Attack of the Killer Water Molecule (AKA hydrolysis)

Water fills our oceans, lakes, rivers and streams. It comes out of our taps. We swim in the stuff, and we drink it. For these reasons we tend to think of water as being inert. It isn’t. Water is an aggressively reactive molecule. Consider what happens when you stir salt into water: the water destroys the salt crystals. But why is water so ferocious? Read on to find out.

The Nature of Water 

In a molecule of water (H2O), two hydrogen atoms are attached to an oxygen atom. Oxygen has a much greater pull on electrons, or electronegativity, than hydrogen. In part this is because the oxygen atom has eight positively charged protons in its nucleus, and hydrogen only has one. Oxygen’s highly charged nucleus tugs electrons away from hydrogen, resulting in a partial negative charge (δ-) on the oxygen and a partial positive charge (δ+) on each hydrogen.

Figure 1 shows two views of a water molecule. Notice that the oxygen atom is surrounded by four pairs of electrons: two bonding pairs and two nonbonding, or lone pairs (Figure 1, left). The four electron pairs, which have a negative charge, repel each other and exist as far apart as possible around the spherical oxygen atom. In effect, to two bonding pairs are pushed to one side by the lone pairs, giving water a bent, or V shape (Figure 1, right).

Figure 1. Water has a bent shape

A partial negative charge exists at the point of the V and a partial positive charge exists at each tip. As a result, each water molecule behaves like a tiny magnet and is attracted to other charged particles. In some cases this attraction is so strong that water is able to break the bonds that hold other particles together. A reaction in which water attacks another compound is called hydrolysis. Hydro means “water” and lysis means “to unbind”, so hydrolysis literally means “to unbind with water”.

Hydrolysis Reactions

Hydrolysis of salt Consider a crystal of table salt (NaCl). The crystal contains positively charged sodium (Na+) ions and negatively charged chlorine (Cl-) ions. These charged particles interact with each other, holding the crystal together. Water, which is attracted to the charged particles, can disrupt this interaction and cause the salt crystal to dissolve (Figure 2).

Figure 2. Salt dissolving in water

Our oceans are salty because water attacks salt, but salt isn’t the only compound that water attacks. Water can also attack certain molecules.

Hydrolysis of molecules We have already established that oxygen is an electron bully. But oxygen doesn’t just tug electrons away from hydrogen. It tugs electrons away from other atoms as well, including carbon. A bond between oxygen and carbon possesses a dipole moment that can attract molecules of water. In some cases water attacks the molecule, causing it to split apart.

If you’ve eaten recently, hydrolysis is occurring right now, in your stomach and intestines. Digestion of fats, carbohydrates, and proteins is a hydrolysis reaction. Figure 3 shows hydrolysis of the ester bond in a fat.

Figure 3. Hydrolysis of the ester bond in a fat.

Of course, our bodies don’t just break down fats, carbohydrates, and proteins; they build new ones, too. To build new molecules water must be added in a type of reaction called dehydration. Dehydration is the reverse of hydrolysis.

So it turns out that water is a very good solvent for life, not because it is inert, but because it is highly reactive. Our very clever cells have learned to harness this reactivity so that they can build or break molecules on an as-needed basis.