Notes on Hydrogen Bonding

Introduction to Hydrogen Bonds

• Definition: A hydrogen bond is a special type of dipole-dipole interaction.
• Characteristics:
  • Stronger than regular dipole-dipole interactions.
  • Occurs when hydrogen is directly attached to highly electronegative atoms: nitrogen (N), oxygen (O), or fluorine (F).

Comparison of Bonds

• Electronegativity Values:

  • Hydrogen (H): 2.1
  • Fluorine (F): 4.0
  • Carbon (C): 2.5
  • Oxygen (O): 3.5

• Polar Molecules:

  • Carbon monoxide (CO) has dipole-dipole interactions.
  • Hydrogen fluoride (HF) has stronger hydrogen bonds due to its higher polarity.

Reasons for Strength of Hydrogen Bonds

1. High Polarization:
   • HF bond has a larger electronegativity difference (1.9) compared to CO bond (1.0).
2. Small Atom Size:
   • Average atomic radius:
     • Hydrogen: 37 picometers
     • Carbon: 77 picometers
   • Smaller atomic radius leads to stronger forces of attraction.

Electrostatic Forces of Attraction

• Force Relation: Force (F) is inversely related to the square of the distance (r):
  • F ∝ 1/r²
  • Reducing distance increases force significantly.

Distinction Between Bonds

• Intramolecular Force:
  • Covalent bond (within the molecule).
• Intermolecular Force:
  • Hydrogen bond (between molecules).
  • Example: In HF, the hydrogen bond exists between separate HF molecules.

Hydrogen Bonds in Water

• Structure:
  • Oxygen carries a partial negative charge, while hydrogen carries a partial positive charge.
  • Water molecules attract each other through hydrogen bonds.

Breaking Bonds and Energy Requirements

• Endothermic Process:
  • Breaking a bond requires heat input (endothermic).
• Enthalpy of Vaporization:
  • Energy needed to break hydrogen bonds in water: 40.7 kJ/mol.
• Covalent Bond Energy:
  • Average energy required to break an OH bond: 467 kJ/mol.
  • Water has 2 OH bonds, requiring 934 kJ to break all covalent bonds in 1 mole of water.

Strength Comparison

• Covalent Bond vs Hydrogen Bond:
  • Covalent bonds (OH) are approximately 23 times stronger than hydrogen bonds in water molecules, as 934 kJ/40.7 kJ ≈ 23.

Summary

• Intermolecular forces (like hydrogen bonds) are generally weaker than intramolecular forces (like covalent bonds).
• Understanding these principles is crucial in both chemistry and biology.