Anhydrous Aluminium Chloride ($AlCl_3$)

By Chemca Editorial Team • Last Updated: January 2026 • 9 min read

Anhydrous Aluminium Chloride ($AlCl_3$) is one of the most versatile reagents in organic chemistry. It is a covalent compound that behaves as a strong Lewis Acid due to the incomplete octet of the aluminium atom (6 valence electrons).

1. Preparation

A. From Aluminium Metal

Heating aluminium turnings in a stream of dry chlorine gas or dry HCl gas.

$$ 2Al(s) + 3Cl_2(g) \xrightarrow{\Delta} 2AlCl_3(s) $$

B. MacAfee Process (From Alumina)

Heating a mixture of Alumina ($Al_2O_3$) and coke in a current of dry chlorine.

$$ Al_2O_3 + 3C + 3Cl_2 \xrightarrow{1000^\circ C} 2AlCl_3 + 3CO $$

2. Structure and Bonding

Anhydrous $AlCl_3$ shows different structures in different phases.

Solid/Vapor Phase: The Dimer ($Al_2Cl_6$)

To complete its octet, $AlCl_3$ exists as a dimer. Each Aluminium atom forms 4 bonds (tetrahedral geometry).

Two bridging chlorine atoms form coordinate bonds ($Cl \rightarrow Al$).
Four terminal chlorine atoms form normal covalent bonds.
Hybridization of Al changes from $sp^2$ (in monomer) to $sp^3$ (in dimer).

Cl Cl
\ /
Al
/ \
Cl Cl

(Structure involves 3c-4e bonds in bridging)

Aqueous Solution

In water, the dimer breaks down. Due to high hydration energy, it forms the octahedral complex: $$ [Al(H_2O)_6]^{3+} + 3Cl^- $$ The solution is acidic due to hydrolysis.

3. Chemical Properties

A. Fuming Nature

Anhydrous $AlCl_3$ fumes in moist air because it hydrolyzes to form HCl gas.

$$ AlCl_3 + 3H_2O \rightarrow Al(OH)_3 + 3HCl \uparrow \text{ (White fumes)} $$

B. Action as Lewis Acid

It forms adducts with Lewis bases like Ammonia or Phosphine.

$$ AlCl_3 + :NH_3 \rightarrow [Cl_3Al \leftarrow NH_3] $$

4. Applications as Catalyst

$AlCl_3$ is the quintessential catalyst for generating electrophiles (Carbocations, Acylium ions) in organic reactions.

Reaction	Role of $AlCl_3$
Friedel-Crafts Alkylation	Generates Carbocation ($R^+$) from $R-X$.
Friedel-Crafts Acylation	Generates Acylium ion ($RCO^+$) from $RCOCl$.
Gattermann-Koch	Generates Formyl cation ($HCO^+$).
Fries Rearrangement	Complexes with ester oxygen to facilitate rearrangement.

5. Crucial Precautions

Must be kept Dry!

Anhydrous $AlCl_3$ must be stored in tightly closed bottles. If it absorbs moisture, it converts to hydrated $AlCl_3 \cdot 6H_2O$. Hydrated $AlCl_3$ cannot be used as a Friedel-Crafts catalyst because the water molecules occupy the coordination sites of Aluminium, destroying its Lewis acid character.

AlCl3 Mastery Quiz

Test your understanding of this key reagent. 10 MCQs with explanations.

Anhydrous AlCl3: Structure, Properties & Uses | Chemca