16 In-Depth Notes on Amides and Peptide Bonds

Learning Outcomes

  • Hydrogen Bonding in Amides

    • Understand the effects of hydrogen bonding on physical properties of amides.

  • Properties of Amide/Peptide Bond

    • Geometry around the amide bond: trigonal planar.
    • Length of C-N peptide bond is shorter than other C-N bonds due to resonance.
    • Non-basicity of nitrogen in amides.
    • Reduced electrophilicity of the carbonyl carbon.

  • Amide (Peptide) Bond Formation

    • Describe the formation process of amide bonds.

Introduction to Amides

  • Amides are derivatives of carboxylic acids.
  • General structure: R1CONR2R3 (where R = hydrogen or alkyl/aryl groups).
    • Primary Amide: R2 and R3 = H
    • Secondary Amide: R3 = alkyl or aryl
    • Tertiary Amide: R2 and R3 = alkyl or aryl

Classes of Carboxylic Acid Derivatives

  • Types include:
    • Carboxylic Acids
    • Acid Anhydrides
    • Amides
    • Esters
    • Thioesters
    • Acid Halides (X = F, Cl, Br, I)

Physical Properties of Amides

  • Boiling Points:

    • Primary and secondary amides exhibit higher melting points (mp) and boiling points (bp) due to hydrogen bonding.
    • Ethanol: $bp = 78 °C$
    • Ethanolamide (C2H5CONH2): $bp = 204 °C$
    • Tertiary amides (like N,N-Dimethylethanamide) are not H-bond donors with $bp = 164 °C$.

  • Hydrogen Bonding:

    • Primary and secondary amides serve as hydrogen bond donors and acceptors.
    • Tertiary amides lack one hydrogen bond donor.

Properties of Amides

  • Least reactive among carboxylic acid derivatives.
  • Geometry of the amide bond is determined as trigonal planar around carbon (C) and nitrogen (N) due to resonance.
  • Bond Characteristics:
    • C-N bond: Shorter than usual due to resonance interaction.
    • Electrophilicity: Reduced due to resonance; nitrogen decreases the positive character of the carbonyl carbon through electron donation.

Amide/Peptide Bond Formation

  • Significance:
    • Essential in the construction of proteins, whereby amino acids are linked via peptide bonds.
  • Chemical Reaction:
    • Peptide bond is formed from an amino group and a carboxylic acid group during protein synthesis.
    • Shorter than a single bond and longer than a double bond in character.
  • Properties:
    • The amide bond is largely stable in aqueous environments, limiting reactivity with water and contributing to protein stability.

Resonance and Stability

  • Resonance Stabilization:
    • Partial double bond character (approximately 40%) contributes to stability in biological settings.
    • Restricted rotation allows for defined 3-D structures in proteins necessary for proper biological activity.

Synthesis of Amides

  • Reaction Types: Amides can be formed from:
    • Acid Chlorides with ammonia or amines.
    • Acid Anhydrides with ammonia or amines, yielding carboxylic acids as byproducts.
    • Esters are reacting with ammonia or amines, producing alcohols as byproducts.

General Summary**

  • The properties and characteristics of amides play a crucial role in the understanding of biological macromolecules like proteins, affecting their structure and function due to unique bonding and stability in aqueous environments.