Copper salts, cation reductions

These observations are satisfactorily explained by a mechanism in which salt decomposition is completed through two consecutive reactions during which there is stepwise cation reduction Cu " Cu" -> Cu . The appearance of a liquid phase during acetate formation, when a is less than 0.5 and Cu " - Cu, accounts for the first acceleratory rate process which fits the autocatalytic exponential law (da/df = ka) with = 200 15 kJ mol" between 468 and 505 K. Reaction occurs in an intracrystalline fiised material but the maintenance of an outer unreactive surface prevents particle coalescence or comprehensive fusion. The product of this rate process, believed to be predominantly copper(I) malonate, then decomposes by a first order reaction for which = 188 6 kJ moT between 477 and 528 K. This probably occurs in the solid state to give a residue identified as a dispersion of copper metal crystallites on a coherent carbon substrate. Structures and properties, including the catalytic activity of this material, have been reported [124]. 

The thermal decompositions of copper(II) fiimarate and of copper(II) maleate [125] showed some important chemical similarities with the reaction of the malonate. The most notable common feature was that the Cu " content in both salts diminished to 5% of the original value when a= 0.5, so that all three decompositions proceed with stepwise cation reduction Cu " Cu" Cu . The first reaction in the copper(II) maleate decomposition was accompanied by melting and or-time values fitted the Prout-Tompkins equation with ii, = 225 6 kJ moT between 473 and 508 K. During these reactions the maleate anion isomerized to the fiimarate and the extent of the second, deceleratory, rate process (f, = 139 15 kJ mol ) decreased as the reaction temperature was increased in the range 509 to 528 K. 

The behaviour of some copper salts is complicated by the concurrent volatilization of metal, attributed to the formation of unstable gaseous copper(I) intermediates. There is strong evidence that reactions proceed to completion through stepwise reduction of the cation (Cu " - Cu" -> Cu ) [72]. 

Stepwise cation reduction has been recognised as a common mechanistic feature in the decompositions of a variety of copper(II) salts. This was reported by Lomovsky et al. [74] for copper(II) hypophosphite (Cu(H2P02)2 Citii2P02 - Cu). Later work on the thermal reactions of the copper(II) salts of a range of organic acids provided evidence that decompositions proceeded with - Cu - Cu°. The mechanisms of these reactions, containing this common constituent, have been reviewed [75]. 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

An alternative redox mechanism leads to the formation of phenols under rather mild conditions. This reaction is initiated by CU2O, which effects reductive formation of an aryl radical. In the presence of Cu salts, the radical is oxidized to the phenyl cation by a reaction presumably taking place in the copper coordination sphere. The reaction is very rapid and gives good yields of phenols over a range of structural types. Equations 27-29 show some examples of this type of transformation. ... 

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