6.3 Hydrogen Bonding and Ionic Bonding

In the previous section, we saw that water molecules are polar and attract each other like magnets. The attractions between hydrogen and oxygen atoms of different molecules of water are examples of hydrogen bonds. A hydrogen atom of one molecule is the hydrogen bond donor; the oxygen atom of another molecule is the hydrogen bond acceptor.

Hydrogen bonds most often happen when a hydrogen atom is between two oxygen atoms, two nitrogen atoms, or a nitrogen and an oxygen. It's easy to understand the mechanism behind hydrogen bonds. Remember that oxygen and nitrogen are usually tetrahedral, and that the unbonded spaces are occupied by electron pairs. The hydrogen atoms in water, ammonia, alcohols, amines, and so on, are all partially deprived of their electrons by their heavy atoms, due to the difference in electronegativity. This leaves the hydrogen nuclei - protons - partially unveiled and able to attract the lone pairs of other atoms.

Hydrogen bonds typically have strengths of 8 to 30 kJ/mol or 2 to 7 kcal/mol, though a few rare kinds can be much, much stronger. Hydrogen bonds even do an unusual thing where the two heavy atoms partially share the proton between them, causing the hydrogen to spend a small portion of the time bonded to the other molecule's heavy atom.

Ionic bonds are attractive forces between ions. If a carboxylic acid donates its spare proton to an amine, it forms a salt. The resulting ions have opposite electric charges and attract each other strongly, forming ionic bonds with a strength of about 30 kJ/mol or 7 kcal/mol. In biological molecules, an ionic bond is sometimes called a salt bridge.

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