Experiment 1: Two Base Extraction                             Jeremy Wolf

6/30/2008                                                         TA: Stefanie Lenz

Introduction:

The purpose of this experiment is to use a two-base extraction method to separate a sample of three immiscible compounds. We converted both benzoic acid and 2-naphthol to their conjugate bases, which are soluble in water, in two separate steps, with two separate bases. The weak base (bicarbonate) converted the stronger acid (benzoic acid, a carboxylic acid) into its conjugate base (benzoate) while leaving the other species in the immiscible phase. This reaction was favored because the conjugate acid of bicarbonate (carbonic acid) has a pKa that was higher than that of benzoic acid, but lower than the pKa of 2-naphthol. In this step there was a separation of the components based on the solubility of the phases. The phase containing the benzoate ions was aqueous, while the naphthalene and 2-naphthol were dissolved in the organic phase.

In the second step of the extraction the 2-naphthol was removed from the organic phase by conversion to its conjugate base by hydroxide. This reaction was favored because the conjugate acid of hydroxide (water) has a higher pKa than that of 2-naphthol. There was another separation based on solubility, where the aqueous 2-naphthoxide was removed from the organic phase containing the neutral naphthalene.

To extract the desired components from their aqueous phases, they were converted back to their conjugate acids by addition of strong acid. In this case, hydrochloric acid was used. Each component precipitated out of the aqueous solution with acidification. In the final organic phase, containing just naphthalene, the solvent was removed by rotary evaporation to isolate that component.

Experimental Procedure:

1. The mass of the mixture was obtained
2. The sample is dissolved in 30-mL of diethyl ether
3. The first base extraction was conducted with two separate 20-mL volumes of 10% sodium bicarbonate solution. Each 20-mL sample was shaken in the separatory funnel, the carbon dioxide gas vented, and then allowed to settle. The bottom aqueous phase was drained.
4. A second extraction was performed with 20 mL of cold 10% sodium hydroxide solution in the separatory funnel and the bottom aqueous layer was drained.
5. About 0.7 grams of anhydrous sodium sulfate was added to the ether and naphthalene mixture to remove any water.
6. The benzoic acid and 2-naphthol were precipitated from their respective aqueous solutions with addition of 3 M HCl. The extracted solids were cooled in an ice bath before they were collected by vacuum filtration.
7. The naphthalene mixture was filtered and the solvent was removed by rotary evaporation.
8. All solids extracted were allowed to air dry in the lab locker for one week, then weighed and their melting points determined.

Table of Reagents

Experimental Flow Chart:

Observations and Results:

4.00 grams of Sample #128 was weighed out and transferred to a 125-mL flask. Since unknown #128 was twice as large as was recommended, the flask needed to be re-rinsed with an additional 5 mL of diethyl ether. The unknown mixture was light tan when it was dissolved.

When the bicarbonate solution was added to the organic phase, the bicarbonate solution stayed clear. Carbon dioxide gas readily evolved and was vented from the separatory funnel. The aqueous layer is more dense than the organic layer and settles on the bottom of the funnel.

Because of the larger initial sample size, 25 mL of 10% ice bath chilled sodium hydroxide solution was added to the separatory funnel. Upon shaking, the bottom aqueous layer was light yellow. The aqueous layer was drained into a 125-mL flask.

Calculation for acid neutralization:

NaHCO3 + HCl => NaCl + H2O + CO2

10% NaHCO3 = 10g NaHCO3/100mL H2O = 4g NaHCO3/40mL H2O

4g NaHCO3 x   = 0.05 mol NaHCO3 = 0.05 mol HCO3-

0.05 mol HCO3- = 0.05 mol H+/3.0 = 0.0167 L = 16.7 mL

NaOH + HCl => NaCl + H2O

10% NaOH = 10g NaOH/100mL H2O = 2g NaOH/20mL H2O

2g NaOH x   = 0.05 mol NaOH = 0.05 mol OH-

0.05 mol OH- = 0.05 mol H+/3.0   = 0.0167 L = 16.7 mL

The benzoic acid recrystallization took approximately the estimated amount of acid to turn acidic. About 20 mL of acid was added to the benzoate solution to pass the endpoint of the titration. The 2-naphthol took much more than the estimated amount of acid to complete the precipitation. About 30 mL of 3 M HCl was added to the solution containing the 2-naphthoxide. This excess is proably caused by the excess NaOH that was added to extract all of the 2-naphthol from the mixture.

The benzoic acid and 2-naphthol crystals were collected and rinsed with vacuum filtration. The vacuum filtration set-up didn't work efficiently because the rubber adapters were not used to complete a seal from the Buchner funnel to the Erlenmeyer flask. Consequently the drying process of the crystals was not finished by the end of the lab period. The moist filter paper was pried from the funnel and set on a watch glass to complete the drying process. When the samples were retrieved after a week of air-drying, there was no readily-apparent adverse effects noted.

The rotary evaporator was used to evaporate the diethyl ether from the naphthalene mixture. The flask containing the naphthalene was also left to air dry in the lab drawer. All air-dried samples were then weighed.

The mass of the naphthalene was obtained by weighing the round-bottom flask with the naphthalene and subtracting the mass of the flask that had been measured the week before. The mass of the other samples were obtained by weighing a watch glass, then scraping the dried samples from the filter paper onto the watch glass and re-weighing.

After the mass of all samples was recorded, the melting point of all samples was measured. The samples were packed into a capillary tube, heated, and the melting point was observed and recorded. Refer to the results table for all data.

Interpretation of Instrumental Data

Total yield = 0.97 grams Naphthalene + 1.55 grams Benzoic acid + 0.80 grams 2-Naphthol = 3.32 grams total

 = 83% Recovered

 = 29% Naphthalene

 = 47% Benzoic acid

 = 24% 2-Naphthol

Conclusion:

The purpose of the laboratory experiment was achieved. According to the results, particularly the melting point analysis, purified compounds were obtained from the initial mixture by two-base extraction. In the extraction process, the naphthalene is expected to be the most impure, because any non-extracted benzoic acid or 2-naphthol will remain in the organic phase. This was reflected in the melting point analysis, since the naphthalene was the most impure of the substances. The melting point of pure naphthalene is 800C and the extracted naphthalene had a melting point of 750C. The difference is 6.25% between the observed and expected melting points. Considering that the melting points of the benzoic acid was exactly the same as the book value for benzoic acid (according to G & M) and the melting point for the extracted 2-naphthol was 1210C, which is 1.6% different than the book value of 1230C. Since the benzoic acid and 2-naphthol samples were considerably pure, the impurities in the extracted naphthalene could be benzoic acid or 2-naphthol that had not been extracted. The 2-naphthol should also have impurities because the strong base (hydroxide) would remove all of the leftover benzoic acid along with the 2-naphthol from the organic phase.

The reactions for the extractions were as follows:

The percentage yield for these extractions was 83%. The loss of 17% could have come from a variety of sources. With every transfer of liquid or aqueous phase to another container, some or the extract was lost. Some extract was lost because it came out of solution and some was lost when the liquid would drip down the mouth of the container. Some of the extracted solids remained on the filter paper and that is another source of loss. All of these sources of error resulted in a loss of product and a reduction in the yield of the reaction.

Overall the reaction was a success. The two base extraction method worked to differentiate between the compounds that were dissolved in the organic phase. The reaction differentiated between the two organic acids based on their difference in pKa. The bicarbonate solution extracted the stronger base because the reaction was favored with benzoic acid, but the base was not strong enough to extract the 2-naphthol. Then the 2-naphthol was able to be extracted from the solution by hydroxide because the pKa of the conjugate acid of hydroxide is water, with a pKa of 15.7. This reaction also heavily favored the products. Finally, after the two extractions, the naphthalene was the only component left in the organic phase, which was easily evaporated. This method was shown to be a viable method for extracting compounds based on the pKa of the organic acid.

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