Amines: Properties and Reactions

Introduction

Amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. They are important functional groups found in many natural products, pharmaceuticals, and polymers. Amines exhibit a range of properties and undergo characteristic reactions due to the presence of the nitrogen atom with its lone pair of electrons.

Classification of Amines

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

• Primary (1^o) Amines: The nitrogen atom is bonded to one alkyl or aryl group and two hydrogen atoms.
• Secondary (2^o) Amines: The nitrogen atom is bonded to two alkyl or aryl groups and one hydrogen atom.
• Tertiary (3^o) Amines: The nitrogen atom is bonded to three alkyl or aryl groups.
• Quaternary Ammonium Salts: The nitrogen atom is bonded to four alkyl or aryl groups and carries a positive charge.

Properties of Amines

Physical Properties

The physical properties of amines depend on the size and nature of the alkyl/aryl substituents. Generally:

• Boiling Point: Amines have higher boiling points than comparable alkanes due to stronger intermolecular forces – hydrogen bonding. The boiling point increases with increasing molecular weight and the presence of hydrogen bonding.
• Solubility: Lower molecular weight amines are soluble in water due to hydrogen bonding. Solubility decreases with increasing molecular weight as the non-polar character dominates.
• Odor: Low molecular weight amines often have a fishy or ammonia-like odor.

Chemical Properties

The lone pair of electrons on the nitrogen atom makes amines basic. They can accept a proton to form ammonium ions.

$$NH_3(g) + H^+(aq) \rightleftharpoons NH_4^+(aq)$$

The basicity of amines is affected by the substituents on the nitrogen atom. Electron-donating groups increase basicity, while electron-withdrawing groups decrease it.

Reactions of Amines

Basicity

Amines act as bases and react with acids to form salts.

$$R_2NH + HCl \rightarrow R_2NH_3Cl$$

The strength of the base depends on the electron density around the nitrogen atom. The more electron-donating groups attached to the nitrogen, the stronger the base.

Acylation

Amines react with acyl chlorides or anhydrides to form amides.

$$R_2NH + R'COCl \rightarrow R_2NHCOR' + HCl$$

This reaction is typically carried out in the presence of a base (e.g., pyridine) to neutralize the HCl produced.

Alkylation

Amines react with alkyl halides to form substituted amines. This reaction can lead to a mixture of primary, secondary, and tertiary amines.

$$R_2NH + R'X \rightarrow R_2NR' + HX$$

Controlling the reaction conditions (e.g., using excess amine or a bulky alkyl halide) can favour the formation of a specific amine.

Reaction with Concentrated Nitric Acid

Primary and secondary amines react with concentrated nitric acid to form mixtures of products, including oxides, nitro compounds, and ammonium salts. This reaction is often vigorous and can be hazardous.

Diazotization and Coupling

Primary aromatic amines react with nitrous acid (HNO₂) at low temperatures (0-5 °C) to form diazonium salts. These salts are versatile intermediates in organic synthesis and can undergo a variety of reactions, including coupling with aromatic compounds to form azo dyes.

$$R-NH_2 + HNO_2 + HCl \rightarrow R-N_2^+Cl^- + 2H_2O$$

The diazonium salt can then react with an electron-rich aromatic compound (e.g., phenol, aniline) to form a coloured azo dye.

Applications of Amines

Amines are used in a wide range of applications, including:

• Pharmaceuticals: Many drugs contain amine functional groups.
• Dyes and Pigments: Azo dyes, formed from diazonium salts, are widely used as colourants.
• Polymers: Amines are used in the production of polymers such as nylon.
• Agrochemicals: Some pesticides and herbicides contain amine groups.

Amine Type	Chemical Formula	Basicity	Typical Reactions
Primary	R-NH2	Moderate	Acylation, Alkylation, Diazotization
Secondary	R2-NH	Moderate	Acylation, Alkylation, Diazotization
Tertiary	R3-N	Low	Less reactive towards acylation and alkylation
Quaternary Ammonium Salt	R4-N+X-	Very Low	Generally do not undergo acylation or alkylation