A relative extrusibility scale and its application to the thermal fragmentation patterns of 5-membered ring diazenes

Abstract

Extrusion of a stable molecular fragment (like N2, CO2, SO2, CO, etc.) is a common chemical process. Using thermodynamic additivity concepts, group values, and heat of formation data, a Relative Extrusibility Scale (ΔHex0) has been defined for these fragmentations:[Formula: see text]Twenty-four such extrusions spanning 150 kcal mol−1 have been compiled. Typical values include N2 (−42.0), CO2 (−2.6), HCN (13.8), CO (15.4), H2CO (18.3), C2H4 (32.2), and 1CH2 (109). Among other theoretical and mechanistic applications, this list should be useful when designing synthetic routes involving extrusion reactions.In a particularly effective illustration of the usefulness of the scale, it rationalizes, for the first time, the variety of product types obtained from thermolyses of 14 different kinds of 5-membered ring diazenes. Heat of formation data on most of these compounds do not exist and they contain such sufficiently complicated functional group arrangements that most of the thermodynamic group equivalents are also unknown. This problem can be circumvented by thermodynamic cycles based on the Relative Extrusibility Scale, suitable models, and interfragment resonance corrections. Within an error framework of less than ± 5 kcal mol−1, enthalpy estimates are sufficiently accurate to give heats of reaction capable of predicting which of the potential fragmentation routes actually occur. Formation of a 3-membered ring must be exothermic by at least 30 kcal mol−1 to contribute to the products, but 3-piece fragmentation and 1,3-dipolar cycloreversion will contribute to the observed products when the process is either thermoneutral or exothermic. The unimportance of entropy is also discussed.

ESR spin-trap study of the radicals present during the thermolysis of some novel alkyl peroxy esters

“…CH 2 R ž < CHR ž 2 < CR ž 3 ). This trend is the same as for decarbonylation of acyl radicals but is less sensitive to radical structure owing to the higher extrusibility of CO 2 compared with CO. 13,15 The radicals released during thermolysis initiate polymerization and are, of course, incorporated into the polymer as end groups. Consequently, they can have a profound influence on the thermal and light stability of the resulting polymer.…”