Koelsch radical

Examples of radicals which are reported to meet these criteria are diphenylpicrylhydrazyl [DPPH, (22)], Koelsch radical (26), nitroxides [e.g. TEMPO (23), Fremy s Salt (24)], triphenylmethyl (25), galvinoxyl (27), and verdazyl radicals [e.g. triphenylverdazyl (28)]. These reagents have seen practical application in a number of contexts. They have been widely utilized in the determination of initiator efficiency (Section 3.3.1.1.3) and in mechanistic investigations (Section 3.5.2). 

The kinetics of these pyrolysis reactions were followed by several complementary methods under conditions as close to the product studies as possible. The most frequently-used ampule technique14 17) with gc analysis of 5 and the scavenger technique, with chloranil or Koelsch radical as scavenger 18), for very labile compounds 5 were complemented by the DSC method, in which the heat flow under conditions of linear temperature increase is analysed. It proved to be a particularly convenient and reliable technique 18- 21). Rates were followed over a temperature span of at least 40 °C with temperature control of 0.1-0.2 °C. All rate data and activation parameters were subjected to a thorough statistical analysis including statistical weights of errors. The maximum statistical errors in k were 3%, in AH 1 kcal mol-1 in AS 513 e.u. and in AG (at the temperature of measurement) g0.5 kcal mol-1. 

Koelsch radical Koelsch, C.F. (1957). Syntheses with triarylvinylmagnesium bromides. v, -bisdiphenylcnc-/i-phcnylallyl, a stable free radical. J. Am. Chem, Soc. 79, 4439 4441... 

Kelimide, polyimide resins, 512 Ketazines, reaction with MA, 219 Ketones, permaleic oxidations, 77 Kinel, polyimide resins, 512 Koelsch radical, styrene-MA copolymerization inhibitor, 396... 

The reviews by Rondestvedt (1960, 1976) are outdated so far as the mechanism of the Meerwein reaction is concerned. This statement is substantiated by Rondestvedt s own comment in his 1976 review (p. 226) in which he states that the generally accepted mechanism involves the aryl radical. .., though the manner of its formation and its subsequent reaction are still controversial . Meerwein et al., in their original paper (1939), expressed the opinion that the reaction is ionic in nature. A radical mechanism was first proposed by Koelsch in 1943 (see also Koelsch and Bockelheide, 1944). He received immediate support from Bergmann et al. (1944) and Bergmann and Weizmann (1944), in spite of the fact that Koelsch s claim was based on rather uncertain and vague arguments. 

This stable radical was prepared by Koelsch by reaction of the corresponding chloride with mercury, and by Kuhn and Neugebauer by the process formulated. 

The radical crystallizes from benzene as a green 1 1 complex, m.p. 231-233°, or without solvent from acetic acid as brown needles, m.p. 188-191°. It is highly dissociated but remarkably stable to oxygen. In fact the original paper describing the substance and submitted by Koelsch in 1932 was initially rejected because a referee held that the properties were not those of a radical. Of the available radicals, BDPA was found to be the most suitable as a radical scavenger in the decomposition of cyclohexylformyl and isobutyryl peroxides. ... 

Another commonly used stable radical is 1,3,5-triphenylverdazyl 19 (446, 447). It is less thermally stable than TEMPO. Both TEMPO and the verdazyl radical do not react with oxygen-centered radicals or oxygen. If an initiator generates an oxygen-centered radical the nitroxide will capture the carbon-centered radical that is generated via the first addition step involving a monomer. Galvinoxyl 20 and l,3-bisdiphenylene-2-phenylallyl (or Koelsch s) radical 21 can also be used as inhibitors. Diphenylpicrylhydrazyl 22 is used much less frequently because of its complicated reaction mode of inhibition (448). 

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