Chapter 2: Water- Hydrogen Bonding.

All living cells depend absolutely on water for their existence. In most living cell, water is the most abundant molecule, accounting for 60% to 90% of the mass of the cell. The macromolecule components of cells-proteins, polysaccharides, nucleic acids, and membranes- get their characteristics shapes in response to interactions with water and much of the metabolic processes of cells has to operate in an aqueous environment because water is an essential solvent as well as a substrate for many cellular reactions.

A water molecule (H2O) is V-shaped, with an angle of 104.5° between the two covalent O-H bonds.

A water molecule. Click on image for credit.

An oxygen atom has six electrons in the outer shell, but the outer shell can potentially accommodate four pairs of electrons in four sp3 orbitals. This means that oxygen can form covalent bonds involving two different hydrogen atoms, each sharing a single electron with the oxygen atom.

An oxygen nucleus (because it contains more protons or positive charge) attracts electrons more strongly towards it than the single proton in the hydrogen nucleus. This attraction of electrons defines oxygen atoms as being more electronegative than hydrogen atoms. As a result, an uneven distribution of charge occurs within each O-H bond of the water molecule, with oxygen bearing a partial negative charge and hydrogen bearing a partial positive charge (+). This uneven distribution of charge within a bond is known as a dipole, and the bond is said to be polar.

Dipolarity of water bonds.

The polarity of a molecule depends on both the polarity of its covalent bonds and its geometry. The angled arrangement of the polar O-H bonds of water creates a permanent dipole for the molecule as a whole.

A molecule of ammonia also contains a permanent diploe. Thus, even though water and gaseous ammonia are electrically neutral, both molecules are polar. The high solubility of the polar ammonia molecules in water is facilitated by strong interactions with polar water molecules.

Ammonia in the form of ammonium ions. Click on image for credit.

The reason why there’s an extra proton (H+) with ammonia is because due to the fact that it is highly polar, it will attract a hydrogen atom of a water molecule (which it has dissolved in; aqueous solution) and form a fourth hydrogen bond with it. In water, the attraction between a slightly positive hydrogen atom of one water molecule and the slightly negative oxygen atom of another produces what is referred to as a hydrogen bond.

Water is not the only molecule capable of forming hydrogen bonds; these interactions can occur between any electronegative atom and a hydrogen atom attached to another electronegative atom. In the case of ammonia, the N (Nitrogen atom) is the slightly electronegative atom and the hydrogen atom of a water molecule forms a hydrogen bond with it, converting it to ammonium ions (NH4+).

The distance between this hydrogen atom and the other oxygen atom, is about twice the length of the covalent bond and hydrogen bonds are much weaker than typical covalent bonds. A single water molecule can form hydrogen bonds with up to four other water molecules.

Hydrogen Bonds in water molecules. Click on image for credit.

Orientation is important in hydrogen bonding and the bonding is most stable when a hydrogen atom and the two electronegative atoms associated with it (the two oxygen atoms in the case of water) are aligned or nearly in line.

Linear H-Bonds. Click on image for credit.

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