NEET Chapter - Amines

Amines can be found in proteins, vitamins, alkaloids, and hormones in nature. Polymers, dyestuffs, and pharmaceuticals are examples of synthetic materials. To raise blood pressure, two biologically active chemicals, adrenaline and ephedrine, both of which contain a secondary amino group, are utilized. In dentistry, novocain, a synthetic amino molecule, is used as an anesthetic.

In this article, we have covered all of the main aspects of amine, amide, and nitro compounds. Students can refer to this amine neet notes and clear their basic knowledge on this chapter.

Important Topics of Amines

• Aliphatic amine

• Carbylamine reaction

• Coupling reaction

• Amide

• Hoffman bromamide reaction

• Aromatic amine

• Nitro camide compounds

• Nitration

• Diazonium salt

Important Concepts of Amines

What are Amines?

Amines are ammonia derivatives that are made by replacing one, two, or all three hydrogen atoms with alkyl and/or aryl groups.

Structure of Amine

Pyramidal Shape of Trimethylamine

Types of Amine

• Depending on the quantity of hydrogen atoms in ammonia molecules that are replaced by alkyl or aryl groups. RNH2 or ArNH2, a primary amine, is formed when one hydrogen atom of ammonia is replaced by R or Ar (1o).

• If two hydrogen atoms of ammonia or one hydrogen atom of R-NH2 are substituted by another alkyl/aryl(R') group, secondary (2o) amines (R2NH) are formed.

• Tertiary (3o) Amines- Tertiary amines are formed when another hydrogen atom of secondary amine is replaced by an alkyl/aryl group (R3N).

Properties of Organic Compounds Containing Nitrogen (Amines)

• Lower (amines with less carbon) aliphatic amines exist in gaseous state with a fishy odor.

• Aniline and other arylamines are generally colourless, but due to air oxidation, they become coloured with storage.

• Because they can establish hydrogen bonds with water molecules, lower aliphatic amines are soluble in water. However, when the molecular mass of amines (R-NH2) increases, solubility declines due to the increased size of the hydrophobic alkyl component.

• Because of hydrogen bonding between the nitrogen of one molecule and the hydrogen of another, primary and secondary amines form intermolecular associations. Because primary amines have two hydrogen atoms available for hydrogen bond formation, they have higher intermolecular interaction than secondary amines. Because there is no hydrogen atom available for hydrogen bond formation, tertiary amines do not exhibit intermolecular interaction. As a result, the boiling temperatures of isomeric amines are in the following order:

Primary > Secondary > Tertiary

• Benzenediazonium chloride is a crystalline substance that is colourless. It is generally water (H2O) soluble and generally stable at room temperature, but when warmed, it reacts with water. In the dry state, it decomposes quickly. Water insoluble, benzenediazonium fluoroborate is stable at ambient temperature.

• Amines are reactive because of the difference in electronegativity between nitrogen and hydrogen atoms, as well as the presence of an unshared pair of electrons above the nitrogen atom.

Basic Character of Amines

Basic character of amines can be explained as follows:

• They react with acids to generate salts. 

• Amine salts regenerate the parent amine when treated with a base such as NaOH.

R+NH3X- + OH- →RNH2 + H2O + X-

• Amines react with mineral acids to generate ammonium salts, indicating that they are basic in nature. 

• Amines are Lewis bases because they have an unshared pair of electrons on nitrogen atoms. 

• The stronger the base, the higher the value of Kb or the lower the value of pKb.

PKb= -log Kb

Preparation Reaction for Amines

1. Reduction of Nitro Compounds

By passing hydrogen gas through finely divided nickel, palladium, or platinum, nitro compounds are reduced to amines, as well as by reduction with metals in an acidic media.

2. Ammonolysis of Alkyl Halide

A nucleophile can easily cleave carbon-halogen bonds in alkyl or benzyl halides. As a result, when an alkyl or benzyl halide reacts with an ethanolic solution of ammonia, the halogen atom is replaced by an amino (–NH2) group in a nucleophilic substitution reaction.

RNH2 + RX → R2NH

R2NH + RX → R3N

R3N + RX → R4+NX-

Treatment with a strong base yields the free amine from the ammonium salt:

R+NH3X- + NaOH → R-NH2 + H2O + NaX

3. Reduction of Nitriles

Primary amines are formed when nitriles are reduced with lithium aluminium hydride (LiAlH4) or hydrogenated catalytically.

R-CN + H2/Ni + Na(Hg)/C2H5OH → R-CH2-NH2

4. Reduction of Amides

When amides are reduced with lithium aluminium hydride, amines are formed.

R-C(O)-NH2 + LiAlH4 + H2O→  R-CH2-NH2

5. Gabriel Phthalimide Reaction

Primary amines are made via the Gabriel synthesis method. When phthalimide is treated with ethanolic potassium hydroxide, it generates a potassium salt, which when heated with an alkyl halide and then alkaline hydrolyzed yields the primary amine.

Because aryl halides do not undergo nucleophilic substitution with the anion generated by phthalimide, this approach cannot be used to make aromatic primary amines.

6. Hoffmann Bromamide Degradation Reaction

By treating an amide with bromine in an aqueous or ethanolic solution of sodium hydroxide, Hoffmann created a method for preparing primary amines.

R-C(O)-NH2 + Br2 + 4NaOH→ R-NH2 + Na2CO3 + 2NaBr + 2H2O

Chemical Reactions of Amine

1. Carbylamine Reaction

When primary amines are heated with chloroform and ethanolic potassium hydroxide, they generate isocyanides or carbylamines, which have a horrible odour. This reaction does not occur in secondary or tertiary amines.

R-NH2 + CHCl3 + 3KOH → R-NC + 3KCl + 3H2O     (on heating)

2. Reaction with Nitrous Acid

When primary aliphatic amines react with nitrous acid, they produce aliphatic diazonium salts, which are unstable and release nitrogen gas and alcohols in large quantities.

R-NH2 + HNO2 (NaNO2 + HCl) → RN+2Cl-

RN+2Cl- + H2O → ROH + N2 + HCl

At low temperatures (273-278 K), aromatic amines react with nitrous acid to create diazonium ions.

C6H5-NH2 + HNO2 (NaNO2 + HCl) → C6H5N+2Cl- + NaCl + 2H2O

3. Reaction with Arylsulphonyl Chloride

Hinsberg's reagent, benzenesulphonyl chloride (C6H5SO2Cl), interacts with primary and secondary amines to produce sulphonamides.

(a) N-ethylbenzenesulphonyl amide is formed by reacting benzenesulphonyl chloride with primary amine.

Due to the presence of a strong electron withdrawing sulfonyl group, the hydrogen linked to nitrogen in sulphonamide is extremely acidic. As a result, it is alkali soluble.

(b) N,N-diethylbenzenesulphonamide is produced in the reaction with secondary amine.

Tertiary amines do not react with benzenesulfonyl chloride.

4. Electrophilic Substitution Reaction

An ortho-para directing group is the Amine group. It is subjected to a number of electrophilic substitution processes.

(a) Bromination:

At room temperature, aniline reacts with bromine water to form a white precipitate of 2,4,6-tribromoaniline. In the presence of an acetyl group, controlled bromination can occur. It shields the amine group and keeps uncontrolled halogenation under control (bromination).

(b) Nitration

Aniline is protonated to create the meta directing anilinium ion in a very acidic solution. As a result, in addition to ortho and para derivatives, a considerable number of meta derivatives are generated.

The nitration reaction can be regulated and the p-nitro derivative obtained as the primary product by protecting the –NH2 group via an acetylation reaction with acetic anhydride.

(C) Sulphonation

Aniline combines with concentrated sulphuric acid to make anilinium hydrogensulphate, which produces p-aminobenzene sulphonic acid when heated with sulphuric acid at 453-473K.

Why Aniline do not undergo Friedel Crafts Reaction

Because the Lewis acid, aluminium chloride, is utilised as a catalyst, aniline does not undergo Friedel-Crafts reaction (alkylation and acetylation). As a result, the nitrogen in aniline gains a positive charge, acting as a powerful deactivate group for subsequent reactions.

Resonance in Diazonium Salt

Preparation Method of Diazonium Salt

The reaction of aniline with nitrous acid at 273-278K produces benzene diazonium chloride. The reaction of sodium nitrite with hydrochloric acid produces nitrous acid in the reaction mixture. Diazotization is the process of converting primary aromatic amines into diazonium ions.

C6H5NH2 + NaNO2 + 2HCl →C6H5N+2Cl- + NaCl + 2H2O

Reactions of Diazonium Salt

1. Sandmayer’s Reaction

Replacement by halide or cyanide ion: In the presence of Cu(I) ions, the nucleophiles Cl–, Br–, and CN– can easily be introduced into the benzene ring. The Sandmeyer reaction is the name for this reaction.

2. Gatterman Reaction 

By treating the diazonium salt solution with the appropriate halogen acid in the presence of copper powder, chlorine or bromine can be introduced into the benzene ring. The Gatterman reaction is the name for this.

3. Replacement by Iodide Ion:

ArN+2Cl- + KI → ArI + KCl + N2

4. Replacement by Fluoride ion:

ArN+2Cl- + HBF4 → Ar-N+2BF-4

Ar-N+2BF-4 → Ar-F + BF3 + N2

5. Replacement by H:

ArN+2Cl- + H3PO2 + H2O → ArH + N2 + H3PO3 + HCl

ArN+2Cl- + CH3CH2OH  → ArH + N2 + CH3CHO + HCl

6. Replacement by Hydroxyl Group:

ArN+2Cl- + H2O → ArOH + N2 + HCl

7. Replacement by –NO2 Group:

8. Coupling Reaction:

Solved Examples/Problems from the Chapter: Amines

1. How will you convert: benzene into aniline

Ans:

Key Point: Nitro group can be converted into amine group with the help of reducing agent. 

2.How will you convert: Cl-(CH2)4-Cl into Hexane -1,6- diamine

Ans:

Key Point: Cyanide group can be converted into amine group with the help of reducing agent. Cyanide group increases one carbon in the parent chain.

Solved Problems of Previous Year Question From the Chapter: Amines

1. Identify the compound that will react with Hinsberg’s reagent to give a solid which dissolves in alkali.

Ans: The correct answer is option 3. Only primary amines react with hinsberg reagents and forms precipitate that dissolve in addition to alkali.

Trick: Hinsberg test is used to distinguish between the amine. Primary and secondary amine give this test (product formed with primary amine is soluble in alkali while with secondary amine is insoluble in base) and tertiary amine do not give this test.

2. Which of the amines will give a carbylamine test?

Ans:  The correct answer is option 1. Carbylamines (or isocyanides) are formed when aliphatic/aromatic primary amines are heated with chloroform and alcoholic KOH.

Ph-NH2 + CHCl3 + 3KOH → Ph-NC + 3KCl + 3H2O

Trick: This test is not given by secondary or tertiary amines.

3. The reagent ‘R’ in the given sequence of chemical reaction is :

(1) H2O

(2) CH3CH2OH

(3) HI

(4) CuCN/KCN

Ans: The correct answer for this question is option 2.

Trick: Some weak reducing agents, such as hypophosphorous acid or ethanol, convert diazonium salts to arene, which is then oxidized to phosphorous acid or ethanal.

Practice Questions

1. Convert: 3 - Methylaniline into3 - nitrotoluene

 Ans:

2. Why Diazonium salts of aromatic amines are more stable than those of aliphatic amines.

Ans: Due to resonance 

Conclusion

In this amine's notes we have provided important information regarding the chapter compounds of nitrogen such as important definitions, reactions, and concepts. Students should work on more solved examples and organic compounds of nitrogen examples for securing good grades in the NEET exams.