What Is Aluminum Chloride?
Aluminum chloride, with the chemical formula AlCl3, is either a white or light-yellow solid known for its strong deliquescence. It has a water solubility of 45.8 g/100 ml at 20°C and comes in anhydrous and hexahydrate forms. The anhydrous form can be produced by reacting aluminum with chlorine or hydrogen chloride gas, whereas polyaluminum chloride is synthesized from aluminum hydroxide and hydrochloric acid.
Uses of Aluminum Chloride
Aluminum chloride finds its primary applications in pharmaceuticals and water treatment as a coagulant. In medicine, it treats hyperhidrosis by causing inflammation and blocking sweat glands. As polyaluminum chloride, it aids in water, sewage, and industrial water treatment by settling out particulate matter and suspended solids due to its coagulation and precipitation properties.
Properties of Aluminum Chloride
1. Basic Properties
With a molecular weight of 133.34, a melting point of 190℃, and a specific gravity of 2.44, aluminum chloride reacts in water to produce aluminum hydroxide Al(OH)3 and hydrogen chloride (HCl). Concentrating and heating its aqueous solution dehydrates the hexahydrate form to [Al(H2O)3]Cl3, eventually leading to the formation of alumina Al2O3.
2. Lewis Acid Catalyst Properties
As a Lewis acid catalyst in organic chemistry, aluminum chloride facilitates alkylation and acylation in Friedel-Crafts reactions. During these reactions, AlCl4– forms, aiding in nucleophilic substitution by coordinating to an intramolecular electron pair. The amount of aluminum chloride impacts the reaction’s progress in alkylation, whereas in acylation, its efficacy decreases unless an equivalent or greater amount is used due to complex formation with the carbonyl oxygen of the formed aromatic acyl.
Types of Aluminum Chloride
Besides anhydrous aluminum chloride and its hexahydrate, polyaluminum chloride [Al(OH)nCl6-n]m, where 1≤n≤5 and m≤10) exists. In water treatment, its high positive charge neutralizes the negatively charged surfaces, aggregating turbidity into flocs that precipitate.
Structure of Aluminum Chloride
1. Anhydrous Aluminum Chloride Structure
Anhydrous aluminum chloride features a crystal structure with one aluminum atom surrounded by six chlorine atoms in a distorted octahedral shape, forming a two-dimensional sheet layer. In liquid or gas form, it becomes a dimeric molecule, Al2Cl6, with a cyclic structure linking two Al atoms with two Cl atoms.
2. Aluminum Chloride Hexahydrate Structure
The structure of aluminum chloride hexahydrate, [Al(H2O)6]Cl3 or AlCl3・6H2O, highlights a trivalent complex ion where six water molecules coordinate to the Al atom.
Other Information on Aluminum Chloride
Production Methods
The industrial process for anhydrous aluminum chloride involves introducing chlorine gas into molten aluminum. The resulting aluminum chloride gas sublimates and is then solidified in a cooler.