In a 12-l. earthenware crock equipped with an efficient stirrer and a strong motor are placed 375 g. (2.5 moles) of mandelic acid (p. 336) and 500 cc. of water. The stirrer is started, and a cool solution of 110 g. (2.8 moles) of sodium hydroxide (or 115 g. of technical sodium hydroxide) in 500 cc. of water is added. This is followed by the addition of 2000 g. of cracked ice and then after a few minutes by 275 g. (1.74 moles) of finely ground u.s.p. potassium permanganate in portions over a period of one-half hour. The mixture is stirred for one and one-half hours, the temperature being maintained at −2° to −4°. About 5 kg. of ice is necessary. At the end of one and one-half hours the mixture is tested for excess permanganate by placing a drop on a piece of filter paper; a black or brown center of manganese dioxide is formed, a greenish ring around this, and an outer ring which is colorless in the absence and pink in the presence of excess permanganate. Any excess is removed by adding ethyl alcohol, generally about 200–300 cc.

When the permanganate has been destroyed, the stirrer is stopped and the manganese dioxide allowed to coagulate for one hour. The mixture is filtered through two 20-cm. Büchner funnels; the filter cakes are washed with 1000 cc. of water (Note 1) and sucked dry again (Note 2). The filtrate is evaporated to 800–1000 cc. over a free flame. While the volume of solution is large, two 30-cm. evaporating dishes are used, but toward the end the two solutions are combined in one dish. The evaporation requires two to three hours. Sometimes a little manganese dioxide will be coagulated during the evaporation; this should be removed by filtration.

The yellowish solution is transferred to a 3-l. round-bottomed, wide-necked flask and cooled with running water while 300 cc. of concentrated sulfuric acid is added slowly from a separatory funnel. This acidification process requires thirty minutes, during which time the temperature should not rise much above 25°. The liquid froths and about 30 g. of benzoic acid separates but it is not filtered. Extraction of the reaction mixture with 200-cc. portions of ether until 1500 cc. of extract has been collected is now carried out in a separatory funnel. The ether is distilled on a steam bath from a 1-l. round-bottomed, wide-necked flask connected with a water condenser, the extract being added from a separatory funnel as fast as the ether distils. After most of the ether has been removed, the heating is continued for an additional one-half hour.

The crude liquid acid (about 300 g.) is cooled with running water, and 15 cc. of concentrated sulfuric acid is added; the clear yellow oil becomes cloudy. The acid is esterified by passing the vapor of ethyl alcohol into the hot solution as in the preparation of ethyl fumarate in Org. Syn. 10, 48, or by the method described for ethyl cyanoacetate in Org. Syn. 3, 54, with the following changes: all three flasks, the alcohol flask, the trap, and the esterification flask are 1-l. round-bottomed, wide-necked flasks. The alcohol flask and the trap are set on adjacent steam baths. The esterification flask is placed in an oil bath which is heated by a free flame. A safety tube leads from the alcohol flask and dips under 7.5 cm. of mercury contained in a side-arm tube. This mercury safety valve avoids all dangers which might result from the development of excess pressure within the system (Note 3). The temperature of the esterification mixture is kept at 105–110° and ethyl alcohol vapor is passed through until 500 cc. of distillate has been collected. This requires about three hours. The distillate consists of alcohol, water, and a little ether.

The crude ester is cooled, an equal volume of benzene is added, then the free acid is neutralized by shaking with about 250 cc. of a 10 per cent solution of sodium carbonate (Note 4). The benzene solution is poured into 1300 cc. of a saturated solution of sodium bisulfite (about 60 g. of technical sodium bisulfite per 100 cc.), contained in a wide-necked bottle equipped with an efficient stirrer, and the mixture stirred for two and one-half hours. The mixture soon warms up a little and becomes semi-solid. It is filtered through a 20-cm. Büchner funnel and carefully washed, first with 200 cc. of a saturated solution of sodium bisulfite, finally with two 150-cc. portions of benzene (Note 5) and (Note 6). The white pearly flakes of the sodium bisulfite addition product are transferred to a 3-l. round-bottomed, wide-necked flask equipped with a mechanical stirrer and containing 700 cc. of water, 175 cc. of concentrated sulfuric acid, and 500 cc. of benzene. The flask is heated on a steam bath under a hood, the temperature being kept at 55°, and the mixture is stirred for thirty minutes (Note 7).

The solution is then poured into a separatory funnel, the benzene separated, and the water layer extracted with a 200-cc. portion of benzene. The combined benzene solution is shaken with excess of 10 per cent sodium carbonate solution to remove free acid and sulfur dioxide (Note 8). The benzene is washed with a little water and then dried over anhydrous potassium carbonate (Note 9). The benzene is distilled at ordinary pressure over a free flame from a 500-cc. Claisen flask, the solution being added from a separatory funnel as fast as the benzene distils. It is advisable to distil the ester under reduced pressure although it can be done under ordinary pressure. The fraction distilling around 118°/5 mm., 130°/10 mm., 138°/15 mm., 148°/25 mm., 155°35 mm., or 254°/760 mm. is collected. The yield of ethyl benzoylformate is 155–175 g. (35–40 per cent of the theoretical amount) (Note 10) and (Note 11).