Why do alcohols that show little tendency to dissolve in water appear to dissolve readily in 85% phosphoric acid? Would you describe the latter behavior as solubility or a chemical reaction? Explain
To understand why alcohols that have little tendency to dissolve in water can appear to dissolve readily in 85% phosphoric acid, we need to explore the properties of both substances.
Water is a polar molecule, meaning it has a partial positive charge (δ+) on its hydrogen atoms and a partial negative charge (δ-) on its oxygen atom. This polarity allows water to form hydrogen bonds with other polar molecules or ions. Alcohols, on the other hand, can be either polar or nonpolar depending on their molecular structures. Nonpolar alcohols, like alkanols (e.g., hexanol, octanol), have hydrocarbon chains that do not interact favorably with water due to the difference in polarity.
However, when alcohols are mixed with 85% phosphoric acid (H₃PO₄), a unique phenomenon occurs. Phosphoric acid is a strong acid that can effectively donate three protons (H⁺ ions). When it dissociates in water, it releases these H⁺ ions, resulting in a highly acidic solution. The high acidity of phosphoric acid changes the behavior of water and makes it a better solvent for many substances, including alcohols.
The presence of the H⁺ ions in phosphoric acid leads to acid-catalyzed esterification reactions with alcohols. Esterification is a chemical reaction that produces esters, which are compounds formed by combining an alcohol with an acid. In this case, the acid is the phosphoric acid, and the alcohol is the nonpolar alcohol that typically shows little solubility in water.
During the esterification reaction, the H⁺ ions from the phosphoric acid protonate the oxygen atom of the alcohol, making it more electrophilic. This protonation enhances the solubility of the alcohol in the highly acidic solution. The polar phosphoric acid also helps to stabilize the charged intermediate species formed during the reaction.
Therefore, the behavior of nonpolar alcohols appearing to dissolve readily in 85% phosphoric acid is primarily due to the acid-catalyzed esterification reactions between the alcohol and the acid. This behavior can be described as a chemical reaction rather than a simple solubility phenomenon.
To summarize, the high acidity of phosphoric acid enables acid-catalyzed esterification reactions with alcohols, resulting in the apparent dissolution of nonpolar alcohols that have low solubility in water.