Org. Synth. 1956, 36, 36
DOI: 10.15227/orgsyn.036.0036
2-FUROIC ACID
Submitted by R. J. Harrisson and M. Moyle
1.
Checked by James Cason and W. N. Baxter.
1. Procedure
A 1-l. flask (Note 1) is fitted with a condenser, an efficient stirrer, two dropping funnels, a thermometer extending well into the flask, and a delivery tube which extends far enough to be below the surface of the stirred liquid.
To the flask is added 250 ml. of a 2.5% solution of sodium hydroxide and a cuprous oxide-silver oxide catalyst (Note 2). In the two dropping funnels are placed, respectively, 96 g. (1.0 mole) of furfural (Note 3) and a solution of 40 g. (1.0 mole) of sodium hydroxide in 100 ml. of water. The contents of the flask are heated to about 55°, vigorous stirring is started, and a rapid stream of oxygen is bubbled through as the contents of the two funnels are added simultaneously (Note 4) to the reaction mixture at such a rate (20–25 min.) as to maintain the temperature at 50–55° (Note 5) without external heating. After the additions have been completed, vigorous stirring and passage of oxygen are continued until the temperature drops below 40° (15–30 min., depending on the rate of oxygen flow).
The catalyst is separated by filtration, and the aqueous solution is extracted with three 30-ml. portions of ether (Note 6), acidified with 30% sulfuric acid, boiled for 45 min. (Note 7) with 6–7 g. of carbon, then filtered hot. The filtrate is cooled to 0° and allowed to stand at this temperature for 1 hour or longer. The 2-furoic acid, which separates as pale pink needles, m.p. 130–132°, is collected by suction filtration and washed with small portions of ice water (Note 8). The yield is 96–101 g. (86–90%).
2. Notes
1.
Since
oxygen is passed through the solution at 55°, it is best to operate under a
reflux condenser. Ground joint fittings are not essential but convenient. The checkers used a
4-necked flask with ground joints; the thermometer was hung through the condenser, and one neck was arranged for attachment of the
gas delivery tube and one of the separatory funnels.
2.
The catalyst used is commercial
cuprous oxide (9.6 g., 10% of the weight of the furfural) and commercial
silver nitrate (0.5 g., 0.5% of the weight of the furfural) in 15 ml. of water. The
cuprous oxide is suspended in the rapidly stirred
2.5% sodium hydroxide solution, and the solution of
silver nitrate is added to give a dark brown suspension of
cuprous oxide and
silver oxide which is used directly. So long as
oxygen is being passed through the reaction mixture, the life of the catalyst appears to be unlimited.
3.
A sample of commercial
furfural is purified by simple distillation or steam distillation immediately before use. The submitters have carried out this reaction on an 8-mole scale following the same procedure except that a total reaction time of about 4 hours was required.
4.
The simultaneous addition of
furfural and
sodium hydroxide maintains the concentration of
sodium hydroxide at about 2.5%, at which concentration only a small percentage of
furfuryl alcohol is formed in the accompanying Cannizzaro reaction.
5.
Below 50°, the reaction is too slow; above 55°, the reaction becomes violent and cooling becomes necessary. Frothing may be checked by addition of small amounts of
benzene.
6.
Ether extraction at this stage removes
furfuryl alcohol (about 12 g.). If this is not removed, the reaction mixture becomes resinous on acidification and heating.
7.
During the period of boiling in acid solution a small amount of polymeric material forms, and this is separated when the charcoal is removed by filtration.
8.
The solubility of
2-furoic acid in water is 2.8 g. per 100 ml. at 0°; however, its solubility in the salt solution resulting from the reaction is quite low. Usually it is not possible to obtain further material from the filtrate by salting out with
sodium chloride; however, in case a lowered yield is obtained in the first crop it may be profitable to investigate the possibility of obtaining additional material from the filtrate.
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3. Discussion
2-Furoic acid has been made by oxidation of
lactose followed by pyrolysis, by the oxidation of
2-acetylfuran,
2-methylfuran, or
furfuryl alcohol using
potassium ferricyanide in alkaline medium, and by other methods already listed.
2 In addition,
furfural has been oxidized to
furoic acid by air in the presence of
sodium hydroxide and
silver nitrate,
3 by
hydrogen peroxide in the presence of
pyridine or
picoline,
4 by
sodium or potassium hypochlorite,
5 by means of
sodium hydroxide in
methanol or
sodium hydroxide in
methanol followed by
hydrogen peroxide,
6 and by
sodium hydroxide in the presence of a
copper oxide-
silver oxide-
carbon catalyst.
7
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
sulfuric acid (7664-93-9)
Benzene (71-43-2)
methanol (67-56-1)
ether (60-29-7)
sodium hydroxide (1310-73-2)
sodium chloride (7647-14-5)
silver oxide (20667-12-3)
silver nitrate (7761-88-8)
oxygen (7782-44-7)
carbon (7782-42-5)
pyridine (110-86-1)
cuprous oxide
hydrogen peroxide (7722-84-1)
2-Furoic acid,
furoic acid (88-14-2)
copper oxide (1317-38-0)
Furfural (98-01-1)
Furfuryl alcohol (98-00-0)
sodium hypochlorite (7681-52-9)
potassium hypochlorite
potassium ferricyanide (13746-66-2)
picoline (109-06-8)
lactose
2-acetylfuran (1192-62-7)
2-methylfuran (534-22-5)
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