Amines Formula – Classification, Preparation, Properties

Amines are derivatives of ammonia where one or more of the hydrogen atoms are replaced by alkyl or aryl groups. They are basic in nature due to the presence of lone pair electrons on the nitrogen atom.

Classification of Amines

Amines are classified based on the number of alkyl or aryl groups attached to the nitrogen atom:

Primary amines (1°): Have one alkyl/aryl group. Example: CH₃NH₂ (Methylamine) Secondary amines (2°): Have two alkyl/aryl groups. Example: (CH₃)₂NH (Dimethylamine)

Tertiary amines (3°): Have three alkyl/aryl groups. Example: (CH₃)₃N (Trimethylamine)

Nomenclature:

For alkyl amines: Name the alkyl group followed by “amine”. Eg: CH₃NH₂ is Methylamine. For complex structures, use the prefix “amino-” to denote the NH₂ group.

Preparation of Amines

Reduction of Nitro Compounds:

R-NO₂ + 6[H] → R-NH₂ + 2H₂O 

Amination of Alkyl Halides:

R-X + 2NH₃ → R-NH₂ + NH₄X (X is a halide)

Reduction of Nitro Compounds:

R−NO₂ ​+ 6[H] → R−NH₂ ​+ 2H₂O 

Amination of Alkyl Halides:

R−X + 2NH₃ → R−NH₂ ​+ NH₄ X 

Reduction of Amides (with LiAlH₄ or other reducing agents):

2 RCONH₂ ​+ 4[H] → 2R−NH₂ ​+ CH₃OH 

Gabriel Phthalimide Synthesis:

Phthalimide + KOH + R−X → 2 R−NH₂ ​ (after hydrolysis with dilute HCl or dilute H₂SO₄)

Reduction of Nitriles:

RCN + 4[H] → R−NH₂ ​

Hofmann Bromamide Reaction: Converts amides to primary amines.

RCONH₂ + Br₂ + 4NaOH → RNH₂ + 2NaBr + 2H₂O + Na₂CO₃

Properties of Amines

 Basicity:

Amines are basic due to the presence of lone pairs on nitrogen. Their basic strength can vary based on substituents.

 Alkylation:

Amines can undergo alkylation with alkyl halides.

Eg: R-NH₂ + R’-X → R-NHR’ + HX

Acylation: 

Amines can react with acyl chlorides to form amides.

Eg: R-NH₂ + RCOCl → RCONHR + HCl 

Alkylation (Amine Salt Formation):

2 R−NH₂ + R′−X → R−NHR ′ + HX 

Acylation (Formation of Amides):

R−NH₂ ​+ RCOCl → RCONHR + HCl 

Reaction with Carbylamine (Isocyanide Test):

2R−NH₂ ​+ CHCl₃ + 3KOH → RNC (foul-smelling isocyanide) + 3KCl + 3H₂O 

Reaction with Nitrous Acid: For primary amines:

R−NH₂ ​+ HNO₂ ​ → Alcohol + N₂ ​+ H₂O

For secondary amines: 

R₂NH + HNO₂ ​→ R₂N−NO (nitrosamine)

Hofmann Elimination (From quaternary ammonium hydroxides):

R₃N+ CH₃CH₂Br− + KOH (heat) → R₃N + Alkene + KBr +  H₂ ​  

Basicity: 

R−NH₂ ​ + H₂O ↔ R−NH₃⁺ ​ + OH ^– 

Also Check – Aluminium Nitrate Formula

Diazonium Salts

They are an important class of compounds in organic chemistry, particularly in the context of aryl compounds.

They have the general formula ArN₂⁺ ​ X⁻, where Ar represents an aryl group, usually benzene, and X is a counter ion, most commonly chloride or bromide.

The most common diazonium salt is benzene diazonium chloride,

C₆H₅N₂⁺ ​Cl⁻ 

Preparation of Diazonium Salts:

Diazonium salts are typically prepared by the reaction of an aromatic primary amine with nitrous acid at a temperature between 0°C and 5°C.

Formation from Aniline:

C₆H₅NH₂ ​+ HNO₂ ​→ C₆H₅N₂⁺Cl ⁻ + 2H₂O

Nitrous acid, HNO₂, is usually prepared in situ by mixing sodium nitrite, NaNO₂, with a mineral acid like hydrochloric acid, HCl.

2NaNO₂ ​+ HCl → HNO₂ ​+ NaCl 

Also Check – Ammonium Nitrate Formula

Properties and Reactions of Diazonium Salts:

Replacement by Halide (Sandmeyer Reaction):

a. Chlorination:

C₆H₅N₂⁺ ​Cl⁻ + CuCl → C₆H₅Cl + N₂ ​ 

b. Bromination:

 C₆H₅N₂⁺ ​Cl⁻ + CuBr → C₆H₅Br + N₂ ​

• Replacement by Cyanide (Sandmeyer Reaction):

C₆H₅N₂⁺ ​Cl⁻ + CuCN → C₆H₅CN + N₂ ​ + NaCl 

• Replacement by Fluoride (Schiemann Reaction):

C₆H₅N₂⁺ BF₄ ⁻ ​+ H₂O → C₆H₅F​ + N₂ ​ + HBF₄ ​

• Detachment of N 2 ​ to form Phenol:

C₆H₅N₂⁺ ​Cl⁻ + H₂O → C₆H₅OH + N₂ ​+ HCl

• Coupling Reactions:

Aryl diazonium salts can react with phenols and anilines to form brightly colored azo compounds:

C₆H₅N₂⁺ ​Cl⁻ + ArH → C₆H₅N=N−Ar +  HCl 

where Ar represents another aryl group. These azo dyes are of industrial importance in dyeing processes.