N-butanol, or butan-1-ol (\(\text{C}_{4}\text{H}_{9}\text{OH}\)), is a type of alcohol that is only partially soluble in water. Solubility refers to the maximum amount of a substance, the solute, that can dissolve in a solvent to form a uniform solution. When mixed with water, n-butanol exhibits limited miscibility rather than complete dissolution.
Understanding the n-Butanol Molecule
The structure of the n-butanol molecule directly determines its unique behavior when mixed with water. The molecule consists of two distinctly different functional regions that pull the molecule in opposing directions regarding solubility.
One end features the hydroxyl group (\(\text{-OH}\)), a small, polar “head” that is hydrophilic, meaning it has an affinity for water. Attached to this is the much larger butyl group, a chain of four nonpolar carbon and hydrogen atoms.
This four-carbon chain acts as the molecule’s nonpolar, hydrophobic “tail,” actively resisting interaction with water molecules. This dual nature—having both a water-loving head and a water-resisting tail—sets up an internal conflict that dictates its ultimate solubility limit.
The Chemistry of Water Solubility
Water is a highly polar solvent due to its bent molecular shape and the high electronegativity of the oxygen atom. Electrons are unequally shared, creating a partial negative charge near the oxygen and partial positive charges near the hydrogen atoms.
The polarity of water allows it to form strong intermolecular attractions known as hydrogen bonds with other polar molecules. Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom, like oxygen, is attracted to a neighboring electronegative atom. These strong attractions are the primary reason water is such an effective solvent for polar and ionic compounds.
The rule “like dissolves like” governs solubility: polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. For a substance to dissolve in water, the attraction between the solute and water molecules must be strong enough to overcome the attractions within the solute and the strong hydrogen bonds within the water. Water’s extensive network of hydrogen bonds must be broken to accommodate the solute.
Resolving the Solubility Question
The solubility of n-butanol is a direct consequence of the molecular conflict between its two parts. The small hydroxyl group forms hydrogen bonds with surrounding water molecules, favoring dissolution. If the molecule had a shorter carbon chain, like methanol or ethanol, it would be completely miscible with water.
However, the four-carbon chain is long enough to significantly interfere with this process. This nonpolar, hydrophobic chain disrupts the stabilizing hydrogen bond network of the water molecules. The energetic cost of surrounding this large, nonpolar tail outweighs the energy gained from the hydrogen bonds formed by the polar head.
The practical result is limited miscibility. At room temperature, n-butanol is soluble in water up to approximately 7.8% by weight (7.3 to 7.8 grams per 100 milliliters). Mixing a higher concentration results in the mixture separating into two distinct liquid layers.
One layer is water-rich, containing dissolved n-butanol, while the other is n-butanol-rich, containing dissolved water. This separation demonstrates the C4 chain’s hydrophobic nature dominating the interaction. Complete miscibility, where the liquids mix fully, is only achieved above \(127\text{°C}\), known as the upper consolute temperature.