Mechanism of thermal decomposition of dibenzhydryl oxalates

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Abstract
Dibenzhydryl oxalate and several of its para-substituted analogs were thermally decomposed in diphenylmethane, diphenyl ether, and in α-chloronaphthalene solution. Evolution of gas (mainly CO2) was approximately first order, both rate and stoichiometry being poorly reproducible. Rates are correlated with σ+-substituent parameters, with ρ = −1.6 at 230.2°. The 13C/12C and 18O/16O isotope effects involved in CO2 formation were measured. It is concluded that thermolysis is a radical process with considerable polar character at the transition state and that the slow step involves concerted formation of one CO2 molecule, a diarylmethyl radical, and a carbodiarylmethoxy radical. The fate of the latter is primarily decarboxylation, but there is some decarbonylation and some trapping by diarylmethyl radicals. Tetraaryl ethane and CO2 are major products, the yield of the latter approaching 2 moles at high temperatures.

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