Org. Synth. 1930, 10, 78
DOI: 10.15227/orgsyn.010.0078
[Oxalic acid, dimethyl ester]
Submitted by Everett Bowden
Checked by C. S. Marvel and A. T. Koide.
1. Procedure
In a 500-cc. Pyrex flask, fitted with a cork which loosely carries a glass mechanical stirrer and a separatory funnel, are placed 90 g. (1 mole) of anhydrous oxalic acid (Org. Syn. Coll. Vol. I, 1941, 421) and 100 cc. (79 g., 2.5 moles) of methanol (Note 1). Then, while the mixture is rapidly stirred, 35 cc. of pure concentrated sulfuric acid (Note 2) is slowly added through the separatory funnel. The mixture is heated, if necessary (Note 3), nearly to boiling, then filtered as rapidly as possible through a 15-cm. filter paper placed in a slightly heated glass funnel, the filtrate being collected in a 500-cc. wide-mouthed Erlenmeyer flask. The first flask is rinsed with 40 cc. of hot methanol, which is poured through the filter paper. After twenty-four hours at 15° (Note 4) the crystals are filtered with suction, sucked as dry as possible, pressed between filter paper, and air-dried for a few minutes. The filtrate, after cooling to about −10°, is filtered rapidly and the product dried as before. A total of 100–115 g. of material, slightly moist with sulfuric acid and melting at 50–52°, is obtained.
For purification, the crude product is dissolved in 100 cc. of redistilled methanol, filtered through a warm funnel, and allowed to crystallize. After several hours the crystals are filtered, and the filtrate is chilled and filtered as before. A total of 80–90 g. (68–76 per cent of the theoretical amount) of methyl oxalate, melting at 52.5–53.5° (Note 5), is obtained.
2. Notes
1. The methanol used is the commercial (almost acetone-free) grade known as Columbian Spirits. This material is redistilled for recrystallizing the methyl oxalate.
2. Less sulfuric acid than that employed gives a smaller yield; larger quantities sometimes result in a product that causes difficulty in filtration. It is essential that the acid be added slowly and with vigorous stirring to prevent local superheating and darkening of the solution and product. Some commercial grades of methanol become quite dark in contact with sulfuric acid. The grade of methanol used here does not discolor, the filtrates being only light yellow.
3. Solution of the oxalic acid causes a sharp drop in temperature whereas solution of the sulfuric acid raises the temperature.
4. The major portion of the reaction is complete within a few minutes, but several hours are necessary for complete crystallization.
5. Larger batches give the same percentage yield as the one described. When several batches are to be run, the alcohol from the first recrystallization becomes the starting alcohol for the second batch, etc. This increases the yield somewhat.
3. Discussion
Methyl oxalate has been prepared by distilling a mixture of oxalic acid, methyl alcohol, and sulfuric acid;1 by dissolving anhydrous oxalic acid in hot methyl alcohol;2 by esterifying oxalic acid with methyl alcohol, using anhydrous hydrogen chloride as a catalyst;3 by methanolysis of ethyl oxalate;4 by passing vapors of dry methanol through hydrated oxalic acid until the water has been removed;5 and by a process in which the methyl alcohol-water mixture evolved from hydrated oxalic acid and methanol is dried over potassium carbonate and returned to the reaction flask.6 The method described in the procedure is simpler than any of these and gives very satisfactory yields.

References and Notes
  1. Dumas and Peligot, Ann. chim. phys. (2) 58, 44 (1835); Ann. 15, 32 (1835); Wöhler, ibid. 81, 376 (1852).
  2. Erlenmeyer, Jahresb. 1874, 572.
  3. Rising and Stieglitz, J. Am. Chem. Soc. 40, 726 (1918).
  4. Pfannl, Monatsh. 31, 316 (1910); Reimer and Downes, J. Am. Chem. Soc. 43, 950 (1921).
  5. Dutt, J. Chem. Soc. 123, 2714 (1923).
  6. Kenyon, Org. Syn. Coll. Vol. I, 1941, 264.

Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

potassium carbonate (584-08-7)

sulfuric acid (7664-93-9)

hydrogen chloride (7647-01-0)

methyl alcohol,
methanol (67-56-1)

Oxalic acid (144-62-7)

Ethyl oxalate

Methyl oxalate

Oxalic acid, dimethyl ester (553-90-2)