MSVgo app, which is a video library that explains concepts with examples along with explanatory visualizations and animations, which makes the concept easier to understand and also fun to learn and also enables learning the core of the concept which helps the students, their parents and teachers to learn and teach better.

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