[CONTRIBUTION FROM
THE
CHEMICAL LABORATORY OF THEOHIOSTATE UNIVERSITY ]
THE MECHANISM OF CARBOHYDRATE OXIDATION. XVIII.* T H E OXIDATION OF CERTAIN SUGARS WITH SILVER OXIDE I N THE PRESENCE OF POTASSIUM HYDROXIDE K. G.A. BUSCH, J. W. CLARK, L. B. GENUNG, E. F. SCHROEDER, W. L. EVANS
AND
Received February 16, 1986
A clearer understanding of the molecular reaction mechanism involved in the oxidation of various carbohydrates may be obtained through an experimental study of the behavior of these important compounds towards reagents that will yield oxidation products differing respectively both in kind and in number. Glucose may be oxidized completely by alkaline potassium permanganate solutions' to carbon dioxide, and oxalic and traces of acetic acids, while with silver oxide under the same conditions, carbon dioxide, and oxalic, glycolic, and formic acids are the final reaction products. I n the acid medium of copper acetate solutions containing an excess of this salt, glucose may be oxidized to glucosone, carbon dioxide, and formic, oxalic, and glyoxylic acids? A comparative study of the data obtained through the use of reagents3 of differing oxidation potential on the sugars and their various theoretical degradation and oxidation intermediates seems to offer a fruitful method of attack on this important oxidation problem. The reagent chosen for the studies reported in this paper was silver oxide, both alone and in the presence of added alkali. K i l i a ~ ~Nef,6 i,~ Behrend and DreyerlBDenis' and Witzemann* are among those who have studied the action of this reagent on various sugars and their intermediate degradation compounds. The use of silver oxide in the study of carbohydrate oxidation offers certain unique advantages. The oxidation products formed are carbon dioxide, and oxalic, formic and glycolic *Contribution XVII of this series, J. Am. Chem. SOC.,67, 200 (1935). This article was submitted in response to the invitation of the editors. EVANSAND COLLABORATORS, J . Am. Chem. Soc., 47, 3085, 3098, 3102 (1925). AND WARINC,ibid., 60, 2543 (1928). EVANS,NICOLL, STROUSE a KARRER AND PFAEHLER, Helv. Chim. Acta, 17, 363, 766 (1935). KILIANI,Ber., 13,2703 (1880); Ann., 206, 187, 191 (1880). NEF, Ann., 367, 287 (1907). BEHREND AND DREYER, Ann., 416, 203 (1918). DENIS,Am. Chem. J., 38, 578 (1907). * WITZEMAN, Ph.D. dissertation, The Ohio State University, (1912). 1 TEE JOURNAL OF ORGANIC CHDb%IBTEY,VOL.
1, NO. 1
2
BUSCH, CLARK, GENUNG, SCHROEDER, AND EVANS
acids, all of which can be determined quantitatively. The silver remaining after oxidation may be easily separated from the unchanged silver oxide, and from its weight the oxygen consumed may be calculated. The reaction is comparatively rapid and hence does not necessitate a long time for its completion. The main objective of these experiments was to study the behavior of mannose, fructose, arabinose and compounds related to these carbohydrates towards silver oxide in the presence and absence of alkalies under carefully controlled conditions for the purpose of obtaining accurate quantitative data which might shed more light on the mechanism involved in the oxidation reaction. EXPERIMENTAL
Reagents.-All the reagents used in these experiments were examined for their purity by the usual well-known laboratory procedures. Carbohydrates.-The carbohydrates used were of the highest obtainable purity. Their identities were verified by determinations of their specific rotations, and by other means when necessary. Silver Oxide.-A solution of 400 g. of AgNOa in 1200 cc. of distilled water was vigorously stirred and a solution of 150 g. of KOH in 800 cc. of water was added a t the rate of 100 cc. per minute. The size of the batch was later increased to as much as 2000 g. of AgNOa but the same concentrations were always maintained. The brown Ag,O thus precipitated was washed with water by decantation until the wash water was free of Ag+ and a 100-cc. sample required less than 0.3 cc. of 0.1N HC1 to neutralize the alkali present. This usually required about ten washings. The AgzO was then dried at 110°C. under vacuum. When dried a t this temperature, i t was changed from the chocolate-brown color of the freshly precipitated oxide to a dark purplish-brown. If dried a t 85"C., the original brown color was retained. After drying, the oxide was passed through a 100-mesh sieve, placed in brown bottles and stored in the dark. It was believed a t first that different lots of silver oxide would give slightly different results, but i t was later shown that when the above directions were carefully followed, uniform results were always obtained. AgnO was analyzed before using for total silver, ammonia-insoluble matter and carbon dioxide. The analysis of three typical batches of the AgzO thus prepared and labeled (a), (b), and (c) was as follows: Silver, (a) 92.6%; (b) 92.20%; (c) 92.95%; theoretical 93.1%: COz, (a) 0.10%; (b) 0.05%; (c) 0.03%: Ammonia-insoluble, (a) 0.07%; (b) 0.09%; (c) 0.12%. Apparatus and Analytical Procedures.-Seven grams of silver oxide was added to 100 cc. of 1.ON KOH contained in a 150-cc. carbon dioxide flask, fitted with a stopper carrying a thermometer and a small stopcock. The flask was then placed in the thermostat maintained at 50" and the reagent was allowed to come to temperature, after which one four-hundredth of a mole of the sugar (e.g., 0.45 g. of mannose, glucose, or fructose, or 0.375 g. of arabinose) was added, the stopper was inserted in the flask and the stopcock closed. By closing the stopcock after inserting the stopper, any pressure effect caused by forcing in the latter was prevented. When this was done, i t was found unnecessary to wire on the stoppers to prevent them being blown out, and no loss of carbon dioxide was experienced. In the experiments carried out in the absence of added alkali, the only change made in the above procedures waS t h a t of using 11 g. of AgzO instead of 7, and carbon-dioxide-free water
MECHANISM OF CARBOHYDRATE OXIDATION
3
was used in place of the 1.ON alkali. The reaction mixtures were then agitated by a mechanical shaker during the various periods of time indicated in Fig. 1. This technique was attended by a quick rise in temperature with a maximum of 5”, which rapidly subsided to the thermostatic temperature. Determination of Consumed Oxygen.-After the reaction flasks were removed from the thermostat they were cooled and the reaction mixture was decanted through a previously weighed and dried Gooch crucible. The silver-silver oxide mixture remaining in the flask was washed several times by decantation and the filtrate and washings were made up to a volume of 250 cc. This solution was used for the determination of oxalic, formic, and glycolic acids. To the residue in the flask was added 50 cc. of dilute NHaOH (1:3)and the flask was shaken vigorously to hasten solution of the Ag20. The undissolved silver was allowed to settle and the liquid was decanted through the Gooch crucible. The ammoniacal solution, thus obtained, was immediately added to a beaker containing an excess of HC1 to prevent explosions of the kind reported by previous workers. The silver residue was washed with two more 50-cc. portions of the NHaOH, the final washing being tested with HC1 to make certain that all the silver oxide was removed. The Ag residue was now transferred to the Gooch crucible, washed with water, dried in a vacuum oven a t llO”C., and weighed. The oxygen consumed in the oxidation was calculated from the weight of silver thus obtained after correcting for ammonia-insoluble impurities. Cai-bon Dioxide.-The determination of carbon dioxide was made on a second sample obtained in exactly the same manner as the sample used for the determination of the acids. The flask was cooled to a temperature below 40°C. as soon as it was removed from the thermostat and was then connected to a carbon dioxide apparatus aThich was essentially the same as that described by F o ~ l k . ~ O d i c Acid.-Oxalic acid was determined by precipitating as calcium oxalate with calcium acetate in the presence of acetic acid and then titrating the calcium oxalate with potassium permanganate in the usual manner. The calcium content was verified by conversion of the oxalate to the sulfate. Foiemic Acid.-Two-fifths of the filtrate from the oxidrttion mixture was placed in a 500-co. round-bottom flask fitted with a dropping funnel, capillary tube, andKjeldah1 bulb. The latter was connected, with an adapter, to a spiral water condenser, theend of which extended to the bottom of a 500-cc. suction flask. Sufficient6.OMphosphoric acid was added both to neutralize the alkali present in the oxidation mixture and t o form the monopotassium salt. The suction flask was then connected to a water pump and the mixture was distilled under vacuum, the round-bottom flask being placed in n water bath kept a t 50°C. Two successive 50-cc. portions of water were added and distillation was carried to dryness each time to insure the presence of all the formic acid in the distillate. The distillate was titrated first with standard alkali, using thymol blue as the indicator, and then by the Jones10011 method. Usually, the two determinations gave very nearly the same result, but when the temperature a t which the distillation was carried out was allowed to rise above 55’, or when large amounts of glycolic acid were present, the permanganate value was sometimes higher than that given by the alkali titration. This was found to be due to the distillation of small amounts of glycolic acid with the formic acid. Appropriate corrections were made in each case. FOULKS “Notes on Quantitative Analysis,” McGraw-Rill Book Co., 1938, p. 220.
