Detection and Observation of Radicals

Only triphenymethyl and a few other stabilized radicals may be generated in concentrations suitable for examination by traditional laboratory methods. Evidence for the transient existence of more reactive radical species in chemical reactions usually requires special techniques, including low-temperature isolation in solids and high speed spectroscopic probes. However, an interesting chemical detection of the methyl radical was carried out by the Austrian chemist Fritz Paneth not long after Gomberg's preparation of triphenylmethyl radical. The Paneth experiment involved gas phase thermal decomposition of tetramethyllead to methyl radicals and lead atoms in a glass tube. The initial flow of the lead compound through the tube is shown in the following illustration, and the consequences of applying strong heat to the tube will be shown in repetitive as below . The text box beneath the diagram provides commentary.

The Paneth Experiment
Gaseous tetramethyllead is carried through the glass reactor tube in a stream of nitrogen.

Heating one section of the reaction tube causes the lead compound to break down into lead atoms and methyl radicals.
The lead deposits on the walls of the tube as a metallic mirror. The methyl radicals combine to give ethane, which may be detected in the exit gases (blue arrow).

Removal of the heat source leaves the lead mirror as a marker of the thermal reaction.

Heat was then applied at a new location nearer to the inlet of the tube.
The same breakdown of tetramethyllead occurs here (top equation). A new lead mirror begins to deposit, but the lead atoms composing the older mirror react with methyl radicals to reform tetramethyllead. (bottom equation and the green arrow)

In this illustration the older lead mirror is almost gone and a substantial new mirror has formed. The exit gases contain both tetramethylead (green arrow) and ethane (blue arrow).

Finally, the old mirror has disappeared, the heat is removed, and the new mirror remains in place.