Why is caffeine only soluble in water that is strongly acidic?

Is that true? I think the solubility of caffeine is about 180 mg/L in water @ 100 Celsius. I guess it depends upon your definition of solubility. At any rate, look at the structure of caffeine. There is a N in one of the heterocyclic rings that has a pair of electrons that are unused. That makes caffeine a base. The pKa for caffeine is about 10.5 or so and Kb = about 1 x 10-4--not exactly a strong base but stronger than NH3 at 10^-5.

Caffeine is a molecule that contains a nitrogen atom with a pair of electrons that are not involved in any bonds. This makes caffeine a weak base. When caffeine comes into contact with water, it can act as a base and accept a proton (H+) from the water molecules. This forms a positively charged caffeine ion, known as a cation. This process is called protonation.

In general, the solubility of a compound in a given solvent is determined by the balance between the attractive forces between the solute and solvent molecules and the attractive forces within the solute and solvent themselves. The solute-solvent interactions are influenced by factors such as polarity, size, and charge.

Since caffeine is a weak base, it can form stronger interactions with water molecules that are strongly acidic. In strongly acidic water, the concentration of H+ ions is high, and these ions can readily protonate the caffeine molecules. The resulting caffeine cations can then form stronger interactions with the water molecules, enhancing the solubility of caffeine in acidic water.

It is worth noting that the solubility of caffeine is also influenced by other factors such as temperature, pressure, and the presence of other solutes. The solubility you mentioned, 180 mg/L at 100 degrees Celsius, is specific to that condition. At lower temperatures, the solubility of caffeine in water decreases. Also, the actual solubility values can vary depending on the measurements and experimental conditions.

In conclusion, while caffeine can dissolve in water regardless of its acidity, it has higher solubility in strongly acidic water due to its ability to form stronger interactions with water molecules in an acidic environment.