Hydrogen bonding

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

  • an electronegative atom with a lone pair of electrons E.g. oxygen, nitrogen, or fluorine
  • 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 intermolecular forces
• The hydrogen bond is shown by a dashed line
• The shape around the hydrogen atom involved in the hydrogen bond is linear
• Hydrogen bonding gives water some unique and anomalous properties that support the existence of life on earth
• Solid ice is less dense than liquid water

  • hydrogen bonds hold water molecules apart in an open lattice structure
  • the water molecules in ice are further apart than in water
  • solid ice floats on water because it is less dense than the liquid water
• With two lone pairs on the oxygen atom and two hydrogen atoms, each water molecule can form four hydrogen bonds.
• Hydrogen bonds extend outwards, holding water molecules slightly apart and forming an open tetrahedral lattice full of holes. These holes decrease the density of water on freezing. When ice melts, the ice lattice collapses and the molecules move closer together so liquid water is denser than solid ice
• The bond angle about the hydrogen atom involved in the hydrogen bond is close to 180 degrees
• As with all molecules, water has London forces between molecules. The hydrogen bonds are extra forces, over and above the London forces
• An appreciable quantity of energy is needed to break the hydrogen bonds in water, so water has much higher melting and boiling points than would be expected from just London forces
• When the ice lattice breaks, the rigid arrangement of hydrogen bonds in the ice is broken. When water boils, the hydrogen bonds break completely.
• Without hydrogen bonds, water would have a boiling point of about -75 degrees C and exist as a gas at room temperature and pressure. There would be no liquid water in most places on Earth and there would be no life as we know it
• Other examples of anomalous properties of water include a relatively high surface tension and viscosity
• Detergents reduce surface tension making water 'wetter'
• In DNA, A and T pair by forming two hydrogen bonds whilst G and C pair by forming three hydrogen bonds. The chemical structure and shape of these four bases ensure correct pairing:

  • Adenine and Guanine are both purine bases with two ringed structures
  • Thymine and cytosine are both pyrimidine bases with single ringed structures
  • hydrogen bonding 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 (O2 or N2) from the other are aligned correctly to maximise hydrogen bonding