Few weeks ago Swid bought Vogel's textbook of practical organic chemistry and found an interesting synthesis written inside. Let's look at it.
5-ethyl-5-phenylbarbituric acid (Phenobarbitone)
In a 250 mL RB flask, fitted with an efficient reflux condenser and guard-tube, prepare a solution of sodium methoxide from 4.6 g (0.2 mol) of clean sodium metal and 50 mL of anhydrous methanol. Add 15 g (0.25 mol) of urea (previously dried at 60° C for 4 hours), and then 26.4 g (0.1 mol) of diethyl ethylphenylmalonate dropwise down the condenser. Reflux the mixture for 6 hours and then remove the excess methanol under the redused pressure (rotavap) from a water bath held below 60° C. Transfer the residue to a small beaker cooled in a freezing mixture and add 100 mL of ice-water with mechanical stirring: the temperature of the reaction mixture must be kept below 5° C since barbiturates are decomposed by a concentrated alkali into the salt of the corresponding malonic acid, Na
2CO
3 and NH
3. Filter and extract the filtrate with two 50 mL portions of toluene in order to remove esters; acidify the aqueous solution cautiously to Congo red, allow to stand for a few hours and filter off the crude phenobarbitone at the pump. The yield after drying at 90-100° C is 13 g (56%). Recrystallisation from hot water yields reasonably pure ethylphenylbarbituric acid, m.p. 171° C. A somewhat higher m.p (175-176° C) is obtained if rectified spirit is employed for recrystallisation, but the recovery is considerably less.
Note. (1) The rate of addition should be controlled so that the reaction mixture does not bocome strongly basic.
References:Vogel's textbook of practical organic chemistry, 5
th edition, p. 1177
That's all for now. Synthesis of diethyl ethylphenylmalonate will be posted soon. If you can't wait, there is one at the Rhodium's site:
https://www.rhodium.ws
Give me some chemicals and I'll give you pleasure.
