Aluminium chloride, also known as aluminium trichloride, is a powerful Lewis acid and an essential industrial chemical. It appears as a white to pale yellow solid that fumes vigorously in moist air. Its primary application lies in its role as a master catalyst in organic chemistry, specifically for Friedel-Crafts alkylation and acylation. Due to its intense reactivity with water and corrosive nature, strict handling and storage protocols are mandatory.
1. Basic Identification
Chemical Formula: AlCl₃ (monomer); typically exists as the dimer Al₂Cl₆ in both solid and liquid phases.
Alternative Names: Aluminum trichloride, trichloroalumane.
Molecular Weight: 133.34 g/mol (anhydrous).
CAS Number: 7446-70-0 (anhydrous).
Appearance: White or pale yellow solid. The yellowish tint is often due to iron chloride impurities. It fumes strongly in damp air.
2. Physical Properties
Understanding the physical properties of aluminum chloride is critical due to its phase-changing behavior and hygroscopic nature.
2.1 Key Data Table
| Property | Anhydrous AlCl₃ |
|---|---|
| Melting Point | 192.4 °C (378.3 °F) – only melts under pressure (2.5 atm) |
| Boiling Point | 180 °C (356 °F) – sublimes at atmospheric pressure |
| Density | 2.48 g/cm³ |
| Vapor Pressure | 1 mmHg at 100 °C |
| Solubility | Reacts violently with water. Soluble in hydrogen chloride, ethanol, chloroform, carbon tetrachloride, and mildly in benzene. |
2.2 Physical Description
Anhydrous aluminum chloride typically forms a powdery or slightly granular solid. It is intensely hygroscopic. When exposed to ambient humidity, it quickly draws moisture from the atmosphere to form a hydrated compound while releasing dense, highly corrosive white fumes of hydrogen chloride (HCl) gas.
3. Chemical Behavior and Reactions
Like its bromine counterpart, aluminium chloride is a strong Lewis acid. The aluminum core is electron‑deficient, seeking electron pairs from other atoms, making it highly reactive.
3.1 Reaction with Water
This is the most critical hazard associated with AlCl₃.
AlCl₃ + 3H₂O → Al(OH)₃ + 3HCl ↑
Observation: A highly exothermic reaction when unmanaged. Releases significant heat and thick clouds of choking hydrogen chloride gas.
Result: It destroys itself in water, prohibiting the creation of an anhydrous aqueous solution.
3.2 Synthesis (How It Is Made)
2Al + 3Cl₂ → 2AlCl₃
Process: It is manufactured commercially by the exothermic reaction of chlorine gas or hydrogen chloride with molten aluminum metal, or sometimes by reacting alumina with carbon and chlorine at high temperatures.
3.3 Reaction with Organic Molecules
In the presence of aromatic compounds and alkyl halides, it pulls the halogen atom away, creating a highly reactive carbocation that can bind to the aromatic ring.
4. Industrial and Laboratory Applications
Aluminum chloride is incredibly impactful, largely driving the petrochemical and synthetic dye industries.
4.1 Friedel-Crafts Catalyst (Primary Use)
AlCl₃ is the definitive catalyst for Friedel-Crafts alkylation and acylation reactions.
- What it does: Facilitates the bonding of alkyl or acyl groups to benzene and other aromatic rings.
- Why it matters: This is a required step for producing:
- Ethylbenzene: The immediate precursor to styrene, used for making polystyrene plastics.
- Dyes & Pigments: Crucial in synthesizing anthraquinone dyes.
- Detergents: Essential for producing alkylbenzenes, the foundation for modern synthetic detergents.
4.2 Polymerization and Isomerization
It acts as a strong catalyst in the isomerization of lightweight hydrocarbons – for example, converting straight‑chain alkanes into branched alkanes to produce high‑octane gasoline components. It is also used to polymerize complex resins.
4.3 Pharmaceuticals
It serves as an essential intermediate and catalytic agent in synthesizing complex medical molecules, including various analgesics and antibiotics.
5. Safety and Hazard Management
⚠️
GHS05
Corrosive
❗
GHS07
Harmful
Critical Warning: Aluminum chloride is extremely corrosive and water‑reactive. The heat generated upon contact with water can boil the water instantly, splattering corrosive acid.
5.1 Health Effects
| Route of Exposure | Effect |
|---|---|
| Inhalation | Immediate burning sensation in respiratory tract. Inhaled fumes form hydrochloric acid in the lungs, potentially leading to pulmonary edema. |
| Skin Contact | Severe chemical burns and deep tissue damage. Reacts with sweat to form acid directly on the skin. |
| Eye Contact | Extreme pain, leading to severe corneal lesions and potential permanent blindness if not immediately treated. |
| Ingestion | Corrosion of the mouth, esophagus, and stomach walls, leading to intense pain and potentially fatal hemorrhaging. |
5.2 Personal Protective Equipment (PPE)
Always use adequate safety gear when handling the anhydrous compound.
- Respiratory: Full‑face respirator with acid gas cartridges or supplied air.
- Hands: Thick chemical‑resistant gloves (butyl rubber).
- Eyes: Tight‑sealing chemical safety goggles and a face shield.
- Body: Complete acid‑resistant suit and chemically impervious footwear.
5.3 First Aid Measures
- Skin: Quickly brush off any dry powder before washing. Then deluge with water for a minimum of 15 minutes. Seek emergency medical care.
- Eyes: Flush immediately with massive quantities of water for 20–30 minutes. Seek immediate emergency ophthalmological care.
- Inhalation: Relocate victim to fresh air. Provide oxygen if breathing is labored. Call an ambulance.
5.4 Firefighting Information
- Suitable Extinguishers: Class D fire extinguishers, dry sand, or dry powder.
- DO NOT USE: Water or water‑based foams. Contact with water intensifies danger and produces toxic HCl gas.
6. Storage and Handling Guidelines
6.1 Storage Conditions
- Container: Tightly sealed, moisture‑proof containers of glass, specialized plastics, or coated steel.
- Atmosphere: Keep securely under a dry, inert gas blanket (nitrogen or argon).
- Location: Store in a cool, dry, exceptionally well‑ventilated area completely isolated from water sources.
- Incompatibles: Water, alcohols, bases, strong oxidizers, organic materials.
6.2 Disposal Considerations
Never dispose into standard waste streams.
- Suspend in a non‑reactive hydrocarbon solvent in a fume hood.
- Carefully and slowly add the suspension to a large volume of crushed ice and water with robust stirring.
- Neutralize the resulting low pH mixture using soda ash or sodium hydroxide until pH stabilizes.
- Dispose as regulated chemical waste through a certified handler.
7. Environmental Impact
Because it rapidly hydrolyzes in moisture, AlCl₃ itself does not persist in the environment. However, its breakdown products – aluminum hydroxide and hydrochloric acid (HCl) – are significantly harmful. The generation of HCl causes sharp drops in aquatic pH, resulting in toxicity to fish and other aquatic life. Spills must be physically contained and rapidly neutralized to prevent runoff.
8. Comparison with Other Aluminium Halides
| Compound | Formula | Melting Point | Lewis Acidity | Industrial Note |
|---|---|---|---|---|
| Aluminium Fluoride | AlF₃ | 1291 °C | Weak | Highly stable, used in aluminum metallurgy. |
| Aluminium Chloride | AlCl₃ | 192.4 °C (at 2.5 atm) | Very Strong | The most heavily utilized industrial catalyst in this class. |
| Aluminium Bromide | AlBr₃ | 97.5 °C | Very Strong | Often selected for delicate laboratory‑scale synthetic precision. |
| Aluminium Iodide | AlI₃ | 191 °C | Strong | Used in niche ether cleavage and specialty reductions. |
9. Frequently Asked Questions
Q: Can I use aluminum chloride from my deodorant for an experiment?
A: No. Antiperspirants use aluminum chlorohydrate, a hydrated and far less reactive variant. Anhydrous aluminum chloride is fundamentally different and dangerously reactive.
Q: Why does my container of AlCl₃ have a yellow tint?
A: Pure anhydrous AlCl₃ is white. A yellow or grayish tint indicates trace impurities, most commonly iron(III) chloride formed during manufacturing.
Q: What happens if I heat aluminum chloride at atmospheric pressure?
A: It sublimes (turns directly from solid to vapor) at approximately 180 °C. It only melts into a liquid if heated under elevated pressure (about 2.5 atm).
Q: Is aluminum chloride explosive?
A: The compound itself is not fundamentally explosive. However, the pressure generated by its violent reaction with water in a sealed container can cause a catastrophic explosion.
10. Summary Data Sheet
| Chemical Name | Aluminum Chloride (Anhydrous) |
|---|---|
| Formula | AlCl₃ / Al₂Cl₆ |
| Appearance | White or pale yellow solid |
| Sublimation Point | 180 °C |
| Solubility | Violently hydrolyzes in water; soluble in organic solvents |
| Primary Hazard | Corrosive, Water‑Reactive (Releases HCl gas) |
| Primary Use | Industrial catalyst for Friedel‑Crafts and polymerization |
| Storage | Cool, dry area under inert atmosphere, completely devoid of moisture |
