Aluminum chloride is an important chemical compound used extensively in industrial processes and chemical synthesis. Its properties make it a versatile material, ranging from a powerful catalyst in organic chemistry to a component in everyday consumer products. Understanding its chemical structure is key to appreciating its significance.
Determining the Chemical Formula
The chemical formula for aluminum chloride is derived from the charges of its constituent elements. Aluminum (Al) is a metal that typically forms a cation with a positive three charge, Al³⁺, as it readily gives up its three valence electrons. Chlorine (Cl) is a nonmetal that forms an anion with a negative one charge, Cl⁻, by accepting one electron.
To achieve a neutral compound, one Al³⁺ ion must combine with three Cl⁻ ions. This requirement for charge neutrality establishes the chemical formula as AlCl₃. The compound is formally named aluminum chloride, but it is also commonly referred to as aluminum trichloride.
Physical Characteristics and Common Forms
In its pure, anhydrous form, aluminum chloride exists as a white or colorless crystalline solid. Commercial samples frequently appear yellowish due to contamination with iron(III) chloride. The anhydrous solid tends to sublime, transitioning directly from a solid to a gas, at about 180°C under atmospheric pressure.
The anhydrous form of AlCl₃ is extremely hygroscopic, meaning it readily absorbs moisture from the air and often fumes in humid conditions. When it reacts with water, it forms the stable hexahydrate, AlCl₃ · 6H₂O. This hydrated form is a colorless or white solid that decomposes at approximately 100°C rather than subliming. The hexahydrate is the form commonly found in consumer products.
Unique Bonding Behavior
Aluminum chloride displays a structural complexity that changes based on its physical state. In the solid state, it exists as an ionic lattice where each aluminum ion is surrounded by six chloride ions in an octahedral arrangement. However, when heated or dissolved in non-polar solvents, the compound shifts to a more covalent structure.
In the gas phase and when molten, AlCl₃ forms a dimer molecule with the formula Al₂Cl₆. This dimer consists of two AlCl₃ units joined by two bridging chlorine atoms. This arrangement allows both aluminum atoms to achieve a more stable electron configuration, driven by the aluminum atom’s inherent electron deficiency. This deficiency makes the compound a potent Lewis acid, capable of accepting an electron pair, which drives its chemical reactivity.
Practical Applications
The strong Lewis acid character of aluminum chloride makes it a catalyst in organic chemistry, particularly for Friedel-Crafts reactions. These reactions attach alkyl or acyl groups to aromatic compounds, a foundational process in synthesizing petrochemicals, pharmaceuticals, and dyes. The anhydrous form is preferred for these industrial catalytic applications.
Aluminum chloride is also used in the production of pure aluminum metal as an intermediate in the electrolytic process. In consumer goods, the hexahydrate form is widely used in antiperspirants. It functions by reacting with moisture on the skin to form a temporary gel plug that physically blocks the sweat ducts, preventing the release of sweat.