EXPERIMENT 5 PREPARATION OF OXALIC ACID

I. Introduction

Oxalic acid with the molecular formula, H2C2O4, is known systematically as acid ethandiova. It contains two carboxylic acids making it a comparatively strong and acidic organic compound. Oxalic acid naturally occurs in fleshy fruits of buckwheat, black pepper, parsley, poppy seed, spinach, most nuts and beans.

It exists as a solid, water-soluble white crystalline substance. It is poisonous and highly corrosive and hence, should be handled carefully. Because of its acidity, it is widely used as an acid rinse where it could form a soluble complex with insoluble iron compounds.

Figure 1 Oxalic Acid structure

Oxalic acid can be synthesized by the oxidation of carbohydrates such as glucose, sucrose, starch, dextrin, cellulose with nitric acid. This oxidation process split carbon atoms off in pairs to give oxalic acid.

C12H22O11 + 9O2 ----> 6(COOH)2 + 5H2O

Figure 2 Oxidation of sucrose to yield oxalic acid

II. DATA AND RESULTS

Table 1 Data Gathered from the Preparation of Oxalic AcidAmount/Mass

Sucrose 5.01gHNO3 26mLFilter paper [a] 0.5908gFilter paper + oxalic acid [b] 0.6774gOxalic acid product/ experimental yield [b-a] 0.0866g

Calculation of Theoretical Yield

Balanced Reaction:

C12H22O11 + 9O2 ----> 6(COOH)2 + 5H2O

Based on the balanced reaction, 1 mol of sucrose will produce 6 mol of oxalic acid.

T heoretical Yield=5.01g Sucrose×1mol Sucrose

342.30008g Sucrose×

6molOxalic acid1mol Sucrose

×90.03548 gOxalic acid

1molOxalic acid=7.91g Sucros e

PercentageYield=(Experimental YieldTheoretical Yield )×100 %=( 0.0866gOxalic acid7.91gOxalic acid )×100 %=1.09%

III. DISCUSSION

Oxalic acid was prepared by oxidizing sucrose with nitric acid. Sucrose (figure 3) is a naturally occurring carbohydrate most commonly known as table sugar. It is a nonreducing disaccharide with a molecular formula of C12H22O11. When hydrolysed, it breaks down into monosaccharide glucose and fructose.

Figure 3 Sucrose’s chemical structure. The glucose and fructose are linked by an acetal oxygen bridge in the alpha orientation.

Nitric acid is a strong acid and a powerful oxidizing agent. It oxidizes organic compounds such as carbohydrates to their highest oxidation states with the formation of nitrogen dioxide for concentrated acid and nitric oxide for dilute acid. Nitric acid oxidizes an aldose into a dicarboxylic acid, aldaric acid. The aldehyde carbonyl and the terminal –CH2OH are oxidized in this reaction.

Figure 4 Oxidation of glucose which is an example of an aldose into glucaric acid, an aldaric acid.

Sucrose was made to oxidize with nitric acid. Around 5 grams of sucrose and 26 ml of concentrated nitric acid were allowed to react in a beaker and were heated until red fumes appeared. These red fumes are NO2 and NO gases which are highly toxic. Proper ventilation

is needed. The mixture was then set to evaporate to approximately 10 ml when the fumes disappeared. However in the experiment, the red fumes in the mixture took too long to disappear that led to an excessive evaporation of the mixture. The mixture was then taken out of the hot plate and was added 10 ml even if there are still red fumes generating. It was again evaporated to 10 ml to finish off the reaction.

C6H12O6 + 12HNO3 3C2H2O4●2H2O + 3H2O + 3NO + 9NO2

Oxalic acid dihydrate Brown/Red Gas

Figure 5 Oxidation reaction of glucose and nitric acid forming oxalic acid dehydrate and NO2

gas.

The mixture was then crystalized to collect pure oxalic acid. In the experiment, however, instead of getting pure white crystal like solid, the oxalic acid was yellowish in color. This would indicate that the oxalic acid must have had some nitric acid that did not completely react as the consequence of the red fumes still generating when the 10 ml water was added and was again evaporated. Personal errors might also take account in the experiment such not proper handling of the chemicals. Vacuum filtration was used to separate the oxalic acid crystals from its liquid impurities. The impurities remained in the liquid solution and were passed though the Buchner funnel. The obtained oxalic acid was weighed and the percentage yield was calculated. The data is summarized on table 1 (see Data and Results). Only 1.09% percentage yield was attained in the experiment. This low percentage yield might be caused of the excessive evaporation that occurred in the first few steps in the experiment. Excessive washings during vacuum filtration is also a factor in this low percentage yield.

Conclusion

Oxalic acid is the simplest dicarboxylic acid is prepared by oxidizing sucrose with nitric acid. Sucrose is a nonreducing disaccharide compromising monosaccharaides glucose and fructose. Nitric acid is a strong oxidizing agent which converts aldose (glucose) into a dicarboxylic acid. Sucrose undergoes the same oxidation process with nitric acid forming an oxalic acid, a dicarboxylic acid. Vacuum filtration was used to isolate the pure oxalic acid separating it from its liquid impurities. Only 1.09% oxalic acid percentage yield was obtained in the experiment.

References

McMurry, J. (2012). Biomolecules: Carbohydrates. Organic Chemistry, 8th Edition. Canada: Cengage Learning.

Smith, J. (2011). Carbohydrates. Organic Chemistry, 3rd Edition. New York: McGraw-Hill Companies.