Lab 4: Base Extraction of Benzoic Acid from Acetanilide; Recrystallization of Products PDF

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Lab report for Lab 4: Base Extraction of Benzoic Acid from Acetanilide; Recrystallization of Products...

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Lab 4: Base Extraction of Benzoic Acid from Acetanilide; Recrystallization of Products Introduction The goals of this lab are to perform a base extraction of benzoic acid from acetanilide, purify the solid by recrystallization, isolate the crystals, and obtain a melting point of the crystalline solid. Extraction is the method of isolating compounds from one liquid to another. When trying to successfully complete the procedure of liquid to liquid extraction, the two substances chosen must not be reactive with each other. Additionally, the extracting solvent must be immiscible with the original solution as well as able to remove the desired components. Properties of both compounds should also be looked at prior to extraction with benzoic acid having a molecular weight of 122.12 grams per mole and a melting point of 122.3C while acetanilide has a molecular weight of 135.17 grams per mole and a melting point of 114.3C. Through these properties, it can be determined that the organic layer will be made up of acetanilide while the aqueous layer will be made up of benzoic acid. Solutions that are less dense and will be on top while the solution that is denser will be on bottom. Crystallization is a technique used to purify solid compounds. Compounds tend to be more soluble in hot solvents than they are in cold solvents. When the hot solution is given time to cool, the compound is no longer soluble in the solvent and forms crystals of a pure compound. The precipitation ability of the solid is fully dependent on the solubility of the solvent. This is combined with temperature, where the boiling point is the highest temperature while the lower temperature in the lab is the cold water. Moreover, the crystallization solvent must not be reactive with the product, the product must be slightly or completely insoluble at room temperature, the impurities of the product are highly soluble at all temperatures, and the product has to be easily separated by vacuum filtration. When looking at acid base reactions in the lab, benzoic acid is more acidic in comparison to acetanilide which is more neutral. The hydroxide used in the lab is a base that deprotonates a carboxylic acid and leads to form a benzoate. As a result of benzoate being soluble in water, neutral acetanilide will reside in the organic layer. Acetanilide is unable to contain itself in the aqueous layer leading to the separation of the two chemicals. At the end of the process, benzoate is protonated using hydrochloric acid (which is a strong acid) and reverts back to benzoic acid. Overall, benzoic acid will be separated from acetanilide through the process of base extraction, purified through recrystallization, isolated, and weighed to determine the percent yields of both benzoic acid and acetanilide.

Reaction Table

Figure 1. Deprotonation of benzoic acid

Compound Formula

Structure

Molar Mass (g)

Sodium Hydroxide

NaOH

40.00

Boiling Point (C) 1388

Acetanilide

C8H9NO

135.17

304

114.3

Benzoic Acid

C7H6O2

122.12

250

122.3

Melting Point (C) 318

Methylene Chloride

CH2Cl2

84.93

39.6

-96.7

Sodium Sulfate

Na2SO4

142.04

1429

884

Procedure Extraction First, one gram of a 50:50 acetanilide and benzoic acid mixture was attained. The portion was then weighed on a scale. Afterwards, the mixture was dissolved in 10 mL of methylene chloride and was put into a separatory funnel. Next, the organic layer was obtained by adding two individual 3 mL shares of 3M sodium hydroxide. Both the aqueous and organic layers were separated from each other and saved in separate Erlenmeyer flasks. Then the aqueous extracts were put back into the separatory funnel and the aqueous layer was separated once more with two 10 mL solutions of methylene chloride. Leftover water was removed through drying with the addition of sodium sulfate. The mixture was then whirled while all of the methylene chloride was boiled off. The aqueous solution was then transferred into a 100 mL beaker and cooled in an ice bath. The sodium benzoate was then Figure 2. Extraction using mixed with 3M hydrochloric acid. Finally, using a Buchner funnel, the a separatory funnel solid was vacuum filtered and washed with a couple mL of cold water.

Recrystallization First, the benzoic acid sample was weighed on a scale. It was then mixed with 10 mL of boiling water. Hot water was continuously added to the sample until the solid completely dissolved. The solution was then given time to cool down to room temperature and then to 0 degrees Celsius in an ice bath. Benzoic acid crystals were obtained through suction filtration and allowed to air dry for 5 minutes. The crystals were then weighed and a melting point was recorded. The same procedure was done with acetanilide to obtain a weight and melting point of the sample. Calculations Percent Yield = Actual Yield / Theoretical Yield * 100% Percent Yield of Benzoic Acid = .46 g / 1.00 g * 100% = 46% Percent Yield of Acetanilide = .25 g / 1.00 g * 100% = 25% At the end of the lab, the crystals obtained of both benzoic acid and acetanilide were used to get a melting point of both compounds. Benzoic acid had a melting point range of 105-112 degrees Celsius, while acetanilide had a melting point range of 75-88 degrees Celsius. Conclusion The goals of this lab were to perform a base extraction of benzoic acid from acetanilide, absolve the solid by recrystallization, separate the crystals, and attain a melting point of the crystalline solid. The data results obtained from the lab indicated that the goals of the experiment were successfully met. The process of extraction of benzoic acid from acetanilide was successful because when the 50:50 mixture was dissolved and put into a separatory funnel, there were two distinct layers in the funnel with the acetanilide (organic layer) being on the bottom while the benzoic acid (aqueous layer) was on top. Since acetanilide has a molecular weight of 135.17 grams per mole and benzoic acid has a molecular weight of 122.12 grams per mole, then it was expected that the acetanilide would be at the bottom of the separatory funnel. This is a result of solutions that are less dense being on the top of the separatory funnel while solutions that are denser will be at the bottom of the separatory funnel. Furthermore the crystallization portion of the lab was successful because crystals formed in both the benzoic acid solution as well as the acetanilide solution. This was due to dissolving the benzoic acid and acetanilide in boiling water and then allowing it to cool in an ice bath. The theory behind this process is that compounds tend to be more soluble in hotter solvents than they are in colder solvents. Once the solution is completely dissolved and given time to cool, the original compound (benzoic acid or acetanilide) is no longer soluble in the solvent (water), thus resulting in crystals forming of the pure compound. For the process to be successful, the solvent must not be reactive with the product, the product must be slightly or completely insoluble at room temperature, and the product has to be easily separable through vacuum filtration.

Additionally, the melting points of benzoic acid and acetanilide obtained in the lab (105-112C, 75-88C) were slightly lower than the actual values of the melting points (122.3C, 114.3C). This could be due to the crystals containing a small amount of impurities or solvent, which would be the reason behind the melting point ranges being slightly lower than the reported values. Finally, when looking at the acid-base reactions in the lab, it was known prior to the lab that benzoic acid is more acidic in comparison to acetanilide, which is more neutral. As a result of these properties, the sodium hydroxide used in the lab is a base that was used to deprotonate a carboxylic acid to form a benzoate. Since the benzoate was soluble in water, the neutral acetanilide resided in the organic layer. As a result of acetanilide not being able to contain itself in the aqueous layer, there was a distinct separation of the two compounds. At the end of the lab, benzoate is protonated using hydrochloric acid which is a strong acid to revert it back to its original compound of benzoic acid. Overall, benzoic acid was successfully extracted and separated from acetanilide, a solid of both compounds was formed through recrystallization, and a melting point was successfully obtained of from the crystals of each compound. Post-Lab Questions 1. K = Concentration of solute in organic layer / concentration of solute in aqueous layer 4.2 = Concentration of caffeine in DCM / (5 mL DCM * 2 mL H2O) / 30 mg caffeine–X) X = 29.30 mg of caffeine will be in the dichloromethane extract 2. You could add NaHCO3 to separate the layers. Acetic acid would be removed from the ether solution since it would be converted to its conjugate base (acetate) by extracting the ether layer with NaHCO3 solution. Sodium acetate would be soluble in water but not soluble in ether, where the acetic acid would be soluble in both water and ether. 3. Since sodium bicarbonate is basic in nature, it can react with acid to neutralize the acid and form water. The organic layer is washed with sodium bicarbonate to neutralize any traces of acid in the organic layer. The reaction of sodium bicarbonate with acid forms CO2 gas so the separatory funnel needs to be aired out. Additionally, when swirling the separatory funnel, it must be tilted in a proper manner while gloves and goggles should be worn at all times. 4. Methanol Crystallization = 59g – 30g = 29 g / 59 g * 100 = 49.15% crystallization Water Crystallization = 7.2g – 0.22g = 6.98 g / 7.2 g = 96.94% crystallization Since water is more efficient in the crystallization of compound X (96.94% vs. 49.15%), it would be ultimately better for the recrystallization of compound X. 5. Circumstances where it is necessary to filter a hot recrystallization solution would be if the solution is cloudy or has insoluble material in it. Do not use vacuum filtration, because the reduced pressure will cause the solvent to boil slightly which will reduce volume and cool it, causing premature precipitation.

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