Hydrogen Bonding

• A hydrogen bond is a special type of permanent dipole-dipole interaction found between molecules containing:

  1. an electronegative atom with a lone pair of electrons, e.g. oxygen, nitrogen, or fluorine
  2. a hydrogen atom attached to an electronegative atom. e.g. H-O, H-N or H-F
• The hydrogen bond acts between a lone pair of electrons on an electronegative atom in one molecule and a hydrogen atom in a different molecule.
• Hydrogen bonds are the strongest type of IMF
• The hydrogen bond is shown by a dashed line.
• The lone pair of electrons on the oxygen in water, and the nitrogen in ammonia play a key role in a hydrogen bond
• The shape around the H atom involved in the hydrogen bond is linear
• Anomalous properties of water: H bonding has a significant influence on the properties of water and many other molecules. H bonding gives water some unique and unusual properties
• Ice is less dense than water:

  1. hydrogen bonds hold water molecules apart in an open lattice structure
  2. the water molecules in ice are further apart than in water
  3. solid ice is less dense than liquid water and floats
• Ice floats instead of sinking in ponds and lakes - forms an insulating layer and prevents water from freezing
• There are 2 lone pairs on the oxygen atom and 2 hydrogen atoms, meaning each water molecule can form 4 hydrogen bonds. The hydrogen bonds extend outwards, holding water molecules tightly apart and forming an open tetrahedral lattice full of holes.
• The bond angle about the hydrogen atom involved in the hydrogen bond in water is close to 180 degrees
• The holes in the open lattice structure decrease the density of water on freezing. When ice melts, the ice lattice collapses and the molecules move closer together - liquid water denser than solid ice
• Water has London forces between molecules
• Water has a relatively high melting and boiling point:

  1. H bonds are extra forces, over and above the London forces
  2. an appreciable quantity of energy is needed to break the H bonds in water, so it has a much higher MP and BP than would be expected from just London forces
  3. when the ice lattice breaks, the rigid arrangement of H bonds in ice is broken
  4. when water boils, the H bons break completely
• Without H bonds, water's boiling point would be -75 degrees, gas at room tempt and pressure - no liquid water in most places - no life as we know it
• The extra IMF in water also contributes to more unusual properties:

  1. relatively high surface tension and viscosity - water droplets are not wet and allows insects to walk on pond surfaces
  2. detergents reduce surface tension - makes water wetter
• Double helix structure in DNA held together by H bonds, enables a single strand of DNA to create a perfect copy of itself - replication
• A-T pair by forming 2 H bonds
• C-G pair by forming 3 H bonds
• The chemical structure and shape of these 4 bases ensure correct pairing - A and G both purine bases with 2-ringed structures
• T and C are both pyrimidine bases with single-ringed structures
• Hydrogen bonding in the double helix can only take place between a purine and pyrimidine base. The bases must fit together so that a hydrogen atom from one molecule and an electronegative atom from the other molecule are aligned correctly to maximise hydrogen bonding