Lab 6 Final (1) (1).docx 4j6w3n

Base Extraction of Benzoic Acid from Acetanilide followed by Recrystallization and melting point Determination Introduction O

H N

OH

CH3 O

Benzoic Acid Chemical Name

Acetanilide Molecular Formula

Molecular Weight (g/mol)

Liquid

Soli d

b.p. ºC

Density g/mL

m.p. ºC

Solubility

Potential Hazards

Acetanilide

C8H9NO

135.17

304

1.21

111115

Slightly Soluble

Slightly toxic

Benzoic Acid

C6H5COOH

122.12

249

1.27

121123

Soluble

Toxic

The goal of this lab is to

perform a base extraction of benzoic acid from acetanilide. This lab also involves identifying immiscible layers in a separatory funnel, operating a separatory funnel, purifying a solid by recrystallization, isolating crystals by vacuum filtration, obtaining a melting point of crystalline solid and differentiating products by IR spectroscopy. This lab incorporates separation of 1:1 mixture of Benzoic acid and acetanilide with base extraction NaOH. Along with the recrystallization, chromatography and distillation methods for separating and purifying organic liquids and solids, extraction is also used to separate immiscible liquids. The separation between the immiscible liquids is known as phase distribution because the liquids are in two different phases. This phase distribution results in partitioning or adsorption phenomenon. Extraction is based on the partitioning which happens due to difference in solubility of liquids concluding selective dissolution. The two immiscible phases in which the solute will be distributed include the extracting phase and the original phase. According to the equation, K=

Solute∈ S ( x ) Solute∈S (o)

, the amount of solid distributed in each phase is quantitatively expressed

through a constant K. K is partition coefficient calculated by dividing S(x) which is the extraction phase to S(o) which is the original phase. If the value of K is greater than 1 that

means the solute will remain largely in the extracting solvent. At equilibrium, the concentration of the solute in both the phases is equal and constant. If value of K is less than 1, then multiple extractions of the solute should be performed. This means multiple smaller extractions are better than one large extraction with the same volume. The fraction of the solute remaining in the original solvent decreases as the number of extractions increases. This is explained by the equation,

F=(

n V (o) ) , where n is the number of extractions, V(o) is the volume of the KV ( f ) +V ( o )

original solvent, V(f) is the volume of extracting solvent per extraction, and K is the partition coefficient. Thus, to perform successful extractions, the extracting solvents must not react irreversibly with the solute, an ideal extracting solvent must be immiscible with the original solvent (i.e. CH2Cl2 and H2O), it must be able to selectively remove the desired component, and it must be easily separated from the solute. For partitioning between nonpolar and polar solvent, the solutes with different polarities would have different coefficient K. For example, if a mixture containing nonpolar neutral compound and ionic polar compound is distributed between a nonpolar and polar solvent, then the solutes will be distributed based on their relative affinities to each solvent. This theory is based on the principle of “like dissolves like”, the neutral compound will partition into the nonpolar phase and the ionic compound will partition into the polar phase. That is why neutral compounds will not dissolve in water rather ionic compounds will dissolve in water due to its high polarity. In In this lab, benzoic acid will be extracted from acetanilide in the presence of an aqueous base like NaOH as shown in Figure 1.

Figure1. Reaction mechanism of Benzoic acid and NaOH to extract acid from acetanilide Reactions between acids and bases are used to extract solutes. So, if an acid is mixed with a basic solvent such as NaOH, then the acid will form its conjugate base and can be separated by extracting the acidic compound into a basic aqueous phase. Thus, reaction can also be vice-versa when a base is placed into an acidic solvent, then the base will form its conjugate acid as shown in Figure 2. When the conjugate base is formed is very polar because originally it comes from an acidic environment makes layers of liquid phase which are easier to be separated. The Keq which is an equilibrium constant can be calculated by Keq = [products] /[ reactants]. Carboxylic acids and phenols are organic compounds that contain functional groups that are polar, hydrophilic but due to their R or aromatic groups, these compounds are insoluble in water and soluble in organic

solvents like CH2Cl2. The reaction of an acid with a base will lie to the right if Keq is greater than one. The extraction methods are used to separate two solvents based on the differences in solubility. The solutions are poured into a separatory funnel and are inverted to enhance the separation of the solutes. The separatory funnel is also timely vented to release any pressure in the funnel built in by the reaction. The bottom layer is the organic layer and the top layer is the aqueous layer containing the conjugate base of benzoic acid. In liquid-liquid base extraction of benzoic acid from acetanilide, neutral compound of benzoic acid is converted to its conjugate base through extraction with an aqueous base like NaOH. This conjugate base of benzoic acid is soluble in water and can be separated from acetanilide which remains dissolved in the organic layer.

Figure 2. . Reaction mechanism of Base and acid to extract base from acid After extraction, drying agents are used to remove water from a solution. The drying agent must not react with the organic liquid and it must easily be separated from the liquid. In this experiment NaSO4 was used as a drying agent because it has neutral acid-base properties. The isolated compound from the different compound may contain impurities which can be removed through the process of recrystallization. In solution recrystallization, the solid is dissolved in an appropriate solvent at an elevated temperature and crystals are allowed to re-form on cooling, whereas the impurities remain dissolved in the solution. Mostly all solids are more soluble in a hot than a cold solvent. But in order to perform recrystallization the solvent must not react with the desired product, the desired product is easily soluble in the solvent at elevated temperatures, but only slightly soluble or insoluble at all or at room temperatures, impurities must be highly soluble at all temperatures, and the solvent must be easily removed from the crystalline product by filtration or evaporation. Filtration is a technique used to separate solids from liquids. The solids are crystals whereas the filtrate is discarded in this lab. It is performed by attaching the Erlenmeyer flask with the vacuum pump letting all the air in the flask out. While the liquid is poured through the ceramic funnel from the filter paper attached to the flask. The solution is purified while the solids stay on the filter paper.

Figure 3. Separatory funnel indicating layers of solvents In this lab experiment, the mixture of benzoic acid and acetanilide was separated by liquid-liquid base extraction and purified via recrystallization. The theoretical yield for both of the solutes was 0.52 g and the actual yield for benzoic acid was 0.13 whereas the actual yield for acetanilide was 0.14 g. The melting point range for benzoic acid was between 123°C and acetanilide was between 111°C – 112°C. IR Procedure: Extraction: A 1:1 mixture of benzoic acid and acetanilide was weighed to 1.04g. The sample was dissolved in 10mL of methylene chloride and transferred to a separatory funnel. 3 ml of 3M NaOH was added to the sample and the separatory funnel was inverted with a stopper while it was frequently vented to remove any pressure. Then, the organic layer (methylene chloride) and aqueous layer were extracted with two portions. Again, the organic layer was transferred into separatory funnel adding 3 ml of 3M NaOH and inverted to extract out two layers. In each case, both the separated aqueous and organic layers were combined individually. Since methylene chloride is denser than water, it will be the bottom layer. The aqueous layer was extracted with two 10mL portions of methylene chloride to ensure removal of any residual acetanilide suspended or dissolved in water. All of the methylene chloride extracts were combined and converted into a beaker. Na2SO4 was added to the organic extract to remove extract water and decant the solution after the suspension by gravity into a clean beaker. The organic layer (Methylene chloride) was boiled off until the total volume was about 2mL and crude acetanilide remained in the beaker while the aqueous solution containing sodium benzoate in the aqueous solution was neutralized with 6.5 ml of 3 M HCl. The conjugate base of benzoic acid was

transferred into a beaker and was cooled in an ice bath. The resulting solid was cooled, was suction filtered on a Buchner funnel, and was washed with a few milliliters of cold water. Recrystallization: To recrystallize the benzoic acid, the sample was mixed with approximately10mL of boiling water per gram of sample and was brought to a gentle boil. The solution was cooled down to room temperature and into the ice water bath to 0 C. Then, the crystals of benzoic acid were collected by suction filtration and were air dried. The crystals of benzoic acid were then weighed and melting point and IR spectrum were obtained. The recrystallization process of acetanilide was performed in the same way like benzoic acid.

Data Acquisition/ Presentation: Equations: Fraction of solute remaining in original solvent n

     

V ( o) Fa=( ) KV ( f ) +V ( o )

Fa= Fraction of solute remaining in original solvent K- partition coefficient Vo-volume of original solvent Vf-volume of extracting solvent (per extraction) n- number of extractions

Coverting pka to Keq Keq=10-pka pKeq= pka(acid left)- pka(acid right) Percent Yield Calculation Percent yield determines whether the separation of benzoic acid from acetanilide was successful or not. Percent Yield = (Actual Yield/ Theoretical Yield) x 100 Yield Actual yield (g) Theoretical yield (g)

Benzoic Acid 0.13 0.52

Acetanilide 0.14 0.52

Percent yield (%)

[(0.13) / (0.52)]*100 = 25%

[(0.14) / (0.52)]*100 = 27%

True melting Point (°C)

121 – 123 °C

111 – 115 °C

Experimental Melting Point (°C)

123

111-112

4. Infrared Spectroscopy Infrared Spectroscopy was carried on to figure out the functional groups present in each compound. As the two compounds are acetanilide and benzoic acid, the peaks of carbonyl groups, benzene, alcohols, and secondary amine groups are more likely to be observed on the IR spectrum. Benzoic Acid Frequency 1677.68 cm-1 3070.90 cm-1

Intensity Strong, sharp Broad, short

Functional Group(s) C=O O-H (Carboxylic acid)

Intensity Strong, Sharp Short Strong, Sharp, Short

Functional Group(s) C=O Benzene Secondary Amine

Acetanilide Frequency 1661.23 cm-1 1538.08 cm-1 3290.61 cm-1 Conclusion The objective of this lab is to perform a base extraction of benzoic acid from acetanilide. This lab also involves identifying immiscible layers in a separatory funnel, operating a separatory funnel, purifying a solid by recrystallization, isolating crystals by vacuum filtration, obtaining a melting point of crystalline solid and differentiating products by IR spectroscopy. This lab incorporates separation of 1:1 mixture of Benzoic acid and acetanilide with base extraction NaOH. This lab was successful in extracting out benzoic acid from acetanilide between two immiscible phases. When NaOH was added to the mixture in the separatory funnel, the mixture was separated in to aqueous and organic layer. Benzoic acid was deprotonated by NaOH, a very strong base, and was present in the aqueous layer (top layer). Whereas acetanilide was nonreactive with strong base, therefore it remained dissolved in the organic layer (CH2Cl2). Again, the aqueous solution was extracted from methylene chloride, a strong acid HCl which

neutralized the solution and protonated the benzoate anion which was crude benzoic acid. Then, the organic extracts of methylene chloride were boiled off and crude acetanilide was obtained. The reason for choosing NaOH and methylene chloride was that their equilibrium constants Keq were found to be much larger than 1 which means NaOH and methylene chloride are effective in extracting acidic and basic solvents. Impurities were isolated from the products through the process of recrystallization. Initially, the experiment started with 0.52 grams of pure acetanilide and 0.52 grams of pure benzoic acid. However, after extraction and recrystallization 0.14 grams of acetanilide and 0.13 grams of benzoic acid were obtained. The percent yield for acetanilide was 27% and for benzoic acid was 25%. Since the percent yield for benzoic acid and acetanilide is not 100%, some of the sample was lost during extraction, recrystallization or filtration processes. There must have been errors in performing extraction which didn’t fully separate acetanilide from the aqueous layer and benzoic acid from the organic layer. The low percent yield of benzoic acid could have resulted due to errors caused during recrystallization procedure. There wasn’t completely neutralized the benzoate anion with HCl which could have lowered the amount of product formed during precipitation. Moreover, during suction filtration if the holes in the funnel are not covered properly with filter paper, some of the sample could have been lost in the filtrate flask rather than sticking to the funnel. Thus, some of the lost sample could have led to inaccurate weight of the sample and therefore, an inaccurate percent yield. The melting points of the each solid sample were determined to be 111-112°C for acetanilide and 123 °C for benzoic acid. However, the true melting point ranges are 111 – 115 °C for acetanilide and 121 – 123 °C for benzoic acid. The inaccurate temperatures of the products indicate the presence of impurities in the sample. Similarly, it should be made sure that the samples were not wet or also rapid heating of the samples because that could completely change the melting points of the sample. In this case, the theoretical values are matching the actual values indicating the precision of the procedures- extractions and recrystallization. The results from the IR indicate alcohol (carboxylic acid) at 3070.90 cm-1 and carbonyl groups at 1677.68 cm-1in benzoic acid sample. The IR spectrum of acetanilide produced three different peaks at 1656.35 cm-1, 1539.88 cm-1, and 3297.68 cm-1. The peak at 1661.23 cm-1 was of C=O ( carboxylic acid) functional group, the peak at 1538.08 cm-1was from the benzene ring, and the peak at 3290.61 cm-1was from the amine group. The results explain that the extracted samples contain crude benzoic acid and crude acetanilide because the indication of functional groups is similar to the actual structures of benzoic acid and acetanilide. Although, the percent yield were not as high as preferred and melting point readings of both the samples indicated the successful extraction of benzoic acid from acetanilide. Finally, the IR spectrum of each sample also indicates the success of recrystallization to remove major impurities from the samples giving the accurate IR spectrum. Thus, the objective of separating 1

gram of 1:1 mixture of benzoic acid and acetanilide by performing liquid-liquid base extraction and purifying the separated compounds by recrystallization was successful. Reference Gilbert, John C., and Stephen F. Martin. Experimental Organic Chemistry. Cengage Learning, Massachusetts, 2011, 5th Ed, pp. 75-81, 153-161, 93-100.