Gomberg free radical reaction

What is Gomberg free radical reaction?

The origins of free radical chemistry can be traced back to Gomberg’s preparation of a triphenylmethyl radical in 1900, which he achieved by abstracting halogens from triphenylmethyl halide using metals. Gomberg claimed the field for himself and thus the production of triphenylmethyl radical became known as the Gomberg reaction.

Gomberg free radical reaction - general reaction scheme
Gomberg free radical reaction

This early work drew attention to trivalent carbon and the concept of free radicals, leading to extensive exploration of their generation and reactions. Triarylmethyl radicals are notable for displaying temperature-dependent colors, such as in quinonoidation, and radicals have been reported to form during the preparation of Grignard reagents using iodine as a catalyst.

Many effective methods for generating free radicals have since been developed, including reduction, oxidation, pyrolysis of peroxides, acyl peroxides, and azo compounds, photo-irradiation, and the use of trialkylaluminium or trialkylboranes. Compounds that easily decompose and form free radical species are used as initiators, while compounds with weak bonds to hydrogen are prepared as proton donors in radical chain reactions and are known as hydride transfer reagents. The transient nature of radicals makes them difficult to study, but their lifetimes can be determined by analyzing the product distribution if they react with a compound undergoing an intramolecular radical rearrangement of known lifetime.

Radical-molecule reaction kinetics is a new field in free radical chemistry that emerged in the 1980s, involving radicals with known lifetimes known as free radical clocks. While most radicals have short lifetimes, bulky-substituent radicals like Gomberg’s triphenylmethyl radical can have longer lifetimes and fall into two categories:

  • Persistent radicals: have relatively long lifetimes, allowing for characterization under the conditions they are generated.
  • Stable radicals: are inherently stable and can be isolated without signs of decomposition under inert conditions at room temperature.

Some radicals are extremely stable, with half-lives of decades, and can withstand typical radical reagents and even highly reactive species like halogens and sodium hydroxide.

These radicals possess very high thermal stability, up to 300 ºC, and are called inert free radicals.

Representative examples of long-living radicals, including persistent, stable, and inert radicals, are summarized.