MALONIC ACID, C3H4O4 or CH2(COOH)2, occurs in the form of its calcium salt in the sugar beet. It was first prepared in 1858 by V. Dessaignes, who obtained it by oxidizing malic acid (Ann., 1858, 107, p. 251). It may also be obtained by oxidizing allylene and propylene with cold potassium permanganate solution, by the hydrolysis of barbituric acid (malonyl urea) with alkalis (A. Baeyer, Ann., 1864, 130, p. 143); by the hydrolysis of cyanacetic acid (H. Kolbe, Ann., 1864, 131, p. 349; H. Muller, Ann., 1864, 131, p. 352), and by the action of silver oxide on β-di-chloracrylic ester at 125° C. (O. Wallach, Ann., 1878, 193, P. 25)
CCl2:CH·COOC2H5 + Ag2O + H2O =
It crystallizes in monoclinic tables, and is readily soluble in water, alcohol and ether. The acid melts at 132° C., and at a higher temperature it rapidly decomposes into acetic acid and carbon dioxide. When heated with bromine and water to 100° C. it forms tribromacetic acid, some bromoform being produced at the same time. Malonic acid, as well as its esters, is characterized by the large number of condensation products it can form. In the presence of a dehydrating agent (such as acetic anhydride), it combines with aldehydes to form compounds of the type R·CH:C(COOH)2, or their decomposition products (formed by loss of CO2) R·CH:CH·COOH.
Many salts of the acid are known and, with the exception of those of the alkali metals, they are difficultly soluble in water. Many esters of malonic acid have been prepared, the most important being the diethyl ester (malonic ester), CH2(COOC2H5)2, which is obtained by dissolving monochloracetic acid in water, neutralizing the solution with potassium carbonate, and then adding potassium cyanide and warming the mixture until the reaction begins. When the reaction has finished, the whole is evaporated and heated to about 130°–140° C. and then allowed to cool. The mass is then covered with two-thirds of its weight of alcohol, and saturated with hydrochloric acid gas. The whole is then poured into ice-cold water, extracted by ether and the ethereal solution distilled (L. Claisen, Ann., 1883, 218, p. 131). It is a colourless liquid boiling at 197°.7–198°.2 C. (W. H. Perkin). It is a most important synthetic reagent; with sodium or sodium ethylate it forms sodio-malonic ester, which reacts readily with alkyl halides, forming alkyl malonic esters, which are again capable of forming sodium derivatives, that by further treatment with alkyl halides yield the di-alkyl malonic esters. These esters are readily hydrolysed and yield the mono- and di-alkyl malonic acids which, on heating, are readily decomposed, with evolution of carbon dioxide and the formation of mono- and di-alkyl acetic acids. The scheme of reactions is shown thus:
| R′I | |
| CH2(COOR)2→CHNa(COOR)2 | →CHR′(COOR)2 |
| ↓ NaOH | |
| CO2 + CH2R′·COOH ← | CHR′(COOH)2 |
| R″I | |
| [CHR′(COOH)2NaOH → | CNaR′(COOR)2 → CR′R″(COOR)2 |
| ↓ NaOH | |
| CO2 + | CHR′R″·COOH ← CR′R″(COOH)2 |
When sodio-malonic ester is heated to 145° C., it undergoes condensation,
with elimination of alcohol and formation of the benzene
derivative, phloroglucin tricarboxylic ester. The addition of urea
to an alcoholic solution of sodio-malonic ester results in the formation
of barbituric acid (A. Michael, Jour. pr. Chem., 1887 [2], 35, p. 456)
The half nitrile of malonic acid is cyanacetic acid, CN·CH·2 COOH,
which, in the form of its ester, may be obtained by the action of a
solution of potassium cyanide on monochloracetic acid. The solution
obtained is neutralized, concentrated on the water-bath, acidified
by sulphuric acid and extracted with ether. It is then converted
into the lead salt, which is decomposed by sulphuretted hydrogen
and the solution is carefully concentrated (Th. Meves, Ann., 1867,
143, p. 201). It melts at 70° C. and at higher temperatures decomposes,
with evolution of carbon dioxide and formation of aceto-nitrile,
CH3·CN. The true nitrile of malonic acid is methylene cyanide,
CH2(CN)2, which is obtained by distilling a mixture of cyanacetamide
and phosphorus pentoxide. It is a crystalline solid, which
melts at 29°–30° C. and boils at 218°–219° C., and is readily soluble
in alcohol and ether.