Ionic Bonds – Weak or Strong? (thanks for asking!)

This "discussion" stemmed from a question asked by a student who was taking organic and cell bio at the same time. In cell bio, the student was told that ionic bonds are very weak. But in organic lecture, the student was told that a strong Li-O ionic bond helped drive a particular reaction. So, did one prof lie? No, not really. Read on.

Ionic bonding in a cell is indeed very weak. Here's a statement from Karp's 2nd edition Cell and Molecular Biology text (pp. 34-35):

Ionic bonds within a salt crystal may be quite strong. However, if the crystal of salt is dissolved in water, each of the individual ions becomes surrounded by water molecules, which inhibit oppositely charged ions from approaching one another closely enough to form ionic bonds. Since cells are composed primarily of water, bonds between free ions are of little importance.

So, within an aqueous environment, the interaction between ions of opposite charge is minimal and ionic bonds can be considered weak. To add a little perspective, consider Karp's next statements:

In contrast, weak ionic bonds between oppositely charged groups of large biological molecules are of considerable importance. For example, when negatively charged phosphate atoms in a DNA molecule are closely associated with positively charged groups on the surface of a protein, ionic bonds between them help hold the complex together. ... The strength of ionic bonds in a cell is generally weak (about 3 kcal/mole) due to the presence of water, but deep within the core of a protein, where water is often excluded, such bonds can be influential."

In water, ionic bonds are very weak. (Your cell bio prof didn’t lie to you!) However, in an aprotic solvent or in an anhydrous environment, you have a totally different situation. When using LDA as a base, there is no water around. If you form an enolate anion, the O^- will be closely associated with the Li⁺ and will form a strong ionic bond. Ionic bonding in crystals will be even stronger than in solution ... but that’s a topic for an inorganic chemistry class!