Aluminum chloride, also known as aluminum trichloride, is an inorganic compound widely used in chemical and industrial applications. The chemical formula for this substance is \(\text{AlCl}_3\). While \(\text{AlCl}_3\) represents the basic unit, this molecule often exists as a dimer, \(\text{Al}_2\text{Cl}_6\), depending on its physical state.
Understanding the Chemical Structure
The formula \(\text{AlCl}_3\) is determined by the valence states of the constituent atoms. Aluminum (Al) is a metal found in Group 13 of the periodic table, possessing three valence electrons. Aluminum typically loses these three electrons to form a positively charged ion, or cation, with a \(3+\) charge (\(\text{Al}^{3+}\)).
Chlorine (Cl) is a nonmetal from Group 17, the halogens, which needs one electron to complete its outer electron shell. Chlorine accepts a single electron to form a negatively charged ion, or anion, with a \(1-\) charge (\(\text{Cl}^-\)). To achieve electrical neutrality, the \(3+\) charge of the single aluminum ion must be balanced by three \(1-\) charges from three chloride ions, dictating the chemical formula \(\text{AlCl}_3\).
Appearance and Unique Physical Properties
Anhydrous aluminum chloride typically appears as a white or yellowish crystalline solid. The yellowish tint often results from trace amounts of an impurity, iron(III) chloride. This compound exhibits an unusual phase behavior at standard atmospheric pressure.
Instead of melting, aluminum chloride sublimes at a temperature around \(178^\circ\text{C}\), converting directly from a solid to a gas without passing through a liquid phase. In the gas phase, the \(\text{AlCl}_3\) units combine to form a larger, more stable molecule called a dimer. This dimerization occurs because the aluminum atom in \(\text{AlCl}_3\) is electron-deficient, allowing the atoms to complete their electron octets and lending the molecule significant covalent character.
Common Uses and Applications
Aluminum chloride is primarily valued for its function as a Lewis acid catalyst in organic chemistry. A Lewis acid is a substance that can accept an electron pair, and the electron-deficient nature of the aluminum center in \(\text{AlCl}_3\) makes it particularly reactive. This catalytic property is utilized in the Friedel-Crafts reaction, a fundamental process for creating carbon-carbon bonds in the production of petrochemicals, dyes, and pharmaceuticals.
The compound also has a common consumer application, notably in antiperspirant products. It is used to treat hyperhidrosis, or excessive sweating, by reacting with the moisture in sweat to form a gel-like plug. This temporary physical barrier blocks the sweat ducts near the skin’s surface, effectively reducing the amount of visible perspiration. It is an active ingredient in many clinical-strength antiperspirant formulations.