Balsohn alkylation

What is Balsohn alkylation?

The discovery of Balsohn alkylation dates back to 1879, only two years after Friedel and Crafts introduced the Friedel-Crafts acylation and Friedel-Crafts alkylation with AlCl3.

Initially, Balsohn directly alkylated benzene with ethylene in the presence of AlCl3, and the extended reaction time favored higher yields of polyethylbenzenes, including pentaethylbenzene.

Balsohn alkylation
Balsohn alkylation

Since then, various acidic catalysts, such as sulfuric acid, phosphorus acid, phosphorus pentoxide, and boron trifluoride, have been applied to similar reactions. It is worth noting that Balsohn’s alkylation is not restricted to ethylene but works for other alkenes, including acetylene, and other aromatics such as phenol. However, when higher alkenes react with aromatics, the less substituted end of the alkenes attaches to the aromatic ring, while diphenylethane and 9,10-dimethyl-9,10-dihydroanthracene form with acetylene. Additionally, when sulfuric acid is used as a catalyst, polymerization and ester formation compete with the actual alkylation. Despite these challenges, some of these reactions have been utilized for industrial-scale production.

Moreover, Balsohn alkylation demonstrates para/ortho regio-selectivity akin to Friedel-Crafts acylation and Friedel-Crafts alkylation.

References

  • Balsohn, M., Bull. Soc. Chim. Fr., 1879, 31, 539.
  • Friedel, C. and Crafts, J. M., Compt. Rend., 1877, 84, 1392.
  • Friedel, C. and Crafts, J. M., Compt. Rend., 1877, 84, 1450.
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