An Introduction to Aluminum Chloride

Aluminum chloride is a crystalline compound formed by combining chlorine and aluminum atoms. It is usually white in color, but iron trichloride contamination often gives it a characteristic yellow color. The compound is extensively used in metallurgy, chemical and pharmaceutical products and in the petrochemical industry. When exposed even to a mild increase in temperature, it cracks, changing from polymer to molecule.

The Synthesis of Aluminum Chloride

Chemically depicted as AlCl3, the compound is manufactured through an exothermic reaction that takes place at 650 to 700 degree Celsius between aluminum metal and chlorine. Alternatively, hydrogen chloride may also be used in the manufacture of the compound. Given the large-scale use of the compound in various industries, more than 21,000 tons of aluminum chloride are produced in the US alone; this does not include the consumption of the compound by the domestic metallurgical industry.

Hydrous and Anhydrous Forms

The anhydrous crystalline form of the compound is a very powerful Lewis Acid with wide scale applications. However, hydrous aluminum chloride is also used in many manufacturing processes. To prepare the hydrous form of the compound, aluminum oxides are dissolved in hydrochloric acid; the resultant solid, when heated, curiously does not form the anhydrous form of the compound. Instead, the hydrous forms, with six molecules of water in it, which then decomposes into aluminum oxide.

The Three States of Aluminum Chloride

Depending in the temperature and pressure applied to AlCl3, the compound changes states and exists in its solid, liquid, and gaseous forms. In its solid state, the compound has a sheet like structure made of cubic layers packed close together. At 180 degrees the crystals sublime and in the vapor state the Al2Cl6 dimmers disassociate, assuming a trigonal planar molecular structure. The compound is in its liquid form when a pressure of 2.5 atm is applied to the crystals while raising the temperature to 190 degrees.

Reaction as a Lewis Acid

In its anhydrous form, AlCl3 is a one of the most powerful Lewis acids, so it is frequently used in the formation of base adducts even with other compounds such as benzophenone and mesitylene, which are considered to comparatively weak Lewis bases. When exposed to chloride ions, the compound takes on a fourth chloride atom forming tetrachloroaluminate AlCl4. The compound also interacts with the magnesium and calcium hydrates found in tertrahydrofuran

Interaction with Water

AlCl3 reacts very violently with water and has an affinity towards H2O; the molecules combine to form a hexahydrate aqueous solution of AlCl3 depicted by the formula AlCl3+6H2O. This aqueous solution is acidic in nature and is found to conduct electricity. The acidic nature of the solution points to the partial hydrolysis of the Al3 ion. Combining any aluminum salt that has hydrated Al3 ions with any aqueous solution will lead to a similar reaction. An interaction with sodium hydroxide will produce a precipitate of aluminum hydroxide

Storing aluminum hydroxide can be a task, as the compound can also react with atmospheric moisture, generating heat and fumes, which may lead to a fire if the compound is placed near inflammable substances. Even when bought in small quantities, the packaging has to be perfect to prevent sample degradation. During bulk handling, a separate, waterproof area will have to be created, and a dry air system should be already in place for unloading stocks.