A Publication
of Reliable Methods
for the Preparation
of Organic Compounds
Annual Volume
Org. Synth. 1951, 31, 23
DOI: 10.15227/orgsyn.031.0023
Submitted by Richard H. Wiley and Newton R. Smith1,2.
Checked by C. F. H. Allen and George A. Reynolds.
1. Procedure
In a 2-l. round-bottomed flask are placed 200 g. (1.49 moles) of powdered malic acid (Note 1) and 170 ml. of concentrated sulfuric acid. To this suspension are added three 50-ml. portions of 20–30% fuming sulfuric acid at 45-minute intervals. After the evolution of gas has slackened, the solution is heated on a water bath for 2 hours with occasional shaking. The reaction mixture is then cooled and poured slowly onto 800 g. of crushed ice with stirring. After standing 24 hours, the acid is filtered on a Büchner funnel, washed with three 50-ml. portions of ice-cold water, and dried on a water bath. The yield of crude acid, melting at 195–200°, is 75–80 g. (Note 2) and (Note 3).
One-half of the crude product is dissolved in five times its weight of hot methanol, and the solution is boiled with 3 g. of Norit or decolorizing carbon. The solution is filtered while hot and cooled in an ice bath. The precipitate is collected on a filter and washed with 25 ml. of cold methanol. The mother liquors are used to recrystallize the remaining crude material. The yield of bright yellow coumalic acid, melting at 206–209°, is 68–73 g. (65–70%) (Note 4).
2. Notes
1. A technical free-flowing powder, melting at 126–128°, was used.
2. This washing is essential to remove the mineral acid and to avoid partial esterification that otherwise takes place during the methanol recrystallization step.
3. The submitters state that an additional 10–12 g. of crude acid can be obtained from the filtrate by extraction with ether in a continuous extractor.
4. Depending on the color of the crude acid, several additional recrystallizations may be required to obtain a colorless product.
3. Discussion
This procedure is essentially that of von Pechmann.3 Esters of coumalic acid may be obtained by heating the sulfuric acid solution with the appropriate alcohol.4
This preparation is referenced from:

References and Notes
  1. University of Louisville, Louisville, Kentucky.
  2. The submitters wish to thank the Research Corporation for a grant under which this work was done.
  3. von Pechmann, Ann., 264, 272 (1891).
  4. Campbell and Hunt, J. Chem. Soc., 1947, 1176; Gilman and Burtner, J. Am. Chem. Soc., 55, 2903 (1933); Ruzicka, Helv. Chim. Acta, 4, 504 (1921).

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

sulfuric acid (7664-93-9)

methanol (67-56-1)

ether (60-29-7)

Norit (7782-42-5)

malic acid (617-48-1)

Coumalic acid (500-05-0)