Reagent Friday: Aluminum Chloride (AlCl3)
Last updated: January 29th, 2020 |
Aluminum Chloride (AlCl3) – A Lewis Acid Reagent For Aromatic Halogenation And The Friedel-Crafts Reactions
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 alkylation and acylation reactions. 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: As A Lewis-Acid Catalyst For 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: As A Reagent For 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. (Note: since AlCl3 tends to coordinate to the ketone product of these reactions, it’s typically used in stoichiometric amounts and therefore is not technically a “catalyst” in this case.)
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.
AlCl3 As A Lewis Acid Catalyst: 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.
(Advanced) References and Further Reading
- The kinetics of aromatic halogen substitution. Part IV. The 1-halogenonaphthalenes and related compounds
P. B. D. de la Mare and P. W. Robertson
J. Chem. Soc., 1948, 100-106
A paper studying the kinetics of electrophilic aromatic halogenation.A lot of chemists, including some influential figures such as Profs. H. C. Brown and G. A. Olah, put in a significant amount of work into studying the electrophilic aromatic chlorination reaction. This is important since chlorinated aromatics are useful end products and intermediates for synthesis. The following papers are a highlight of all the work that has been done in the last century.
- The kinetics of aromatic halogen substitution. Part IX. Relative reactivities of monosubstituted benzenes
P. W. Robertson, P. B. D. de la Mare and B. E. Swedlund
J. Chem. Soc., 1953, 782-788
- Rates of Chlorination of Benzene, Toluene and the Xylenes. Partial Rate Factors for the Chlorination Reaction
Herbert C. Brown and Leon M. Stock
Journal of the American Chemical Society 1957, 79 (19), 5175-5179
- Rates and Isomer Distributions in the Chlorination of Benzene, Toluene and t-Butylbenzene in Aqueous Acetic Acid Solvents. The Influence of Solvent on the Reaction and the Baker-Nathan Effect
Leon M. Stock and Albert Himoe
Journal of the American Chemical Society 1961, 83 (8), 1937-1944
- Aromatic Substitution. XVII. Ferric Chloride and Aluminum Chloride Catalyzed Chlorination of Benzene, Alkylbenzenes, and Halobenzenes
George A. Olah, Stephen J. Kuhn, and Barbara A. Hardie
Journal of the American Chemical Society 1964, 86 (6), 1055-1060
Org. Synth. 1963, 43, 15
Since anthracene is significantly more electron-rich than benzene, it is more reactive in electrophilic reactions and therefore reacts with much milder conditions – in this case chlorination is effected simply with CuCl2 rather than with the harsher combination of Cl2+FeCl3. Additionally, this procedure can be used as a convenient synthesis of CuCl, which is prone to oxidation in air.
- Production of hexachlorobenzene
Inventor: Bernard H Nicolaisen, Olin Mathieson Chemical Corporation
US Patent US2777003A
This patent details an industrial process for the synthesis of hexachlorobenzene from benzene and Cl2. FeCl3 can be used as the catalyst.
- N-Halosuccinimide/BF3−H2O, Efficient Electrophilic Halogenating Systems for Aromatics
K. Surya Prakash, Thomas Mathew, Dushyanthi Hoole, Pierre M. Esteves, Qi Wang, Golam Rasul, and George A. Olah
Journal of the American Chemical Society 2004, 126 (48), 15770-15776
A modern approach to electrophilic aromatic substitution using N-halosuccinimides, which are easier to handle than the elemental halogens. This uses BF3-H2O as the acid, which was developed by Hans Meerwein in the early 20th century.