Benzoic acid is highly soluble in ethanol, especially compared to its low solubility in water. A significant amount of benzoic acid can be mixed into ethanol to form a homogeneous solution. This high solubility is a direct consequence of the chemical structures of both compounds and the forces that govern their interaction.
Chemical Nature of Benzoic Acid and Ethanol
Ethanol is a simple alcohol that functions as a polar organic solvent. Its structure includes a small, nonpolar ethyl group and a highly polar hydroxyl group. This hydroxyl group allows ethanol to readily participate in strong intermolecular forces, particularly hydrogen bonding.
Benzoic acid is an aromatic carboxylic acid classified as a polar organic compound. The molecule has two distinct parts: a nonpolar benzene ring and a polar carboxyl functional group attached to it. The overall polarity of benzoic acid stems from the carboxyl group, which contains atoms with significantly different electronegativities.
Understanding the Intermolecular Attraction
The principle governing solubility is “like dissolves like,” meaning substances with similar intermolecular forces dissolve in each other. This is why benzoic acid is highly soluble in ethanol: both compounds engage in strong interactions known as hydrogen bonds.
Ethanol molecules, via their hydroxyl group, are excellent hydrogen bond donors and acceptors. Benzoic acid’s carboxyl group also allows it to form strong hydrogen bonds with ethanol molecules. When benzoic acid dissolves, ethanol surrounds the solute, forming new, energetically favorable hydrogen bonds between the solvent and the solute.
Although benzoic acid contains a large nonpolar benzene ring, the polar carboxyl group’s strong hydrogen-bonding capability dominates its interaction with ethanol. The strong attractive forces formed between the polar groups overcome the energy required to separate the molecules. This allows the compounds to mix completely.
Utilizing Ethanol Solubility in Purification
The significant solubility of benzoic acid in ethanol is particularly useful in purification methods, most notably recrystallization. This technique purifies solid compounds by relying on the difference in a substance’s solubility at various temperatures.
Benzoic acid is much more soluble in hot ethanol than in cold ethanol. The impure solid is dissolved in a minimum amount of hot ethanol to create a saturated solution. As this solution cools slowly, the solubility of the benzoic acid decreases sharply, causing the pure compound to crystallize out. Impurities either remain dissolved in the cold ethanol or are filtered out while the solution is hot.