Chapter 19: Amines

Introduction to Amines

Amines are organic derivatives of ammonia (NH3) in which one or more hydrogen atoms are replaced by alkyl or aryl groups. They are a fundamental class of organic compounds with significant roles in biology, medicine, and industry.

• General Structure: Amines contain a nitrogen atom bonded to one or more carbon-containing groups (alkyl or aryl).

• Classification: Based on the number of organic substituents on nitrogen:

  • Primary (1°) amine: One alkyl/aryl group (RNH2)

  • Secondary (2°) amine: Two alkyl/aryl groups (R2NH)

  • Tertiary (3°) amine: Three alkyl/aryl groups (R3N)

  • Quaternary ammonium salt: Four alkyl/aryl groups (R4N+ X-)

Nitrogen Hybridization: The nitrogen atom in amines is sp3 hybridized, with a lone pair of electrons. The bond angle is slightly less than the tetrahedral angle (109.5°) due to the lone pair's repulsion.

Physical Properties of Amines

Amines exhibit unique physical properties due to their structure and the presence of a lone pair on nitrogen.

• Polarity: Amines are polar molecules. The N–H bond is less polar than the O–H bond, resulting in weaker hydrogen bonding compared to alcohols.

• Hydrogen Bonding: Primary and secondary amines can both donate and accept hydrogen bonds, while tertiary amines can only accept hydrogen bonds.

• Boiling Points: For compounds of similar molecular weight, the trend is: Alcohols > 1°/2° Amines > Ethers > 3° Amines. Tertiary amines have lower boiling points due to the absence of N–H bonds.

• Solubility: Amines are generally soluble in water due to hydrogen bonding. Solubility decreases with increasing alkyl chain length. Branching increases solubility.

Basicity of Amines

Amines act as bases because the lone pair on nitrogen can accept a proton (H+). The basicity depends on the availability of this lone pair, which is influenced by electronic and steric effects.

• Order of Basicity: Alkylamines (most basic, pKa ≈ 10–11) > Ammonia (pKa ≈ 9.25) > Arylamines (least basic, pKa ≈ 3–5)

• Effect of Substituents: Electron-donating groups (EDGs) increase basicity; electron-withdrawing groups (EWGs) decrease basicity.

• Resonance: In arylamines, the lone pair is delocalized into the aromatic ring, reducing basicity.

Nomenclature of Amines

Common Names

• Formed by naming the alkyl groups attached to nitrogen, followed by "-amine" (e.g., ethylamine, diphenylamine).

• Prefixes di-, tri-, tetra- are used for multiple identical groups (e.g., trimethylamine).

IUPAC Nomenclature

• Identify the longest carbon chain containing the -NH2 group.

• Replace the "-e" ending of the parent alkane with "-amine" (e.g., hexanamine).

• Number the chain to give the amine the lowest possible number.

• For secondary and tertiary amines, use "N-" to indicate substituents on nitrogen.

• When an amine is a substituent, use the "amino-" prefix.

Spectroscopy of Amines

Infrared (IR) Spectroscopy

• Primary and secondary amines show N–H stretching between 3200–3500 cm-1.

• Broad absorption, sometimes with multiple spikes, can be confused with O–H stretching.

Mass Spectrometry

• Molecules with an odd number of nitrogen atoms have an odd molecular mass.

• Fragment ions often have even mass numbers.

NMR Spectroscopy

• Protons on α-carbons (adjacent to nitrogen) appear in the range δ 1–4.

• Amine protons are exchangeable and may disappear or shift in D2O.

Common Uses of Amines

• Pharmaceuticals: Antihistamines, antidepressants, local anesthetics (e.g., lidocaine), analgesics, vasodilators, antibacterial agents.

• Agricultural Chemicals: Herbicides, pesticides, fungicides, and insecticides.

• Dyes and Pigments: Many dyes contain amine groups (e.g., methylene blue, Congo red).

Synthesis of Amines

1. Alkylation with Alkyl Halides (SN2 Mechanism)

• Amines and ammonia act as nucleophiles in SN2 reactions with alkyl halides, forming higher-order amines and quaternary ammonium salts.

• Multiple alkylations can occur, leading to mixtures of products.

2. Gabriel Synthesis

• Used to prepare primary amines from alkyl halides via phthalimide alkylation.

• The phthalimide anion is a strong nucleophile, allowing selective formation of primary amines.

3. Reduction of Nitro and Nitrile Compounds

• Nitro compounds and nitriles can be reduced to amines using hydrogenation (H2/Pd, H2/Ni), metal hydrides (LiAlH4), or metals in acid (Fe/H+, Sn/H+).

4. Rearrangements: Hofmann and Curtius

• Both rearrangements convert amides or acyl azides to primary amines, with loss of CO2 (and N2 in Curtius).

Reactions of Amines

1. Reactions with Acids

• Amines react with acids to form ammonium salts.

• The equilibrium constant (Kb) and pKb values indicate basicity.

2. Acylation

• Primary and secondary amines react with acid chlorides to form amides (nucleophilic acyl substitution).

3. Sulfonylation

• Primary and secondary amines react with sulfonyl chlorides to form sulfonamides, important in pharmaceuticals (e.g., sulfa drugs).

4. Imine (Schiff Base) Formation

• Primary amines react with aldehydes or ketones to form imines (Schiff bases) via nucleophilic addition and dehydration.

5. Hofmann Elimination

• Quaternary ammonium salts undergo elimination (E2) to form alkenes, favoring the less substituted (Hofmann) product due to steric effects.

Summary Table: Synthesis and Reactions of Amines

Additional info: This summary covers the structure, nomenclature, physical and chemical properties, synthesis, and reactions of amines, as well as their spectroscopic identification and common uses in pharmaceuticals, agriculture, and industry.