By James Ashenhurst
Reagent Friday: Aluminum Chloride (AlCl3)
Last updated: February 12th, 2019
In a blatant plug for the Reagent Guide, each Friday I profile a different reagent that is commonly encountered in Org 1/ Org 2.
Note: there’s going to be an exciting announcement within the next little while on a new development regarding the Reagent Guide…more details to come soon!
Aluminum Chloride (AlCl3)
Also known as: aluminum trichloride
What it’s used for: Aluminum chloride is a strong Lewis acid. It’s most commonly used as a catalyst for the halogenation (especially chlorination) of aromatic groups, as well as in the Friedel Crafts reaction. It’s also used in the Meerwein-Ponndorf-Verley reduction
Similar or equivalent to: Iron chloride (FeCl3) is another reagent which performs many of the same reactions as AlCl3. In addition AlCl3 has essentially the same mode of action as AlBr3 and FeBr3.
Example 1: In electrophilic chlorination
AlCl3 promotes the chlorination of aromatic molecules such as benzene, when chlorine (Cl2) is added. The AlCl3 is regenerated, and HCl is a byproduct.
Example 2: In the Friedel-Crafts acylation reaction
The Friedel-Crafts reaction is also promoted by AlCl3. In Friedel-Crafts acylation, the product is an aromatic ketone, and the byproduct is HCl.
Example 3: In the Friedel-Crafts alkylation reaction
The Friedel-Crafts alkylation reaction is also promoted by AlCl3. Since AlCl3 will lead to the formation of a carbocation, one thing to watch out for in these cases is the possibility for rearrangement to more substituted carbocations.
Example 4: In the Meerwein-Ponndorf-Verley reduction
Finally, AlCl3 will react with alcohols to make aluminum alkoxides. The aluminum alkoxides, once formed, will catalyze the Meerwein-Ponndorf-Verley reduction of ketones to give alcohols. Interestingly, the reductant in this case is not the aluminum, but the alcohol from the aluminum alkoxide. In the process, this alcohol is oxidized.
How it works
AlCl3 (and other Lewis acids like it) will coordinate to halogens, and facilitate the breaking of these bonds. In doing so, it increases the electrophilicity of its binding partner, making it much more reactive.
The aromatic group then attacks the resulting strong electrophile, leading to what is often called the Wheland intermediate. (or “arenium ion”). Finally, this loses a proton to regenerate the aromatic.
You might get some deja vu looking at the reactions of AlCl3, FeCl3, AlBr3, and FeBr3. They all behave essentially identically in these types of reactions. One final question: why do you think AlCl3 or FeCl3 might be preferred for chlorination reactions and AlBr3 or FeBr3 for bromination reactions? Why not use AlCl3 for bromination reactions?
P.S. You can read about the chemistry of AlCl3 and more than 80 other reagents in undergraduate organic chemistry in the “Organic Chemistry Reagent Guide”, available here as a downloadable PDF.