ACETO-ACETIC ESTER, ethyl acetoacetate is an out standing example of a chemical substance having a dual character arising from the possession of two different molecular structures. Such substances are called tautomeric compounds (see ISOMER ISM). The substance is an ester (which see), and has the molecu lar formula The two tautomeric forms are repre sented by and ordinary specimens of the ester as handled in commerce may be regarded as consisting of 93% of the former in equilibrium with 7% of the latter. When treated by reagents which combine with ketones, such as sodium bisulphite, hydroxylamine, phenylhy drazine and hydrocyanic acid, the ketonic character of the ester is manifested, and when acted upon by reagents such as phosphorus pentachloride, diazomethane, ammonia and amines, which detect alcoholic (enolic) groups, then its enolic nature is revealed.
In 1911 L. Knorr separated the two forms of the ester in a State of purity. The ketonic modification was frozen out of the equilibrium mixture at —78° C and the enolic modification was isolated in the liquid condition by decomposing the sodium deriv ative with hydrogen chloride at the same low temperature. In these experiments it was found to be essential to use silica vessels. The ester was first discovered by A. Geuther in 1863 and its ketonic nature was unravelled by E. Frankland and B. F. Duppa in 1865. The original method of production is still em ployed on a manufacturing scale. Sodium, either molten or in wire form, is added to dry ethyl acetate containing a little ethyl alcohol. When all the metal has dissolved the mixture is acidified with dilute acetic or sulphuric acid and the crude acetoacetic ester, which is partially miscible with water, separates as an oil on addition of common salt and is purified by distillation under diminished pressure. The ester is a colourless, fragrant liquid boiling, with slight decomposition, at 181° C under a pressure of 76o mm.
In the sodium condensation a little alcohol is needed to pro mote the solubility of the metal and this action of alcohol is indicated in the following equations :— In the chemical laboratory acetoacetic ester is a valuable syn thetic reagent, for its sodium derivative when acted on by an alkyl iodide yields an alkylacetoacetic ester, and the sodium derivative of the latter ester by similar means furnishes a dialkyl acetoacetic ester. These alkyl- and dialkyl-acetoacetic esters may be employed in producing either higher ketones or higher fatty acids (see CHEMISTRY: Organic, Aliphatic). The ester has also been employed in the synthesis of pyridines (q.v.), quino lines (q.v.), furfuranes (q.v.), pyrazoles (q.v.), pyrroles (q.v.), and compounds of the purine group (q.v.).
In addition to the foregoing scientific uses acetoacetic ester is utilized industrially in the manufacture of synthetic drugs and dyes. Of these drugs the best known is antipyrine (q.v.) which is prepared by methylating phenylmethylpyrazolone, the conden sation product of acetoacetic ester and phenylhydrazine.
Several important dyes belong to the pyrazolone series. Erio chrome red (Chrome fast red B) is manufactured by condensing acetoacetic ester with phenylhydrazine and with diaz otized I-amino-O-naphthol-4-sulphonic acid. A series of yellow dyes and pigment colours is similarly produced by condensing the ester successively with various aromatic hydrazines and diazo compounds (see DYES, SYNTHETIC).