Lab 5 - seperation (fall 2023)

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Experiment 5: Separation of a mixture 1 SEPARATION OF A MIXTURE: EXTRACTION (Adapted from http://wwwchem.uwimona.edu.jm/lab_manuals/c1901exp8.html ) Purpose: Use pH dependent solubility to separate (aka extract) organic compounds from a mixture. Background Information : Extraction is a particularly useful means of separating organic compounds if one compound in the mixture can be chemically converted to an ionic form. The ionic form is soluble in an aqueous layer and can be extracted into it. Other, non-ionic organic compounds in the mixture will remain dissolved in the organic solvent layer. Separation of the two layers results in the separation of the two compounds. The extent to which an acid-base reaction proceeds to completion depends upon the relative acidity of the reactants and products. Reactions occur so that stronger acids and bases are converted into weaker conjugate base and conjugate acids, respectively. The p K a value of the acids gives a measure of the acidity of each compound. Stronger acids have smaller p K a values and their conjugate bases are weaker. The position of an acid-base equilibrium can then be predicted from knowledge of the p K a values of the acids involved. Look at the following acid-base reactions in Figure 1, paying particular attention to the position of the equilibrium and its relationship to the p K a values given. Figure 1: The reactions of a carboxylic acid and a phenol with bicarbonate ion. Note that the carboxylic acid has a lower pKa than the conjugate acid of bicarbonate ion (carbonic acid). The reaction,

Experiment 5: Separation of a mixture 2 therefore, proceeds to products. The reaction of a phenol, however, favors the reactants since the pKa of phenol (10) is larger than that of the carbonic acid (6.4). Acid-base reactions favor the side with the weaker acid (that is, they favor the side with the larger pKa). So, extracting a mixture of these two compounds with bicarbonate results in the ionization and extraction of a carboxylic acid in the presence of phenol thus separating the two compounds from one another. Now, look at the reaction in Figure 2 where we use a stronger base to do the reaction: Figure 2: The reactions of a carboxylic acid and a phenol with hydroxide ion. Note that in both cases, the reactions favor the formation of products. Therefore, extracting with hydroxide ion would result in the ionization and extraction of both compounds at the same time. A close look at these two figures indicates that separating a mixture of a carboxylic acid and a phenol would best be done using bicarbonate ion since only the carboxylic acid is converted into its conjugate base by bicarbonate. The conjugate base of the carboxylic acid, being an ionic species, is soluble in the aqueous layer while the phenol (left unionized) would remain dissolved in the organic layer. However, if we were to extract with hydroxide ion, both the carboxylic acid and the phenol would be converted into their conjugate bases (see figure2). The conjugate bases again are both ionic species and therefore soluble in the aqueous layer. This means that both compounds would be extracted at the same time, resulting in no separation. A neutral compound will not react with either bicarbonate ion or hydroxide ion since a neutral compound does not have protons acidic enough to be removed by these bases. Therefore, such a compound will remain dissolved in the organic layer, no matter which base is added. For

Experiment 5: Separation of a mixture 3 example, a mixture of neutral compound and carboxylic acid can be separated using bicarbonate ion since only carboxylic acid will be ionized by the bicarbonate ion. Once extracted, the carboxylic acid and phenol can both be recovered by adding HCl to the aqueous solutions. The carboxylate ion and phenoxide will both be protonated by HCl, resulting in the formation of the original carboxylic acid and phenol, neither of which is soluble in water, so they precipitate from solution. The solid can then be isolated by filtration. Figure 3 shows this chemistry for you. Figure 3: The reactions of a carboxylate ion and a phenoxide ion with HCl. Since HCl is a stronger acid than either of the conjugate acids, the products are favored in both cases. The products, a carboxylic acid and a phenol, are insoluble in aqueous solutions and precipitate from solution. The resulting solids can be isolated, and their melting points determined. (Note: melting point analysis is not employed in this lab.) The procedure you will use in this experiment exploits the differences in acidity and solubility as described previously. (a) you will dissolve your unknown in ethyl acetate (an organic solvent). All the possible compounds are soluble in ethyl acetate. (b) you will extract with sodium bicarbonate to remove any carboxylic acid that is present. (c) you will extract with sodium hydroxide to remove any phenol that is present. (d) you will acidify both resulting aqueous solutions to cause any compounds that were extracted to precipitate.

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Experiment 5: Separation of a mixture 4 These solids will be isolated by filtration and dried. You will weigh each solid you obtain to determine the percent recovery of your procedure. If a neutral compound was present in your unknown, it will remain in the organic layer throughout the extraction procedure. To isolate it, you will simply evaporate the ethyl acetate to leave a solid. Remember, though, that you only have two compounds in your unknown mixture so that you should not isolate solids from all the extracts. Possible compounds in the unknown mixture. Look up the structures of these compounds and include them in your prelab question 1. benzoic acid, 4-tertbutylphenol, 1,4-dimethoxybenzene

Experiment 5: Separation of a mixture 5 Safety Concentrated NaOH and HCl will cause burns! Gloves must be worn at all times. Notify your TA immediately of any spills! Pressure can build up in the separatory funnel. Handle carefully and vent the gas frequently - as demonstrated by the TA Procedure: Read the entire section of the procedure before starting the corresponding experiment. 1. Label 3 Erlenmeyer flasks:  125 ml flask 1: bicarbonate  125-ml flask 2: hydroxide  50-ml round-bottom flask: ethyl acetate 2. Collect an unknown and record the unknown number. (Without this number, we cannot grade your report.) 3. Dissolve approximately 1.0 g of your unknown mixture in 10 mL of ethyl acetate. Unknown: ________ Mass of unknown: ______________ Mass of mixture: ______________ Extraction 1 – Bicarbonate extraction Prediction: Which compound(s) would be extracted in this step? Show the reaction that occurs that results in the compound(s) becoming water soluble. 1. Pour the solution into a clean separatory funnel. 2. Add 10 mL of 10% aqueous sodium bicarbonate found on the solutions bench. 3. Stopper the funnel, properly grip the neck and stopper to prevent leaks, and invert it. Slowly open the stopcock to release any built-up pressure, then close the stopcock ( Figure 4 ). Don’t lose track of which is which!! A 1 . 099 9 , 409 Beaker = 35 . 259

Experiment 5: Separation of a mixture 6 4. Gently shake the separatory funnel to allow both thorough mixing of the solutions and effective extraction of the compound from the organic mixture. Caution: When shaken, the mixture may develop pressure; be sure to vent it periodically. 5. Clamp the separatory funnel to a ring stand or place it into a ring secured on the ring stand to hold and allow the mixture to separate into two layers ( Figure 5 ). This may take a couple of minutes. Examine the separation line between the aqueous and organic layers for emulsification. 6. Remove the stopper and collect the aqueous layer (the lower layer) in the 125 mL Erlenmeyer flask labeled "bicarbonate". Leave the organic layer (the upper layer) in the separatory funnel. 7. Repeat steps 2-6 two more times draining each lower layer portion successively into the same “bicarbonate” flask. At the end of this sequence, you will have extracted the organic solution with three 10 mL portions of 10% aqueous sodium bicarbonate. Leave the organic layer (the upper layer) in the separatory funnel. 8. Put the Erlenmeyer flask labeled "bicarbonate" aside in a safe place. Later you will isolate any compound that was extracted by the bicarbonate. Extraction 2 – NaOH Extraction Prediction: Which compound(s) would be extracted in this step? Show the reaction that occurs that results in the compound(s) becoming water soluble. 1. Add 10 mL of 5% aqueous NaOH to the separatory funnel with the remaining ethyl acetate.

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Experiment 5: Separation of a mixture 7 2. Stopper the funnel, properly grip the neck and stopper to prevent leaks, and invert it. Slowly open the stopcock to release any built-up pressure, then close the stopcock ( Figure 4 ). 3. Gently shake the separatory funnel to allow both thorough mixing of the solutions and effective extraction of the compound from the organic mixture. 4. Clamp the separatory funnel to a ring stand or place it into a ring secured on the ring stand to hold and allow the mixture to separate into two layers ( Figure 5 ). This may take a several minutes; be patient. Examine the separation line between the aqueous and organic layers for emulsification. 5. Remove the stopper and collect the aqueous layer in the 125 mL Erlenmeyer flask labeled "hydroxide". Leave the organic layer (the upper layer) in the separatory funnel. 6. Repeat steps 1-5 of extraction 2 two more times draining each portion successively into the same “hydroxide” flask. At the end of this sequence, you will have extracted the organic solution with three 10 mL portions of 5 % aqueous sodium hydroxide. 7. Put the Erlenmeyer flask labeled "hydroxide" aside in a safe place. Later, you will isolate any compound that was extracted by the hydroxide. Isolation of any remaining compound Prediction: Which compound(s) would remain in the organic layer? 1. Add 5 mL of saturated aqueous NaCl and 5 mL of distilled H 2 O to the ethyl acetate layer in the separatory funnel. 2. Gently shake the separatory funnel to allow both thorough mixing of the solutions and effective extraction of the compound from the organic mixture. 3. Separate the lower aqueous layer into an Erlenmeyer flask and set aside. This will not be used for workup but should be kept until all the compounds are isolated from other layers in case of mistaken identity. 4. Pour the organic layer out the top of the separatory funnel in a 50 mL Erlenmeyer flask and dry with anhydrous Na 2 SO 4 . (There is no exact amount of Na 2 SO 4 , the amount will vary depending on the extraction success and remaining ethyl acetate.) 5. Filter the dried organic solution into a dry pre-weighed 50 mL round bottom flask and remove the ethyl acetate using the warm water bath in the hood. If a solid remains after evaporation of ethyl acetate, it is a neutral substance. Determine its weight.

Experiment 5: Separation of a mixture 8 Extraction Workup – isolating the organic compounds from aqueous solutions Bicarbonate: 1. Take the Erlenmeyer flask labeled "bicarbonate" to the fume hood and carefully acidify the aqueous solution by the dropwise addition of 6M HCl. ( CAUTION: The bicarbonate solution will vigorously liberate carbon dioxide when neutralized with HCl - that is, it will bubble a lot). Check to make sure the solution is acidic with litmus paper – it should turn the appropriate color shown on the litmus paper guide to indicate a low pH (very acidic). Perform the check by tearing a small piece of litmus paper off the roll, then use a glass stir rod to dab a bit of your solution onto the paper. 2. If a solid precipitates , add a boiling stone and then gently heat the solution to bring most of the solid back into solution (that is, redissolve the solid into solution). Allow to cool slowly to room temperature and then use an ice/water bath to complete the precipitation. If no solid precipitates, your unknown did not contain a carboxylic acid. In that case, skip steps 3-4. 3. When the solution is ice cold, isolate the solid precipitate by filtration. 4. Filter, rinse the solid with ice-cold water, allow to dry on watch glass, and determine the weight. Hydroxide: 5. Take the Erlenmeyer flask labeled "hydroxide" to the fume hood and carefully acidify the aqueous solution in the centrifuge tube by the dropwise addition of 6M HCl. CAUTION: The hydroxide solution will become hot when neutralized with HCl. Check to make sure the solution is acidic with litmus paper – it should turn the appropriate color shown on the litmus paper guide to indicate a low pH (very acidic). Perform the check by tearing a small piece of litmus paper off the roll, then use a glass stir rod to dab a bit of your solution onto the paper. 1. If a solid precipitates , add a boiling stone and then gently heat the solution to bring most of the solid back into solution (that is, redissolve the solid into solution). Allow to cool slowly to room temperature and then use an ice/water bath to complete the precipitation. If no solid precipitates, your unknown did not contain a phenol. In that case, skip steps 3-4. 2. When the solution is ice cold, isolate the solid precipitate by filtration.

Experiment 5: Separation of a mixture 9 3. Filter, rinse the solid with ice cold water, allow to dry on watch glass, and determine the weight. Final Analysis: Determine the percent (by mass) of each compound in the mixture 1. Record the mass recovered of each organic compound. Determine the percentage of your sample lost during the extraction process. Data/Results: Record your data, observations, and results on the report sheet Answer all prelab and post lab questions on the report sheet.

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Exp 5: Separation of a Mixture Report Sheet 10 Name: ______________________________ Pre-Lab Questions: Complete these questions before coming to lab. Your TA will check for completion at the start of the lab. 1. Draw the structures of the 3 possible organic compounds. If the compound will react in an acid-base reaction, draw both the protonated and deprotonated forms. 2. On the flow chart below, predict where each compound would be found. Use the numbers provided to make it easy to read. Indicate if the compound would have a charge using + or – Jason Galvan - - * o + # X-o-tHy Me OH 8 - ↑ - It H + - + Ha X UH NaOH 13 I - 0x - Xi Eor EXH - ~ ~ I I N y I

Exp 5: Separation of a Mixture Report Sheet 11 In Lab: Data and Observations Unknown: _______ Weight of unknown: ___________ Weight of mixture: _____________ Record your observations as you perform the experiment. Use drawing to help communicate what you see in the flask. Be sure to indicate which compound is present in each layer of the separation. Extraction 1: NaHCO 3 Reaction: Possible substance in the Bicarbonate Flask: ______________________________ Extraction 2: NaOH Reaction: Possible substance in the NaOH Flask: _______________________________ A 1 . 899 9 6g - Reaction because there was pressure - small amount of bubbles , showing that there is a slight reaction - small bubbles are forming after shaking - fuggy liquid with bubbles pressure is being released during shaking - second shake showed twice as many bubbles for a small amount of time . OH + - + Ha

Exp 5: Separation of a Mixture Report Sheet 12 Isolation of neutral organic from ethyl acetate: If Any! Mass of round-bottom flask: _______ Mass of flask + compound: _______ Mass of compound: _______ Extraction 1 workup: Mass of watch glass: _______ Mass of filter paper: _______ Mass of watch glass +filter paper + compound: _______ Mass of compound: _______ Extraction 2 workup: Mass of watch glass: _______ Mass of filter paper: _______ Mass of watch glass +filter paper + compound: _______ Mass of compound: _______ Unknown: ______ Extraction 1 – NaHCO 3 Extraction 2 – NaOH Final Organic Compound name present in each layer (NA if no compound present): Mass of recovered compound (NA if no compound present): Percent recovered of initial sample: 14 . 999 45 . 59 · 519 35 . 199 · 89 39 : 929 4 . 739 35 . 199 · 89 37 . 829 2 139 A Benzoic 4 fertbutylphenol 1 - 4 Acid dimethoxybenzene 4 . 739 2 . 43g · 51g + 12 43) + 1 . 51) x 100 722 . 01 . ( . 51) x11 - 8 71 . 1 09 unknown A = Benzoic Acid = 1-4 dimethoxybenzene

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Exp 5: Separation of a Mixture Report Sheet 13 Post lab Questions 1. In the extraction workup, you added acid until you reached a pH of about 2. Explain why it was important to get to a sufficiently low pH value. What would have happened if your pH was too high? How would this change your estimate of the percent compound in the mixture? 2. What are some possible explanations for the loss of mass during your extraction procedure? It is Important to have a low pH-value because the lower the value , the organic compound becomes more water-soluable by taking the acidic functional groups and converting them into a more soluable form . Having a high pH-value would make a lower yeild o the compound , which then results in an under-estimation of what were are calculating . some possible explanations to why there might be loss of mass during the extraction procedure is because of possible spills and and not correctly separating compounds . Not releasing the preasure correctly When separating .