20.4 Amines and Amides

Amines and amides are crucial organic compounds with nitrogen atoms. Amines have a lone pair of electrons, making them basic and nucleophilic. They're classified as primary, secondary, or tertiary based on carbon-nitrogen bonds. Their properties depend on hydrogen bonding ability.

Amides contain a carbonyl group bonded to nitrogen. They're formed by condensing carboxylic acids and amines. Amides have planar structures due to resonance, lower basicity than amines, and higher melting points. Both groups are vital in biological molecules and pharmaceuticals.

Amines

Structure and properties of amines

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• Amines are organic compounds containing a nitrogen atom bonded to carbon atoms
  • Classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbon atoms bonded to the nitrogen (methyl amine, diethyl amine, triethyl amine)
• Nitrogen atom in amines has a lone pair of electrons, making amines basic compounds
  • Can accept a proton ($H^+$) to form an ammonium ion ($R-NH_3^+$)
  • Act as weak bases in aqueous solutions, with basicity increasing from 1° < 2° < 3° (pKa values: 1° ~10, 2° ~11, 3° ~10.5)
• Physical properties of amines are influenced by their ability to form hydrogen bonds
  • Lower molecular weight amines are soluble in water due to hydrogen bonding with water molecules (ethyl amine, propyl amine)
  • Higher molecular weight amines are less soluble in water and have higher boiling points (octyl amine, decyl amine)
• Amines exhibit high nucleophilicity due to the lone pair of electrons on the nitrogen atom

Amides

Formation and characteristics of amides

• Amides are organic compounds containing a carbonyl group (C=O) bonded to a nitrogen atom
  • Formed by the condensation reaction between a carboxylic acid and an amine, releasing water (acetic acid + methylamine → acetamide + water)
  • General formula: R-CO-NH-R' or R-CO-NR'R"
• Amides have a planar structure due to the resonance of the C-N bond
  • Partial double bond character of the C-N bond restricts rotation, leading to higher stability
• Amides exhibit lower basicity compared to amines due to the electron-withdrawing effect of the carbonyl group (pKa ~15 for amides vs ~10 for amines)
• Participate in hydrogen bonding, resulting in higher melting and boiling points compared to similar-sized hydrocarbons (acetamide mp 81°C, propanamide mp 79°C)

Amines vs amides: comparison

• Functional groups:
  • Amines contain a nitrogen atom bonded to carbon atoms ($R-NH_2$, $R-NH-R'$, $R-NR'R"$)
  • Amides contain a carbonyl group bonded to a nitrogen atom ($R-CO-NH_2$, $R-CO-NH-R'$, $R-CO-NR'R"$)
• Reactivity:
  • Amines are basic and can act as nucleophiles in reactions, such as in the formation of amides (nucleophilic acyl substitution)
  • Amides are less reactive compared to amines due to the resonance stabilization of the C-N bond
    • Amides can undergo hydrolysis under acidic or basic conditions to form carboxylic acids and amines (acetamide + $H_2O$ + $H^+$ → acetic acid + ammonia)
• Biological significance:
  • Amines are found in many biologically important molecules, such as neurotransmitters (serotonin, dopamine) and amino acids (glycine, lysine)
  • Amides are present in the peptide bonds that link amino acids in proteins
    • Peptide bonds provide stability to protein structures due to their partial double bond character
  • Many pharmaceuticals and natural products contain amine or amide functional groups, contributing to their biological activity and therapeutic properties (lidocaine, penicillin)