Solvent Extraction Utilized to Separate an Organic Mixture

Written by Karl

Aim/Purpose

This lab is designed to give the student an understanding of laboratory techniques utilized to isolate a certain compound form impurities or mixture of substances. Solvent extraction utilizes the solubility characteristics of organic substances to separate compounds from the various mixtures. Furthermore, Thin Layer Chromatography helps the student identify the mobility of different compounds as well as the purity of the isolated compounds.

Theory

The solvent extraction process is composed of three main steps: dissolving the compound; separating the acid, base and neutral compounds; and, finally, recrystallizing the compound. Dissolving the compound is the main step in creating a compound that can be separated based on the compounds properties in the mixture. Once the mixture is dissolved in ether, the substance can be separated by liquid-liquid extraction. This solution is the key difference between liquid-liquid extraction and liquid-solid extraction. Water is utilized as the opposing solution to the compound because water is able to dissolve HCl and NaOH. Additionally, water has a great aversion to forming a mixture with ether, so when water and ether are combined, water will form a layer below the ether. This is because water is denser than ether and the denser solution goes to the bottom.  The following were the three compounds that made up the mixture, which are all insoluble in water:

Acid: Benzoic acid

Base: Ethyl 4-Aminobenzoate

Neutral: 9-Fluorenone

The second main step in the solvent extraction is separating the acid, base and neutral compounds from the original solution. The basic concept behind this process is to remove the base then the acid then the neutral compound to come up with the mass of each compound in the original solution. To begin with, the base is removed by adding the acid which results in a conjugate acid that is water soluble (Reaction 1). The conjugate acid-eventually to become the base again- then separates into water, so it does not mix with the ether and is easily extractible. The base is then extracted with several rounds of acid to leave only the acid and neutral compound. Several rounds of acid are used rather than one big round because multiple rounds maximizes the exposure time and portioning of the stationary and mobile phase greatly increases the separation. To remove the acid compound from the solution, a strong base is utilized to create the conjugate base by ionizing the acid (Reaction 2). As before, the densities of the two solutions are different and the acid solution is pipetted out of the centrifuge. The base is also applied in cycles for the same reasons as before. After the base and acid are taken out of the solution, only the neutral compound remained.

Reaction 1: Ar-NH₂ + HCL → Ar-NH₃⁺ + Cl^–

Reaction 2: Ar-COOH + NaOH → Ar-COO^– + Na⁺

Once all three compounds are separated, the compounds can be recrystallized. The conjugate base of the acid is the first part to be recrystallized with a concentrated, strong acid- in this case HCl- to protonate the conjugate base to make a benzoic ring, which is insoluble in water (Reaction 3). Since the ring is insoluble, the base precipitates out. By decreasing the temperature around the benzoic acid, the ring becomes even more insoluble so filtration techniques can be utilized to acquire the pure form of the acid from the solid and liquid combination. The same procedure is done for the base’s conjugate acid. However, a strong, concentrated base is utilized for the purpose of precipitating the ethyl-4-aminobenzoate (Reaction 4). As before, cooling the compound and vacuum filtration separate the compound. The final neutral compound is separated by a completely different method. Water needs to be removed from the solution, so Na₂SO₄ is added to the solution which insures any water with acid or base is removed. After the water is removed, the ether is boiled out which allows the 9-fluorenone to come out of the solution. Once all of the solutions dry, they can be measured and utilized to calculate the percent recovery.

Reaction 3: Ar-NH₃⁺ + Cl^– + NaOH → Ar-NH₂

Reaction 4: Ar-COO^– + Na⁺ + HCl → Ar-COOH
Note: These reactions are simplified to point out the precipitate in each reaction.

As previously mentioned, two techniques are vital to solvent extraction: vacuum filtration and TLC. TLC is used to determine if certain compounds in the mixture have been fully removed. The TLC utilizes chloroform as the mobile phase so that the solutions can be separated by their various polarities. TLC is essential in determining when the nest step in the lab can be taken. The three substances that make up the mixture in this lab have different R_f values because of polarities. Additionally, vacuum filtration is used in the process of recrystallization to separate the solutions that have newly insoluble solids. Therefore, vacuum filtration is vital in separating the compounds to be measured.

Experimental Results

Table 1: Mass of Objects

Mass:  Mass of container and compound – mass of container = mass of recovered compound
Mass filtered acid: 0.145 g – 0.107 g = 0.038 g
Mass filtered base: 0.148 g – 0.108 g = 0.040 g
Mass neutral: 14.200 g – 14.232 g = 0.032 g
Mass total: Mass filtered acid + Mass filtered base+ Mass neutral= Total Mass Recovered
Mass total: 0.038 g + 0.040 g + 0.032 g = .110 g

Table 2: % Recovery

Percent Acid= 0.040g/ 0.05033g = 79.47%
Percent Base= 0.038g/ 0.05033g = 75.50%
Percent Neutral=0.032g/ 0.05033g = 63.58%
Percent Total= 0.110g/ 0.151g = 72.85%

Table 3: R_f Values

 Other Notes:

–When removing solution with the pipet out of the centrifuge, there was some discrepancy about if all of the solution had been removed.

-The basic compound seemed to have a slight tent of yellow color.

– Precipitates took some time to form once the recovery process was started.

-Initially the first TLC showed some base left, so more cycles were completed to remove the entire base compound from the mixture.

Discussion

After examining the TLC plates after each set of reactions, the targeted compounds were isolated from the solutions. Therefore, the basic solvent extraction was at least fairly successful because the acid, base and neutral were all separated according to the TLC plates. Furthermore, the R_f values were all around the expected range because the acid, base and neutral were in order of expected R_f values.  Therefore, the compounds were extracted from the original mixture.

However, the percent recovers were nowhere close to the expected values. This could have come about for a multitude of reasons. To begin with, during vacuum filtration, the entire compound was not adequately transferred onto the filter paper- some was lost to the sides. Therefore, the mass recovered would be lower. Also, during the actual separation of the water mixture from ether solution, some of the solution was spilt while transferring the solution to a beaker from the centrifuge. Similar to before, this would cause the mass recovered to be lower.

The original composition of the mixture obtained was also not known. Though it was assumed to be a perfect 1:1:1 ratio of acid to base to neutral, it cannot be determined if the solution obtained was a perfect 1:1:1 ratio. Therefore, the percentage recovers could be potentially higher for some and lower for others. Lastly, the equipment utilized was not perfectly cleaned. Therefore containments may have been involved with the reaction which would cause dramatic changes in the data if reactions occurred before they were supposed to. If a little HCl was still left after the first initial addition, the HCl could have caused a precipitate to form at the wrong point in time. Therefore, the experiment was not perfect because many uncertainties are present about the errors in the lab but can be considered a success because the compounds were separated from the original mixture.

Conclusion

This lab gave the student adequate knowledge and skills to perform a solvent extraction to separate compounds out of a mixture. Therefore, the student can now effectively go about a liquid-liquid solvent extraction if necessary in research or any field of study. The basic and acid compounds of a mixture can be separated based on their polarities in order create homogenous mixtures of a certain compound. By ionizing the compounds and dissolving them in solutions, salts can be obtained. These salts can be switched to nonpolar forms by reacting with other strong acids and bases to obtain the desired compounds. Some other techniques learned are the simple techniques of vacuum filtration and TLC. Therefore, vacuum filtration can be used to separate compounds and TLC can be utilized to determine the polarities of compounds and to check to see if compounds were completely removed from a mixture.

Waste Disposal

Aqueous Liquid and Basic Chemical Waste
HCl
NaOH

Organic Liquid Hazardous Waste Container
Ether
Acetone

Hazardous Solid Waste Container
Basic Crystals
Acidic Crystals
Neutral Crystals

Broken Glass Container
Glass pipettes

Regular Waste Disposal
Gloves

References

Forbes, David C., Dana W. Mayo, and Ronald M. Pike. Microscale Organic Laboratory.

5^th. Hoboken: John Wiley & Sons, Inc, 2011. Print.

“Solvent Extraction.” University of Kentucky Organic Chemistry Lab. PowerPoint, 2013.