Aliphatic Amines

Aliphatic amines are organic compounds derived from ammonia (NH₃) by replacing one or more hydrogen atoms with alkyl or aryl groups. Unlike their aromatic counterparts, aliphatic amines have an open-chain structure. They’re highly reactive and often form bonds with other molecules. These amines come in primary (1°), secondary (2°), and tertiary (3°) varieties, depending on the number of attached alkyl or aryl groups. So, whether it’s a solo alkyl group (primary), a double date (secondary), or a nitrogen atom surrounded by a lively trio (tertiary). In this article we will see Basicity of amines, effect of substituent on Basicity. Qualitative test, Structure and uses of Ethanolamine, Ethylenediamine, Amphetamine etc.

Table of Contents

Basicity of amines

Aliphatic amines, those nitrogen-containing compounds with alkyl or aryl groups, are more basic than plain ammonia (NH₃). The secret lies in their electron-loving friends—the alkyl groups. Imagine ammonia as the introverted guest at a chemical gathering. Now, swap some of its hydrogen atoms with alkyl molecules (like ethyl, propyl, or butyl). These alkyl groups are electron-donating champs—they share their electron love with nitrogen.

In aliphatic amines, the nitrogen atom gains extra electrons due to the presence of alkyl or aryl groups. These electron-rich species have a strong affinity for protons, readily accepting them. As a result, aliphatic amines become positively charged.

Effects of Substituents on the Basicity of Aliphatic Amines

The basicity of an amine, its ability to donate a lone pair of electrons to accept a proton, is significantly influenced by the nature of the substituents attached to the nitrogen atom.

General Trend

Alkyl groups enhance basicity: Alkyl groups are electron-donating in nature. They increase the electron density on the nitrogen atom, making it more capable of accepting a proton. Thus, aliphatic amines are generally stronger bases than ammonia.
Electron-withdrawing groups decrease basicity: Groups like halogens, nitro, and carbonyl groups are electron-withdrawing. They reduce the electron density on the nitrogen, making it less basic.

Factors Affecting Basicity

Inductive Effect

Alkyl groups: These exert a +I effect, increasing electron density on nitrogen, enhancing basicity.
Electron-withdrawing groups: These exert a -I effect, decreasing electron density on nitrogen, reducing basicity.

Steric Hindrance

Bulky groups around the nitrogen can hinder the approach of a proton, reducing basicity. This is more pronounced in tertiary amines.

Solvation Effects

The ability of the solvent to solvate the ammonium ion (conjugate acid) affects basicity. More stable ammonium ions lead to weaker bases.

Examples

Methylamine (CH3NH2) is a stronger base than ammonia (NH3) due to the +I effect of the methyl group.
Dimethylamine ((CH3)2NH) is even stronger than methylamine due to the increased +I effect of two methyl groups. However, steric hindrance starts to play a role, reducing the difference in basicity compared to methylamine.
Trimethylamine ((CH3)3N) has the highest +I effect but also the highest steric hindrance. The balance of these effects often leads to trimethylamine being slightly less basic than dimethylamine.
Chloroethylamine (ClCH2CH2NH2) is a weaker base than ethylamine (CH3CH2NH2) due to the -I effect of the chlorine atom.

Summary Table

Amine	Basicity	Reason
Ammonia (NH3)	Base	Reference point
Methylamine (CH3NH2)	Stronger	+I effect of methyl group
Dimethylamine ((CH3)2NH)	Stronger	Increased +I effect, but some steric hindrance
Trimethylamine ((CH3)3N)	Slightly weaker than dimethylamine	Maximum +I effect, but significant steric hindrance
Chloroethylamine (ClCH2CH2NH2)	Weaker	-I effect of chlorine atom

Qualitative Tests for Aliphatic Amines

Aliphatic amines can be distinguished from each other (primary, secondary, and tertiary) based on their reactions with different reagents.

Carbylamine Test (Isocyanide Test)

Specific for primary amines (aliphatic and aromatic)
Reagent: Chloroform and alcoholic potassium hydroxide (KOH)
Procedure: The amine is heated with chloroform and alcoholic KOH.
Observation: A foul-smelling isocyanide is formed if a primary amine is present.

Reaction

R-NH₂ + CHCl₃ + 3KOH → RNC + 3KCl + 3H₂O

Nitrous Acid Test

Distinguishes between primary, secondary, and tertiary amines
Reagent: Sodium nitrite (NaNO₂) and hydrochloric acid (HCl)
Procedure: The amine is treated with nitrous acid (formed in situ from NaNO₂ and HCl) at different temperatures.
- Primary aliphatic amines: Liberate nitrogen gas rapidly at room temperature.
- Secondary aliphatic amines: Form yellow oily nitrosoamines.
- Tertiary aliphatic amines: Form soluble nitrite salts.

Hinsberg’s Test

Distinguishes between primary, secondary, and tertiary amines
Reagent: Benzenesulfonyl chloride (C₆H₅SO₂Cl) and aqueous sodium hydroxide (NaOH)
Procedure: The amine is treated with benzenesulfonyl chloride and aqueous NaOH.
- Primary amines: Form water-soluble sodium salts of N-benzenesulfonamide.
- Secondary amines: Form insoluble N-benzenesulfonamides.
- Tertiary amines: Do not react with benzenesulfonyl chloride.

Solubility Test

Distinguishes between different classes of amines
Procedure: The amine is tested for its solubility in water, dilute HCl, and dilute NaOH.
- Lower aliphatic amines (C₁-C₄): Soluble in water due to hydrogen bonding.
- Higher aliphatic amines: Insoluble in water but soluble in dilute HCl due to formation of water-soluble ammonium salts.
- Aromatic amines: Generally insoluble in water but soluble in dilute HCl.

These tests are qualitative and can be used to identify the class of an amine, but other techniques like spectroscopy are often required for confirmation and structural determination.

Some Important Structures and Functions

Ethanolamine

Ethanolamine is a simple amino alcohol with the chemical formula HOCH₂CH₂NH₂. It contains both an alcohol (-OH) and amine (-NH₂) functional group.

Uses

Surfactant: It is used in the production of detergents, emulsifiers, and wetting agents.
Gas purification: It is used to remove carbon dioxide and hydrogen sulfide from natural gas.
Pharmaceuticals: It is used as an intermediate in the synthesis of various drugs and as a pH regulator in pharmaceutical formulations.
Cosmetics: It is used as a pH adjuster and emulsifier in cosmetics.
Industrial applications: It is used in the production of various chemicals, including resins, plasticizers, and corrosion inhibitors.

Ethylenediamine

Ethylenediamine has the chemical formula H₂NCH₂CH₂NH₂. It is a diamine, containing two amine (-NH₂) groups.

Uses

Chelating agent: It is used to form stable complexes with metal ions, making it useful in water treatment, metal cleaning, and catalysis.
Polymers: It is used in the production of polyamides, such as nylon, and epoxy resins.
Pharmaceuticals: It is used as an intermediate in the synthesis of various drugs.
Agriculture: It is used as a precursor for the production of herbicides and fungicides.
Rubber industry: It is used as a vulcanization accelerator.

Amphetamine

Amphetamine is a central nervous system stimulant with the chemical formula C₉H₁₃N. It contains a benzene ring, an amine group, and a methyl group attached to the benzene ring.

Uses

Medically: Amphetamine and its derivatives are used in the treatment of attention deficit hyperactivity disorder (ADHD) and narcolepsy under strict medical supervision.
Abuse: Amphetamine is a controlled substance with high potential for abuse and addiction. It is commonly used illicitly as a recreational drug.

Summary

These organic compounds, containing nitrogen atoms bonded to alkyl groups, exhibit greater basicity than ammonia due to alkyl group electron donation. The more alkyl groups attached to nitrogen, the stronger the base. Notably,

Ethanolamine (MEA): Used in detergents, cosmetics, and gas-scrubbing processes.
Ethylenediamine (EDA): A chelating agent forming stable metal complexes.
Amphetamine: Known for stimulant effects and used in ADHD treatment. Additionally, the Hinsberg test helps identify amines based on sulfonamide formation.