Comprehensive NEET Chemistry Notes for Organic Compounds Containing Nitrogen (Amines)

Organic Compounds Containing Nitrogen (Amines)

1. Introduction

Amines are an important class of organic compounds derived by replacing one or more hydrogen atoms of an ammonia molecule with alkyl or aryl groups. They are found in nature in proteins, vitamins, alkaloids, and hormones. Synthetic examples include polymers, dyes, and drugs such as adrenaline, ephedrine, Novocain, and Benadryl.

Did You Know?

Quaternary ammonium salts are used as surfactants and diazonium salts are intermediates in the preparation of aromatic compounds including dyes.

2. Structure of Amines

Amines can be considered derivatives of ammonia. The nitrogen atom in amines is trivalent and carries an unshared pair of electrons, resulting in sp³ hybridization and a pyramidal geometry. The angle C–N–E (where E is C or H) in amines is less than 109.5°, e.g., 108° in trimethylamine.

NEET Tip:

The geometry of amines is pyramidal due to sp³ hybridization of the nitrogen atom.

3. Classification of Amines

Amines are classified based on the number of hydrogen atoms replaced by alkyl or aryl groups:

Primary (1°) Amines: One hydrogen atom replaced (e.g., RNH₂).
Secondary (2°) Amines: Two hydrogen atoms replaced (e.g., R₂NH).
Tertiary (3°) Amines: All three hydrogen atoms replaced (e.g., R₃N).

Mnemonic:

"Primary one, secondary two, tertiary three."

4. Nomenclature of Amines

Common Names: Prefix alkyl group to "amine" (e.g., methylamine).
IUPAC Names: Replace 'e' of alkane with 'amine' (e.g., methanamine).

For secondary and tertiary amines, use the locant N to designate substituents attached to nitrogen (e.g., N-methylethanamine).

Example:

CH₃NH₂ (methanamine) and CH₃NHCH₃ (N-methylmethanamine).

5. Preparation of Amines

5.1 Reduction of Nitro Compounds

Nitro compounds are reduced to amines using hydrogen in the presence of metal catalysts like nickel, palladium, or platinum.

Equation:

$R - N O_{2} + 3 H_{2} \to R - N H_{2} + 2 H_{2} O$

5.2 Ammonolysis of Alkyl Halides

Alkyl halides react with ammonia to form amines.

Equation:

$R - X + N H_{3} \to R - N H_{2} + H X$

5.3 Reduction of Nitriles

Nitriles are reduced to primary amines using lithium aluminium hydride or catalytic hydrogenation.

Equation:

$R - CN + 4 [H] \to R - C H_{2} N H_{2}$

5.4 Reduction of Amides

Amides are reduced to amines using lithium aluminium hydride.

Equation:

$R - CON H_{2} + 4 [H] \to R - C H_{2} N H_{2} + H_{2} O$

5.5 Gabriel Phthalimide Synthesis

Phthalimide reacts with ethanolic KOH and alkyl halide, followed by hydrolysis to yield primary amines.

5.6 Hoffmann Bromamide Degradation

Amides react with bromine in the presence of NaOH to produce primary amines.

Equation:

$R - CON H_{2} + B r_{2} + 4 N a O H \to R - N H_{2} + N a_{2} C O_{3} + 2 N a B r + 2 H_{2} O$

Common Misconception:

Gabriel phthalimide synthesis cannot be used for aromatic primary amines.

6. Physical Properties

State: Lower aliphatic amines are gases with fishy odor, higher ones are liquids or solids.
Boiling Points: Amines have higher boiling points than hydrocarbons of similar molecular mass due to hydrogen bonding.
Solubility: Lower amines are soluble in water, solubility decreases with increasing molecular mass.

Table: Comparison of Boiling Points of Amines, Alcohols, and Alkanes

Compound	Molar Mass (g/mol)	Boiling Point (K)
n-Butylamine	73	350.8
Diethylamine	73	329.3
Trimethylamine	59	276.6
n-Butanol	74	390.3
n-Pentane	72	309.1

NEET Tip:

Primary and secondary amines exhibit intermolecular hydrogen bonding, which increases their boiling points.

7. Chemical Reactions

7.1 Basicity of Amines

Amines react with acids to form ammonium salts, indicating their basic nature.

Equation:

$R - N H_{2} + H Cl \to R - N H_{3}^{+} C l^{-}$

Table: pK_b Values of Amines

Amine	pK_b
Methanamine	3.38
N-Methylmethanamine	3.27
Aniline	9.38

7.2 Alkylation and Acylation

Amines react with alkyl halides (alkylation) and acyl halides (acylation) to form substituted amines and amides, respectively.

Equation:

$R - N H_{2} + R^{'} - Cl \to R - N H R^{'} + H Cl$

7.3 Carbylamine Reaction

Primary amines react with chloroform and ethanolic KOH to form isocyanides.

Equation:

$R - N H_{2} + C H C l_{3} + 3 K O H \to R - NC + 3 K Cl + 3 H_{2} O$

7.4 Reaction with Nitrous Acid

Primary amines react with nitrous acid to form diazonium salts, which are key intermediates in organic synthesis.

Equation:

$R - N H_{2} + H N O_{2} \to R - N_{2}^{+} + 2 H_{2} O$

NEET Tip:

Diazonium salts are used in coupling reactions to form azo compounds.

8. Applications of Amines

Pharmaceuticals: Amines are found in drugs like adrenaline and ephedrine.
Industrial Use: Amines are used in the manufacture of dyes, polymers, and surfactants.
Diazonium Salts: Used in the synthesis of aromatic compounds and dyes.

Real-life Application:

Novocain, an amino compound, is used as an anesthetic in dentistry.

Quick Recap

Structure: Amines are derivatives of ammonia with a pyramidal geometry.
Classification: Primary, secondary, and tertiary amines based on the number of hydrogen atoms replaced.
Preparation: Reduction of nitro compounds, nitriles, and amides; ammonolysis of alkyl halides; Gabriel synthesis; Hoffmann degradation.
Properties: Higher boiling points due to hydrogen bonding, solubility decreases with increasing molecular mass.
Reactions: Basicity, alkylation, acylation, carbylamine reaction, and reaction with nitrous acid.

Practice Questions

Explain the mechanism of the Hoffmann bromamide degradation reaction.
How does the basicity of amines change with the introduction of alkyl groups?
Describe the Gabriel phthalimide synthesis with an example.
What are the products of the reaction of aniline with nitrous acid?
Compare the boiling points of methylamine, dimethylamine, and trimethylamine.

Answers to Practice Questions

Mechanism of Hoffmann Degradation: Involves the migration of an alkyl or aryl group from the carbonyl carbon to the nitrogen atom.
Basicity of Amines: Increases with electron-donating alkyl groups due to increased electron density on nitrogen.
Gabriel Synthesis: Phthalimide reacts with ethanolic KOH and alkyl halide, followed by hydrolysis to yield primary amines.
Reaction of Aniline with Nitrous Acid: Forms benzenediazonium chloride and nitrogen gas.
Boiling Points Comparison: Methylamine > Dimethylamine > Trimethylamine, due to decreasing hydrogen bonding.

Glossary

Ammonolysis: Replacement of a halogen atom by an amino group in alkyl halides.
Acylation: Reaction of amines with acyl chlorides to form amides.
Diazonium Salts: Compounds formed by the reaction of primary amines with nitrous acid, used in azo coupling reactions.
Carbylamine Reaction: Reaction of primary amines with chloroform and KOH to form isocyanides.