Method 1: Add 160 mL of water, 50 mL of acetone, 37.2 mL of glacial acetic acid and 4.0 g of potassium chlorate into a 500 mL four-neck flask, and install a mechanical stirrer, thermometer, reflux condenser and long-neck dropping funnel. Start the mechanical stirring, control the temperature of the water bath at 60-75 °C, and the temperature in the bottle at about 60 °C, add 35.4 mL of bromine with a dropping funnel, and control the drop rate to prevent unreacted bromine from accumulating. After the addition of bromine, the reaction was continued for 0.5 h. After the color of the solution faded, 80 mL of ice water was added to dilute the reaction solution, cooled to 10 °C, and then 100 g of anhydrous sodium carbonate was added to make the Congo red neutral. The oil layer was separated, dried with 8g of anhydrous calcium chloride, distilled under reduced pressure, and the fractions at 40-42°C/1733Pa were collected, and the yield was 63.5% (the literature was 50%-51%). The collected fractions and DMSO were added into a 250 mL three-necked flask according to an appropriate ratio of ingredients, air was introduced, and the temperature was controlled at a suitable temperature, and the reaction was carried out under reflux for 8 hours under electromagnetic stirring. After the reaction was completed, fractional distillation was carried out at atmospheric pressure, and the fractions at 71.5-73.0 °C were collected, and the refractive index was n24.6D=1.3803 (document n20D=1.4209).
Method 2: Weigh 89.4 g (0.3 mol) of potassium dichromate dihydrate, dissolve it in 400 mL of water, slowly add 63.9 mL (1.2 mol) of concentrated sulfuric acid dropwise while stirring, stir well, and after cooling, transfer to a dropping funnel middle. In a 1000mL three-necked flask, first add 32.9mL (0.45mol) of 1,2-propanediol and 1.0g of manganese sulfate as catalysts, then connect a distillation device to the middle port, a dropping funnel at one port, and a glass stopper at the other port. Glass tube, the lower mouth of the glass tube should be inserted below the liquid level. First, introduce nitrogen gas into the three-necked flask from the glass tube, and then heat the three-necked flask with an electric heating jacket with a pressure regulator. After a few minutes, drip the above prepared solution from the dropping funnel, and the dripping is completed in about 50-70 minutes.
After dripping, continue heating and distillation for 15min (pay attention to closing the piston of the dropping funnel), and collect about 250mL of yellow-green distillate. The distillate was extracted four times with 450 mL of anhydrous ether. After extraction, the aqueous layer was discarded, and the extracts were combined. Wash with saturated aqueous sodium bicarbonate until neutral, separate the aqueous layer, and dry the ether layer with anhydrous magnesium sulfate for 4 h. Filtration, water bath 45～50℃ to evaporate ether, then boiling water bath fractionation, collecting 71～73℃ fractions to obtain yellow-green liquid with special odor, yield 49%～50%. The product was stored at 0°C under nitrogen. After inspection, this fraction can make Schiff’s reagent appear purple, make Fehling’s reagent appear brick-red precipitate, and form orange-red precipitate with 2,4-dinitrobenzene trap (recrystallized in nitrobenzene), measure its melting point It is 308～309 ℃ (the literature is 308～309 ℃). After purification, the boiling point of glyoxal measured by micro method is 72°C (the literature is 72°C).