Aluminum Chloride
Properties
| State | Solid (powder, hygroscopic) |
| Color | Yellowish-white (anhydrous); colorless (hexahydrate) |
| Solubility | Reacts with water (anhydrous); hexahydrate freely soluble |
| Melting Point | 192.4°C (at 2.5 atm; sublimes at 180°C at 1 atm) |
| Boiling Point | 180°C (sublimes) |
About Aluminum Chloride
Aluminum chloride is the Lewis acid that organic chemists reach for whenever a reaction needs an electrophile activated by halide abstraction — the canonical example being Friedel–Crafts alkylation and acylation, where AlCl3 strips chloride off an alkyl or acyl halide to generate the carbocation or acylium ion that attacks the aromatic ring. The activation works because aluminum's empty p-orbital is hungry enough for electron density that even a weakly basic chloride donates lone-pair electrons readily, and the resulting [AlCl4]⁻ counterion is stable enough that the carbocation is genuinely free to react. Beyond the Friedel–Crafts reactions, AlCl3 catalyzes Diels–Alder cycloadditions (where it activates the dienophile carbonyl), various rearrangements, and the Gattermann–Koch formylation. The structural curiosity of the molecule is that it dimerizes — solid AlCl3 has a layered ionic structure, but in the gas phase or in non-coordinating solvents it forms Al2Cl6 with two bridging chlorines, a textbook example of a halide-bridged dimer with three-coordinate geometry filled out by donation from a neighbor's lone pair. The handling reality is that anhydrous AlCl3 reacts with water with an aggressive exotherm, releasing HCl gas and generating a corrosive solution — which is why bottles arrive with desiccant, get opened in a dry box or under a flow of nitrogen, and never sit on the bench longer than they have to.
Where you'll encounter it
If you've done a Friedel–Crafts acylation or alkylation in undergraduate organic chemistry, AlCl3 was the catalyst — it's the standard reagent for installing acetyl groups on benzene rings or branching alkyl groups onto aromatic systems. In industry, the same chemistry feeds production of detergent alkylbenzenes, anthraquinone dye precursors, and a long list of pharmaceutical intermediates. The other place it shows up is in antiperspirants, where the related compound aluminum chlorohydrate (Al2(OH)5Cl) is the sweat-blocking active ingredient — the aluminum ions form a temporary plug in eccrine sweat ducts. The fume hood encounter most chemists remember is opening an old AlCl3 bottle: the trapped HCl from slow hydrolysis vents immediately and the cap rim is usually crusted with corrosion product.
Common Uses
- Friedel–Crafts alkylation and acylation catalyst
- Lewis acid for Diels–Alder and Mukaiyama aldol reactions
- Petroleum-refining isomerization and alkylation catalyst
- Aluminum chlorohydrate (related salt) as antiperspirant active
- Pharmaceutical and dye intermediate synthesis
Safety Information
Reacts violently with water — exotherm and HCl release happen the moment the anhydrous solid contacts moisture. Causes severe skin and eye burns; vapor and dust irritate the airways. Always handle under a glove-bag or in a dry inert atmosphere; quench spills with limited additions of dry sodium carbonate before any aqueous wash. GHS H290, H302, H314.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.