Acid/Base Extraction of a Benzoic Acid, 4-Nitroaniline, and Naphthalene Mixture

I. Introduction

A. Objective

The purpose of this experiment is to separate a prepared mixture of benzoic acid, 4-nitroaniline, and naphthalene by the technique of extraction. The compounds will be extracted on the basis of the solubility properties of the acids, bases, and their salts. The given unknown sample will be dissolved with dichloromethane. Hydrochloric acid will then be added and extraction of p-nitroaniline will be performed in two steps: mixing and separation. The extracted p-nitroaniline will then be isolated by adding aqueous sodium hydroxide which will turn the solution basic which will cause the p-nitroaniline to precipitate. The benzoic acid will be extracted by adding sodium hydroxide to the dichloromethane solution and using the process of separation. The benzoic acid will then be isolated using aqueous hydrochloric acid to turn the solution acidic which will make the benzoic acid precipitate. These two precipitates will be collected using vacuum filtration. The 4-nitroaniline will be recrystallized using boiling water, and will then be weighed and measured for its melting point. The benzoic acid will also be recrystallized using boiling water, and will then be weighed and measured for its melting point. Anhydrous sodium sulfate will be added to the left over dichloromethane solution. The mixture will react and then be filtered, and the solid residue of naphthalene will be weighed.

B. Materials and Safety

C. Experimental Procedure

A prepared mixture of unknown relative amounts benzoic acid, 4-nitroaniline, and naphthalene will first be obtained. The weight of the mixture will be taken and recorded. The sample will then be transferred to a 15 mL glass centrifuge tube. 3.0 mL of dichloromethane will then be added to the tube and the mixture will be dissolved by shaking the tube gently. Next, 1.5 mL of 6 M aqueous hydrochloric acid will be added to the centrifuge tube. The mixture will be mixed by capping the tube and shaking it by hand to thoroughly mix the two phases. The centrifuge tube will then be vented by loosening the cap, and then it will be clamped to allow the two phases to separate. The dichloromethane layer will be separated using a pipet with bulb on top. The dichloromethane layer will be sucked into the pipet and transferred to a reaction tube. The left over aqueous layer will be transferred into a 20 x 80 mm vial labeled “aqueous acid extracts” in the same manner using a new pipet. The dichloromethane layer will then be transferred back to the centrifuge tube using a pipet. This technique will be repeated by adding another 1.5 mL of 6 M aqueous hydrochloric acid to the centrifuge tube as before.

Next, 1.5 mL of cold 3 M sodium hydroxide solution will be added to the centrifuge tube with dichloromethane. The same procedure will be followed as above for mixing and separating, except this time the aqueous layer will be transferred to a 20 x 80 mm vial labeled “aqueous hydroxide extracts”.

After that, 12 M aqueous hydrochloric acid will be added to the 20 x 80 mm vial labeled “aqueous hydroxide extracts” dropwise until the solution is acidic. The vial will be placed in an ice bath while this takes place. Next, 6 M aqueous sodium hydroxide will be added dropwise to the vial labeled “aqueous acid extracts” until the solution is basic. This will also take place in an ice bath. The solid formed by these steps will be separately filtered via vacuum filtration using a 25 mL filter flask. The precipitates, benzoic acid and p-nitroaniline, will be transferred to filter paper and allowed to dry, then will be weighed. If specified, 4-nitroaniline and benzoic acid will be recrystallized from boiling water. These samples would then be dried, weighed, and have their melting points determined.

The dichloromethane solution will then be dried by adding about 0.3 g of anhydrous sodium sulfate to the centrifuge tube. The tube will be briefly shaken and allowed to stand for 5 minutes with occasional swirling. Finally, the dichloromethane solution will be filtered or decanted into a tared reaction tube under the hood. The weight of the solid residue in the reaction tube will be determined and the tube will be stoppered. If specified, the solid residue will be purified by column chromatography.

II. Experiment and Results

A. Data

A sample comprised of a mixture of unknown proportions of benzoic acid, 4-nitroaniline (p-nitroaniline), and naphthalene weighing 0.292 g was obtained. The sample was transferred into a 15 mL glass centrifuge tube along with 3.0 mL of dichloromethane. The mixture was dissolved by gently shaking the tube. Once the mixture was completely dissolved, 1.5 mL of 6 M aqueous hydrochloric acid was added to the centrifuge tube. The tube was capped and shaken to thoroughly mix the two phases. The tube was then uncapped and clamped vertically to a ring stand. A pipet was then used to take a sample of the upper layer of the mixture, which was then put into a separate clean centrifuge tube. An equal amount of water was then put into this test tube to confirm that the upper layer was the aqueous layer. The contents in the second tube were then transferred back into the original tube using the pipet.

The lower layer of the mixture, the dichloromethane layer, was then transferred using the pipet into a clean centrifuge tube. The remaining aqueous layer was transferred using the pipet into a separate clean tube labeled “aqueous acid extracts”. The dichloromethane layer was then transferred back to the original centrifuge tube again using the pipet. Another 1.5 mL of 6 M aqueous hydrochloric acid was added to the tube with dichloromethane and the process of separating the two layers was performed for a second time. The aqueous layer was transferred to the same test tube as before containing aqueous acid extracts.

Next, 1.5 mL of cold 3 M sodium hydroxide solution was added to the centrifuge tube containing dichloromethane. The tube was capped and shaken to thoroughly mix the two phases, just as before. The dichloromethane layer was again separated from the aqueous layer, and this time the aqueous solution was transferred into a clean centrifuge tube labeled “aqueous hydroxide extracts”. This process was repeated a second time by adding an additional 1.5 mL of cold 3 M sodium hydroxide solution to the dichloromethane solution.

After that, about 0.3 g of anhydrous sodium sulfate was added to the left over dichloromethane layer remaining in the centrifuge tube. The tube was shaken allowed to stand with occasional swirling. While the tube was allowed to stand, the tube containing aqueous hydroxide extracts was placed into an ice bath and 12 M aqueous hydrochloric acid was added dropwise to the until the solution turned acidic. A precipitate, benzoic acid, formed which was filtered out via vacuum filtration. The benzoic acid was allowed to air dry for a week. It was then weighed at 0.074 g and its melting point was about 119 ºC.

The tube labeled “aqueous acid extracts” was also placed in an ice bath and 6 M aqueous sodium hydroxide was added to the solution until the solution turned basic. A precipitate, p-nitroaniline, formed which was also filtered via vacuum filtration. The p-nitroaniline was allowed to air dry for a week. The resulting p-nitroaniline was weighed at 0.040 g and its melting point was about 139 ºC. The dichloromethane solution which was allowed to stand was decanted into a clean test tube. The solvent was then removed under a stream of nitrogen in the hood. The solid residue, naphthalene, was allowed to air dry for a week. The naphthalene was weighed at 0.035 g and its melting point was 76 ºC.

III. Conclusions

The compounds extracted seem to be slightly impure. The melting points for benzoic acid, p-nitroaniline, and naphthalene that were found are all slightly lower than the melting points I looked up beforehand. There are not huge discrepancies in the numbers, so this leads me to believe the compounds extracted are the expected compounds, they are just not completely pure, which is to be expected.

Error in this experiment could have come from a few different sources. One step that could have led to error was during separation. It was hard to extract only the dichloromethane layer, so I am sure some of the aqueous layer was picked up with it. This means that the resulting aqueous solution, whether it be acid or hydroxide extracts, would yield slightly less benzoic acid and p-nitroaniline than expected. If the error were to happen the other way around and some of the dichloromethane layer was picked up with the aqueous extracts, which means less naphthalene would have resulted than expected. It was impossible to be precise with the separation, so that means there was going to be error in the final weight either way.

If the 3 M sodium hydroxide added during separation was not cold, I think the benzoic acid would not have been as soluble in the aqueous solution and therefore less benzoic acid would have resulted than expected. A couple other obvious spots where error occurred were during vacuum filtration and decanting of the dichloromethane. Some filtrate could have been lost if it were too close to the edge of the filter paper. During the decanting of dichloromethane, some of the liquid may not have made it would of the tube. Lastly, I know it was hard to get all of the naphthalene out of tube to weigh it, so the weight of naphthalene recovered is low.