CHAPTER 2 (1)

• Octet rule


  There are 4 electron pairs around an Oxygen atom in water
• Structure of water


  1. Electrons are in 4 sp3 orbitals

  2. 2 pairs covalently link 2 hydrogen atoms to a central Oxygen atom

  3. 2 remaining pairs are nonbonding pairs (lone pairs)

  4. Water geometry is a distorted tetrahedron

  5. The electronegativity of the Oxygen atom induces a net dipole moment. Because of the dipole moment, water can serve as both a Hydrogen donor and acceptor
• Hydrogen bonds


  • Strong dipole-dipole or charge-dipole interactions that arise between covalently bound Hydrogen and lone pair of electrons
  • Typically involve Nitrogen or Oxygen
  • Strongest when the bonded molecules allow for linear bonding patterns
• Hydrogen bonding in water

  • Up to 4 H-bonds per water molecule gives water its high boiling point, high melting point, and large surface tension
  • Hydrogen bonding in water is co-operative
  • Hydrogen bonds between neighbouring molecules are weak (20kJ/mol) relative to the H-O covalent bonds (420kJ/mol) IMPORTANCE OF HYDROGEN BONDS ‐ Source of unique properties of water ‐ Structure and function of proteins ‐ Structure and function of DNA ‐ Structure and function of polysaccharides ‐ Binding of substrates to enzymes ‐ Binding of hormones to receptors ‐ Matching of mRNA and tRNA
• Ice


  • Water has many different crystal forms - the hexagonal ice is the most common
  • Hexagonal ice forms an organised lattice and thus has a low entropy
  • Hexagonal ice contains maximal hydrogen bonds/ water molecules, forcing the water molecules into equidistant arrangement
  • Ice has lower density than liquid water
  • Ice floats
• Water as a solvent


  • Good solvent for charged and polar substances (amino acids, peptides, small alcohols, carbohydrates)
  • Poor solvent for nonpolar substances (nonpolar gases, aromatic moieties, aliphatic chains)
• Noncovalent interactions


  • Do not involve sharing a pair of electrons
  • Include ionic (Coulombic) interactions, dipole interactions, van der Waals interactions, and hydrophobic effect
• Ionic (Coulombic) interactions are electrostatic interactions between permanently charged species, or between the ion and a permanent dipole
• Dipole interactions are electrostatic interactions between uncharged but polar molecules
• Van der Waals interactions are weak interactions between all atoms, regardless of polarity, with attractive (dispersion) and repulsive (steric) components
• Hydrophobic effect is a complex phenomenon associated with the ordering of water molecules around nonpolar substances