Benzil, an organic compound with the chemical formula \(C_{14}H_{10}O_2\), is a bright yellow crystalline powder often encountered in organic chemistry as a building block. It is systematically known as 1,2-diphenylethane-1,2-dione, classifying it as an \(\alpha\)-diketone. The direct answer to the question of its water solubility is that Benzil is considered poorly soluble or essentially insoluble in water. Experimental data places its solubility at a very low concentration, approximately \(0.2\) to \(0.5\) grams per liter of water at \(20^\circ\text{C}\).
The Fundamental Rule of Solubility
The ability of one substance to dissolve in another is governed by the core chemical principle known as “Like Dissolves Like.” This rule means that a solute dissolves most effectively in a solvent that shares similar intermolecular forces and polarity. Water is a highly polar solvent, meaning its molecules possess a significant charge separation, or dipole moment, allowing it to form strong interactions like hydrogen bonds. Polar substances, such as table salt or sugar, readily dissolve in water because their attractive forces can overcome the forces holding the solute molecules together.
Conversely, non-polar molecules, like oils or hydrocarbons, lack this significant charge separation and primarily interact through weaker London dispersion forces. When a non-polar solute is introduced to a polar solvent like water, the water molecules preferentially interact with each other. The strong attraction between water molecules excludes the non-polar solute, which cannot form competitive intermolecular bonds, causing it to remain undissolved. For dissolution to occur, the energy required to separate the solute and solvent molecules must be compensated by the energy released when the new solute-solvent interactions form.
Analyzing Benzil’s Molecular Polarity
Applying the solubility rule to Benzil reveals why it is nearly insoluble in water. The Benzil molecule is structurally composed of two distinct parts: two large, non-polar phenyl rings and two polar carbonyl (\(C=O\)) groups. The two carbonyl groups introduce some polarity to the molecule, resulting in a measurable net dipole moment of approximately \(3.8 \text{ Debye}\). These polar regions can interact weakly with water molecules through dipole-dipole forces.
The size and nature of the two phenyl rings significantly outweigh the influence of the two polar carbonyl centers. Each phenyl group is a bulky, non-polar hydrocarbon structure that is strongly hydrophobic, meaning it repels water. The majority of Benzil’s surface area is non-polar, contributing to its overall non-polar character. Furthermore, Benzil lacks the necessary hydrogen bond donor groups, such as an \(O-H\) or \(N-H\) bond, required to form the strong hydrogen bonds needed to break up water’s extensive network of attractions. The dominance of the non-polar hydrocarbon regions prevents the molecule from being effectively surrounded and solvated by polar water molecules, leading to its low solubility.
Practical Solubility in Organic Solvents
Since Benzil is predominantly non-polar, it readily dissolves in organic solvents that share this characteristic, illustrating the “Like Dissolves Like” principle. It exhibits good solubility in less polar organic compounds such as diethyl ether, benzene, and chloroform. These solvents possess intermolecular forces compatible with the London dispersion forces and dipole interactions present in the Benzil molecule.
Benzil also dissolves well in moderately polar organic solvents, including acetone and ethanol. Ethanol, for example, contains a large non-polar ethyl group in addition to a polar hydroxyl group, accommodating Benzil’s mixed structure. This high solubility is often exploited in laboratory settings, such as using ethanol for the recrystallization process to purify the yellow solid. The compatibility with these organic mediums confirms Benzil’s overall non-polar nature despite the presence of its small polar functional groups.