Formation of Radicals in the Gas Phase

Radicals still less stable than the ones discussed thus far are proposed as reaction intermediates. Many of these have been prepared in the gas phase where they may be detected by the well-known mirror technique. When a stream of a carrier gas, such as hydrogen or nitrogen containing lead tetramethyl vapor, is passed over a hot spot in a quartz tube, lead from the decomposing lead tetramethyl forms a lead mirror at the hot spot. But if there is already a lead mirror not too far downstream from the hot spot, the downstream mirror disappears.44 

Only lead tetramethyl, carrier gas, hydrogen, and stable hydrocarbons issue from the downstream end of the tube, and none of these will react with lead. Clearly some unstable and reactive intermediate is 

Rice has shown that the cracking of hydrocarbons at high temperatures gives free radicals.48 Only the methyl and ethyl radicals survive long enough to react with mercury to give dialkylmercury. Presumably the others decomposes by a beta cleavage process. 

The cleavage reaction is the reverse of the reaction of addition to unsaturated molecules which prevails at lower temperatures. 

Photolysis of acetone forms methyl radicals, isolated as dimethyl-mercury, and acetyl radicals isolated as diacetyl. Photolysis of benzo-phenone forms phenyl radicals which remove a tellurium mirror to give diphenyltellurium. 

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