Amines: Class 12 NCERT Chemistry Guide for Preparation & Properties

Introduction to Amines: Definition and Importance

Amines are organic compounds derived from ammonia ($NH_3$) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They play a crucial role in pharmaceuticals, dyes, and polymers.

In Class 12 NCERT Chemistry, understanding amines helps build foundational knowledge for organic chemistry and practical applications. Amines are classified as:

• Primary amines ($R-NH_2$)
• Secondary amines ($R_2-NH$)
• Tertiary amines ($R_3-N$)

where $R$ is an alkyl or aryl group.

Their structure influences physical and chemical properties, including basicity and reactivity.

Methods of Preparation of Amines

Class 12 NCERT Chemistry details several important methods to prepare amines. These methods vary based on starting materials and desired amine type.

1. Reduction of Nitro Compounds

• Nitro compounds ($R-NO_2$) reduce to amines ($R-NH_2$) using catalytic hydrogenation ($H_2$ with Ni, Pd, Pt) or metal-acid reductions (Fe/HCl).
• Example: Nitrobenzene to aniline.

2. Ammonolysis of Alkyl Halides

• Alkyl halides react with ethanolic ammonia at 373 K.
• Halogen is replaced by $-NH_2$ via nucleophilic substitution.
• Excess ammonia favors primary amines; otherwise, secondary, tertiary amines, and quaternary ammonium salts form.
• Reactivity order: $RI > RBr > RCl$.

3. Reduction of Nitriles

• Nitriles ($R-C riplebond N$) reduce with $LiAlH_4$ or catalytic hydrogenation to primary amines ($R-CH_2-NH_2$).
• Useful for increasing carbon chain length by one.

4. Reduction of Amides

• Amides ($R-CONH_2$) reduce with $LiAlH_4$ to primary amines.

5. Gabriel Phthalimide Synthesis

• Potassium phthalimide reacts with alkyl halides, followed by hydrolysis, yielding primary amines.
• Specific for primary amines, not aromatic.

6. Hoffmann Bromamide Degradation

• Amides treated with $Br_2$ and $NaOH$ give primary amines with one less carbon atom.

These methods provide versatile routes to synthesize amines for various applications.

Comparison of Amines Preparation Methods

Understanding the advantages and limitations of each preparation method is essential for Class 12 students.

Choose the method based on the target amine and available reagents.

Basicity of Amines: Factors and Order

Basicity is a key property of amines, influenced by the availability of the nitrogen lone pair to accept a proton.

Factors affecting amine basicity:

• Electron donating groups increase electron density on nitrogen, enhancing basicity.
• Electron withdrawing groups decrease basicity.
• Resonance effects can delocalize the lone pair, reducing basicity.
• Steric hindrance can reduce accessibility of nitrogen.

Typical order of basicity (from NCERT Class 12):

Example:

Aniline is less basic because the lone pair on nitrogen participates in resonance with the aromatic ring, making it less available for protonation.

Worked Example: Preparation of Primary Amine from Alkyl Halide

Problem: Write the chemical equations for the preparation of ethylamine from bromoethane using ammonolysis.

Solution:

1. Bromoethane reacts with excess ethanolic ammonia to form ethylamine:

$$ C_2H_5Br + NH_3 \xrightarrow{ethanol, 373 K} C_2H_5NH_2 + HBr $$

2. Secondary and tertiary amines may form if ammonia is limited, but excess ammonia favours primary amine formation.

This method is widely used in Class 12 NCERT labs and exams for amine synthesis.

Gabriel Phthalimide Synthesis: Stepwise Mechanism

Gabriel synthesis is an important method to prepare primary amines without contamination by secondary or tertiary amines.

Steps:

1. Phthalimide reacts with $KOH$ to form potassium phthalimide salt.

2. This salt undergoes nucleophilic substitution with an alkyl halide ($R-X$) to form N-alkyl phthalimide.

3. Hydrolysis (acidic or basic) of N-alkyl phthalimide yields the primary amine ($R-NH_2$).

Advantages:

• Specific for primary amines.
• Avoids multiple alkylation side products.

Limitations:

• Not suitable for aromatic amines.

Reaction scheme:

$$ \text{Phthalimide} + KOH \rightarrow \text{Potassium phthalimide} $$ $$ \text{Potassium phthalimide} + R-X \rightarrow \text{N-alkyl phthalimide} $$ $$ \text{N-alkyl phthalimide} + H_2O / H^+ \rightarrow R-NH_2 + \text{phthalic acid} $$