Preparation of Ethyl Acetoacetate
Introduction
The condensation of two molecules of an ester (e.g. ethyl acetate), or of two molecules of different esters, or of one molecule of an ester with one molecule of a ketone under the influence of sodium or sodium ethoxide, is termed Claisen condensation, and is one of the best methods for preparing beta-ketonic esters and beta-diketones. For the purpose of the preparation the ethyl acetate must be carefully purified and freed from traces of acetic acid and water, and most of the alcohol. The latter must not be completely removed, for then the reaction proceeds very slowly.
Purification of Ethyl Acetate
- Inglis and Knight recommend the following method of purification: The ester is dried over anhydrous potassium carbonate and then treated with phosphoric anhydride (10g for 100g of ester), and boiled in a reflux apparatus for 20 minutes. It is distilled off, using a fractionating column, the portion boiling between 76.7°C and 77.2°C being collected. This process gives an pure substance which gives good yields of ethyl acetoacetate.
- The ester is allowed to stand for a day over freshly heated granular calcium chloride in a well-stoppered bottle, filtered into a dry distilling flask and redistilled, care being taken that all parts of the apparatus are perfectly dry. The fraction boiling between 76.7-77.2°C is collected.
Preparation of Ethyl Acetoacetate
30g of clean sodium are cut into thin strips, or better, pressed into wire by means of a sodium press (or cut in small pieces under kerosene), and introduced into a clean, dry, round-bottomed flask. The latter is attached to a long reflux condenser inclined obliquely, and fixed on a water bath. 300g of the purified ethyl acetate is introduced through the condenser tube, which is then fitted with a drying tube, and after some 15 minutes the water bath is gradually warmed, the temperature being regulated so that gentle ebullition takes place within the flask. The heating is continued for 3 hours, or until all the sodium is dissolved, when a mixture of 100g of glacial acetic acid with an equal quantity of water is added gradually until the contents of the flask show an acid reaction. If any solid is deposited, it is redissolved by vigorously shaking the flask. The liquid is then poured into a separating funnel containing an equal volume of saturated common salt solution, the mixture shaken, and the lower aqueous layer removed. The upper layer, consisting of ethyl acetate and ethyl acetoacetate, is fractionally distilled. For this purpose the whole is placed in a large distilling flask, one or two pieces of porous pot are added, and the flask heated in a rapidly boiling water bath until the thermometer indicates a temperature of 90-95°C, when the residue in the flask is subjected to distillation under diminished pressure. Small amounts of impurities first pass over, and when the temperature rises to the bp of the procuct the receiver is changed, and the acetoacetic ester collected. Should a large amount of liquid distill over before the correct temperature for the given pressure is attained, the first fraction is redistilled and a further yield of the ester thereby obtained. The yield is 45-50g.
Note: The yield is considerably lowered if the whole experiment is not completed in one day. The ester is a colourless mobile liquid with a pleasant fruity odour. Its boiling-point at atmospherical pressure is 181°C, but it undergoes slight decomposition under these conditions; it is very sparingly soluble in water, but dissolves in alcohol and ether.
