Free radicals from tetramethyllead

On the other hand, the presence of a short-lived free radical was confirmed first from an elegant experiment in 1929 by Paneth and Hofeditz [44], although the existence of paramagnetic species was pointed out in the middle of the 19th century by Faraday [45]. When tetramethyllead was thermolyzed, a methyl radical was postulated to be formed as the reaction intermediate (Eq. 2)... 

A stream of tetramethyllead vapor, (CH3)4Pb, was passed through a quartz tube which was heated at one spot a mirror of metallic lead was deposited at the hot point, and the gas escaping from the tube was found to be chiefly ethane. The tube was next heated upstream of the lead mirror while more tetramethyllead was passed through a new mirror appeared at the hot point, the old mirror disappeared, and the gas escaping from the tube was now found to be chiefly tetramethyllead. Experiments like this, done by Fritz Paneth at the University of Berlin, were considered the first good evidence for the existence of short-lived free radicals like methyl, (a) Show how these experimental results can be accounted for in terms of intermediate free radicals, (b) The farther upstream the tube was heated, the more slowly the old mirror disappeared. Account for this. 

This experiment showed that some volatile component was formed in the thermal decomposition of tetramethyllead and that this compound consumed a cold lead mirror with formation of a volatile product. If, instead, a zinc mirror was first deposited and allowed to be consumed by the volatile product from decomposition of tetramethyllead, dimethylzinc could be identified as the product. Paneth concluded that free methyl radical was formed in the thermal reaction and could determine its half-life to be 0.006 seconds under the reaction conditions employed. Also, free ethyl radicals could be formed in... 

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