A colourless liquid ester with a pleasant odour, CH₃COCH₂COOC₂H₅; r.d. 1.03; m.p. <–80°C; b.p. 180.4°C. It can be prepared by reacting ethyl ethanoate (CH₃COOC₂H₅) with sodium or sodium ethoxide. The compound shows keto–enol tautomerism and contains about 7% of the enol form, CH₃C(OH)=CHCOOC₂H₅, under normal conditions. Sometimes known as acetoacetic ester, it is used in organic synthesis.