6.4 Hydrogen bonding
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What is a hydrogen bond
A special type of permanent dipole-dipole interaction found between molecules containing an electronegative atom with a lone pair of electrons, and a hydrogen atom attached to an electronegative atom
Where does a hydrogen bond act
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 _____ type of intermolecular attractions
Strongest
What is the hydrogen bond shown by
A dashed line
Hydrogen bonds and water
They hold water molecules apart in an open lattice structure The water molecules in ice are further apart than in water Solid ice is less dense than liquid water and floats
Why can ice float on water
The two lone pairs on the oxygen atom and two hydrogen atoms can form four hydrogen bonds. These extend outwards holding water molecules slightly apart and forming an open tetrahedral lattice full of holes. The holes decrease the density of water on freezing. When ice melts, the lattice collapses and the molecules move closer together
Polarity of hydrogen
𝛿+
Bonds in water
Hydrogen bonds are extra forces, over and above the London forces An appreciable quantity of energy is required to break the hydrogen bonds so water has a much higher melting and boiling point than would be expected from just London forces When the ice lattice breaks, the rigid arrangement of the hydrogen bonds in ice is broken, when it boils, the hydrogen bonds break completely
What would the boiling point of water be without hydrogen bonds
-75°C
What properties of water do hydrogen bonds contribute to
Relatively high surface tension and viscosity Droplets that are ‘not wet’ and allow insects to walk on pond surfaces Melting and boiling points
Hydrogen bonds in DNA
Hold together the double helix structure and enable a single DNA to create a perfect copy of itself in replication
What does the replication process of DNA depend on
Four bases being in the correct order A - Adenine T - Thymine C - Cytosine G - Guanine
How does A and T in DNA pair
By forming two hydrogen bonds
How does C and G in DNA pair
By forming three hydrogen bonds
How do the bases in DNA pair
By forming hydrogen bonds A and T C and G
How do the 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 in the double helix can only take place between a purine and a pyrimidine base They must fit together so a hydrogen atom and an electronegative atom are aligned correctly