Blanc chloromethylation

What is Blanc chloromethylation?

Grassi-Cristaldi and Maselli first reported this reaction in 1898, and it was later expanded upon by Blanc in 1923. The Blanc chloromethylation reaction involves introducing a chloromethyl group into aromatic rings by treating aromatic compounds with formaldehyde, CH2O, and hydrogen chloride, HCl, in the presence of zinc chloride, ZnCl2.

Blanc chloromethylation
Blanc chloromethylation

While aliphatic ketones can also be chloromethylated in a similar manner, only aromatic ketones with at least two alkyl groups can undergo chloromethylation in this reaction. However, if combined with subsequent reduction, this reaction can serve as a useful indirect method for the methylation of aromatic compounds. Chloromethylation of phenyl rings is important for the functionalization of resins, providing an alternative to using functionalized monomers in the polymerization process.

Although reports exist of uncatalyzed chloromethylation of aromatic rings using formaldehyde and HCl in acetic acid at 85 ºC, and using chloromethyl methyl ether in acetic acid at 100 ºC, these results are highly disputed and could not be reproduced. The Blanc chloromethylation reaction has been successfully utilized to synthesize a variety of compounds, including benzyl chloride, 6-chloromethyldehydroabietic acid, hydrindene, methylcholanthrene, cyclophane, and Bakelite.

Typically, this reaction is employed to incorporate the chloromethyl group into aromatic compounds.

References

  • Grassi-Cristaldi, G. and Maselli, M., Gazz. Chim. Ital., 1898, 20 , 477
  • Blanc, M. G., Bull. Soc. Chim. Fr., 1923, 33, 313
  • Blanc, M. G., Am. Perfumer, 1923, 17, 541.
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