Paneth technique

What is Paneth technique?

In 1929, Paneth et al. introduced a technique known as the Paneth technique for generating alkyl or aryl radicals, primarily methyl radical, through the thermal decomposition of organolead compounds under vacuum conditions.

Paneth technique - general reaction scheme
Paneth technique

The Paneth technique involves the decomposition of tetramethyl lead in a glass tube heated by either a mobile Bunsen burner or an electric furnace at a pressure of 1-2 mmHg. The generated methyl radical is carried by hydrogen gas at a speed of 16 m/s down the tube, which eliminates the deposited lead mirror by forming tetramethyl lead again. No diffusion against the flow of methyl radical occurs under these conditions.

The methyl radical is clearly generated from tetramethyl lead, as evidenced by the disappearance of deposited lead mirror up to 40 cm from the heated zone, even with the cooling of the space between the heated zone and deposited lead mirror by water. Tetraethyl lead reacts in the same fashion, while other higher-order alkyl lead compounds give a mixture of radicals upon thermal decomposition. Methyl radicals have also been found to react with other metals, including arsenic, antimony, zinc, cadmium, and tellurium.

References

Paneth, F. and Hofeditz, W., Ber., 1929, 62, 1335

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