Amines

Definition and Classification

Definition

Amines: organic compounds containing nitrogen atom bonded to alkyl or aryl groups. Functional group: –NH2, –NHR, –NR2. Derived from ammonia (NH3) by substitution of one or more hydrogens by organic substituents.

Classification

Based on number of organic substituents on nitrogen:

• Primary amines (1°): one alkyl/aryl group + two hydrogens (R–NH2)
• Secondary amines (2°): two alkyl/aryl groups + one hydrogen (R2–NH)
• Tertiary amines (3°): three alkyl/aryl groups (R3–N)
• Quaternary ammonium salts: four organic groups with positive charge (R4N+)

Subtypes

Aliphatic amines: nitrogen bonded to saturated carbons. Aromatic amines: nitrogen bonded to aromatic rings.

Structure and Physical Properties

Molecular Geometry

Tetrahedral electron geometry around nitrogen. Molecular shape: trigonal pyramidal. Lone pair on nitrogen causes bond angles ~107° (less than 109.5°).

Hydrogen Bonding

Amines exhibit hydrogen bonding (primary and secondary), increasing boiling points compared to alkanes of similar mass.

Solubility

Lower molecular weight amines are soluble in water due to hydrogen bonding with solvent. Solubility decreases with increasing alkyl chain length.

Physical State

Low molecular weight amines: gases or liquids. Higher amines: waxy solids. Characteristic fishy odor attributed to volatile amines.

Polarity

Moderate polarity due to nitrogen electronegativity and lone pair; affects solubility and reactivity.

Nomenclature

IUPAC Naming

Suffix –amine added to parent alkane name. Numbering priority to amine group. Example: ethylamine, propan-2-amine.

Common Names

Simple amines named after corresponding alkyl groups followed by "amine." Example: methylamine, dimethylamine.

Substituent Naming

When amine is substituent: prefix "amino–" used. Example: 2-aminoethanol.

Quaternary Ammonium Compounds

Named as substituted ammonium salts. Example: tetramethylammonium chloride.

Basicity and Electronic Effects

Basicity Overview

Amines: Lewis bases due to lone pair on nitrogen. Proton acceptors forming ammonium ions.

Factors Affecting Basicity

Alkyl groups: electron-donating, increase basicity. Aromatic rings: electron-withdrawing, decrease basicity. Steric hindrance reduces accessibility of lone pair.

pKa Values

Typical amines pKa (conjugate acid): 9–11. Aniline lower (~4.6) due to resonance delocalization.

Comparative Basicity Table

Resonance Effects

Aromatic amines: lone pair delocalized into ring, reducing availability for protonation and thus basicity.

Synthesis Methods

Reductive Amination

Carbonyl compounds + ammonia or amines + reducing agent (NaBH3CN, H2/Pd). Forms primary, secondary, tertiary amines selectively.

Alkylation of Ammonia

Alkyl halides + ammonia yield mixture of amines by nucleophilic substitution. Overalkylation common problem.

Gabriel Synthesis

Phthalimide + alkyl halide → alkyl phthalimide; hydrolysis yields primary amine. Selective primary amine preparation.

Reduction of Nitro Compounds

Nitroaromatics reduced by catalytic hydrogenation or metals in acid to give aromatic amines.

Hofmann Rearrangement

Primary amides converted to primary amines with one fewer carbon atom using bromine and base.

RCONH2 + Br2 + 4OH− → RNH2 + CO32− + 2Br− + 2H2O

Reactions of Amines

Acid-Base Reactions

Protonation of amines by acids forming ammonium salts. Equilibrium depends on basicity.

Alkylation and Acylation

Nucleophilic substitution with alkyl halides yields higher amines. Acylation with acid chlorides or anhydrides forms amides.

Diazotization (Primary Aromatic Amines)

Primary aromatic amines react with nitrous acid to form diazonium salts, intermediates for azo coupling.

Hofmann Elimination

Quaternary ammonium hydroxides undergo elimination to form alkenes.

Oxidation

Secondary amines oxidized to N-oxides, tertiary amines to amine oxides with peracids.

R2NH + HNO2 → R2N–N2+ (diazonium salt)

Aromatic Amines

Structure and Resonance

Amines attached to benzene ring: lone pair delocalized, reducing basicity and nucleophilicity.

Preparation

Reduction of nitrobenzene with Fe/HCl or catalytic hydrogenation yields aniline.

Reactivity

Aromatic amines undergo electrophilic substitution ortho/para directing. Diazotization reactions unique to aromatic primary amines.

Applications

Used in dyes, pharmaceuticals, polymers. Example: aniline in polyurethane production.

Spectroscopic Identification

Infrared Spectroscopy (IR)

N–H stretching vibrations at 3300–3500 cm⁻¹; primary amines show two bands, secondary one band.

Proton NMR (¹H NMR)

NH protons appear as broad signals; chemical shift depends on hydrogen bonding and solvent.

Mass Spectrometry (MS)

Characteristic fragmentation patterns: loss of ammonia (NH3) or alkyl groups.

UV-Vis Spectroscopy

Aromatic amines show absorption due to π→π* transitions influenced by amino substituent.

Industrial and Biological Applications

Pharmaceuticals

Amines: core in many drugs (analgesics, antihistamines, antidepressants). Functionality allows interaction with biological targets.

Agrochemicals

Used in pesticides, herbicides synthesis. Amines act as intermediates and active components.

Polymers and Dyes

Building blocks for polyurethanes, epoxy resins, azo dyes, and pigments.

Biological Amines

Neurotransmitters (dopamine, serotonin), amino acids contain amine groups essential for life processes.

Toxicity and Environmental Impact

Human Toxicity

Some amines are irritants, sensitizers, or carcinogens (aromatic amines). Exposure limits regulated.

Environmental Fate

Biodegradable under aerobic conditions. Some persistent in soil/water causing contamination.

Regulatory Guidelines

Strict controls on aromatic amines due to carcinogenicity. Industrial emissions monitored.

Comparison with Related Functional Groups

Amides

Amides: nitrogen bonded to carbonyl carbon. Less basic than amines due to resonance stabilization.

Imine

Imines: C=N functional group; reactive intermediates in amine chemistry.

Ammonium Ions

Protonated amines; charged species with different solubility and reactivity.

Hydrazines

Two nitrogen atoms connected; stronger reducing agents and different reactivity.

Summary

Amines: versatile nitrogen-containing functional groups. Classification: primary, secondary, tertiary. Properties influenced by alkyl/aryl substitution. Basicity modulated by electronic factors. Synthesized via reductive amination, alkylation, reduction. React via acid-base, substitution, diazotization. Aromatic amines show unique resonance effects. Spectroscopy confirms structure. Applications span pharmaceuticals, polymers, biology. Toxicity varies; regulatory oversight critical.

References

• March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley, 4th Ed., 1992, pp. 252-310.
• Smith, M. B. March’s Advanced Organic Chemistry: Reaction Mechanisms. Wiley, 7th Ed., 2013, pp. 1100-1150.
• Clayden, J., Greeves, N., Warren, S., Wothers, P. Organic Chemistry. Oxford University Press, 2nd Ed., 2012, pp. 650-700.
• Carey, F. A., Sundberg, R. J. Advanced Organic Chemistry Part A: Structure and Mechanisms. Springer, 5th Ed., 2007, pp. 500-540.
• Silverstein, R. M., Webster, F. X., Kiemle, D. J. Spectrometric Identification of Organic Compounds. Wiley, 7th Ed., 2005, pp. 120-150.