Malonic anions, decomposition

Comparisons of observations for the decompositions of five metal malonates (Cu(II) [22], Ag [62], Ni [65], Co [66] and Ca [67]) illustrate the behavioural idiosyncrasies and individualities of these closely related compounds. Anion breakdown was accompanied by hydrogen transfer so that four of the reactants gave appreciable yields of acetate or acetic acid. (No analytical measurement was made for the cobalt salt.) The principal kinetic results (Table 18.3.) show the remarkably contrasting behaviours of copper(II) malonate, where decomposition... 

Steroidal a,jS-unsaturated ketones can be dehydrogenated, via n-allyl complexes, to give conjugated dienones. Decomposition of the Jt-allyl complex can be effected either thermally (Howsam and McQuillin, 1968) or by treatment with diethyl malonate anion or cyanide ion (Harrison et al, 1969). 

Less basic malonic ester anions may be employed for the twofold alkylation of dibromides. Cyclic 1,1-dicarboxylic esters are formed, if the reaction is executed in an appropriate manner. In the synthesis of cyclobutane diester A the undesired open-chain tetraester B was always a side product (J.A. Cason, 1949), the malonic ester and its monoalkylation product were always only partially ionized. Alkylation was therefore slow and intermolecular reactions of mono-alkyl intermediates with excess malonic ester prevailed. If the malonic ester was dissolved in ethanol containing sodium ethoxide, and 1,3-dibromopropane as well as more sodium ethoxide were added slowly to the solution, 63% of A and only 7% of B were isolated. The latter operations kept the malonic ester and its monoalkylated product in the ionic form, and the dibromide concentration low, so that the intramolecular reaction was favored against intermolecular reactions. The continuous addition of base during the reaction kept the ethoxide concentration low, which helped to prevent decomposition of the bromide by this nucleophile. 

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. 

Further reactions occur, including processes catalyzed by the solid products, where the different organic anions give various carbonaceous radicals. The participation of acetate in malonate breakdown has already been discussed above. Decompositions of calcium carboxylates have been used as a method of ketone preparation in organic chemistry. 

Calcium salts. The thermal decompositions of calcium oxalate, malonate, maleate and fumarate were studied in significantly higher temperature ranges (above 720, 612 to 653, 733 to 763 and 733 to 803 K, respectively) than those of the same salts of the transition metals. This is evidence of a stabilizing influence on these anions of the strong bond formed with this strongly electropositive cation. 

The initial or rate limiting step for anion breakdown in metal oxalate decompositions has been identified as either the rupture of the C - C bond [4], or electron transfer at a M - O bond [5], This may be an oversimplification, because different controls may operate for different constituent cations. The decomposition of nickel oxalate is probably promoted by the metallic product [68] (the activity of which may be decreased by deposited carbon, compare with nickel malonate mentioned above [65]). No catalytically-active metal product is formed on breakdown of oxalates of the more electropositive elements. Instead, they yield oxide or carbonate and reactions may include secondary processes [27]. There is, however, evidence that the decompositions of transition metal oxalates may be accompanied by electron transfers. The decomposition of copper(II) oxalate [69] (Cu - Cu - Cu°) was not catalytically promoted by the metal and the acceleratory behaviour was ascribed to progressive melting. Similarly, iron(III) oxalate decomposition [61,70] was accompanied by cation reduction (Fe " - Fe ). In contrast, evidence was obtained that the reaction of MnC204 was accompanied by the intervention of Mn believed to be active in anion breakdown [71]. These observations confirm the participation of electron transfer steps in breakdown of the oxalate ion, but other controls influence the overall behaviour. Dollimore has discussed [72] the literature concerned with oxalate pyrolyses, including possible bond rupture steps involved in the decomposition mechanisms... 

Few data are available on the concentration of dicarboxylic acid anions in subsurface waters. C2 through C q saturated acid anions have been reported in addition to maleic acid (cz5-butenedioic acid) (5. 15-16L Oxalic acid (ethanedioic) and malonic acid (propanedioic) appear to be the most abundant. Reported concentrations range widely from 0 to 2540 mg/1 but mostly are less than a few 100 mg/1. Concentrations of these species in formation waters are probably limited by several factors, including the very low solubility of calcium oxalate and calcium malonate (5), and the susceptibility of these dicarboxylic acid anions to thermal decomposition (16). This paper will focus on the monocarboxylic acids because they are much more abundant and widespread, and stability constants for their complexes with metals are better known. We do recognize that dicarboxylic acid anions may be locally important, especially for complexing metals. 

The conclusion reached by Mulliken, that in the electrolysis of certain weak organic acids a portion of the anions unite in pairs without undergoing decomposition, was closely examined by Weems as to its general applicability and as to the exact nature of the chemical changes which take place. All direct attempts to oxidize malonic ester with hydrogen peroxide, potassium permanganate, and chromic add, and to produce an effect similar to that caused by the current, were without success. 

Multidentate Leaving Groups.—The hydrolysis of [Co(ox)a] - and of [Co(ox)2(OH2)2], which ultimately produces cobalt(n) and carbon dioxide, involves the formation of an intermediate containing a unidentate oxalate ligand previous to the rate-determining step. Free radical intermediates are thought unlikely in the decomposition of these oxalato-complexes, but malonate ion-radicals are thought to be intermediates both in the thermal and photochemical hydrolysis of the [Co(mal)3] anion. Kinetics are reported for a third example of these aquation-redox processes, [Co(acac)2] in acidic solution. ... 

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