Calcium malonate, decomposition

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. 

Dissolve 13 g. of sodium in 30 ml. of absolute ethanol in a 250 ml. flask carrying a reflux condenser, then add 10 g. (9 5 ml.) of redistilled ethyl malonate, and place the flask on a boiling water-bath. Without delay, add a solution of 5 3 g. of thiourea in a minimum of boiling absolute ethanol (about 100 ml.). The sodium salt of thiobarbituric acid rapidly begins to separate. Fit the water-condenser with a calcium chloride guard-tube (Fig. 61, p. 105), and boil the mixture on the water-bath for 1 hour. Cool the mixture, filter off the sodium salt at the pump and wash it with a small quantity of cold acetone. Dissolve the salt in warm water and liberate the acid by the addition of 30 ml. of concentrated hydrochloric acid diluted with 30 ml. of water. Cool the mixture, filter off the thiobarbituric acid, and recrystallise it from hot water. Colourless crystals, m.p. 245 with decomposition (immersed at 230°). Yield, 3 5 -4 0 g. 

In a 2-litre round-bottomed flask, fitted with a double surface reflux condenser, place 11.5 g (0.5 mol) of clean sodium. Add 250 ml of absolute ethanol in one portion if the reaction is unduly vigorous, immerse the flask momentarily in ice. When all the sodium has reacted, add 80 g (76 ml, 0.5 mol) of diethyl malonate, followed by a solution of 30 g (0.5 mol) of dry urea in 250 ml of hot (c. 70 °C) absolute ethanol. Shake the mixture well, fit a calcium chloride guard-tube to the top of the condenser and reflux the mixture for 7 hours in an oil bath heated to 110°C. A white solid separates. Treat the reaction mixture with 450 ml of hot (50 °C) water and then with concentrated hydrochloric acid, with stirring, until the solution is acid (about 45 ml). Filter the resulting almost clear solution and leave it in the refrigerator overnight. Filter the solid at the pump, wash it with 25 ml of cold water, drain well and then dry at 100 °C for 4 hours. The yield of barbituric acid is 50 g (78%). It melts with decomposition at 245 °C. 

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. 

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