Construct a flow chart describing the seperation of the
mixture and the isolation of each compound in this experiment. (Lab
steps/procedures includes for reference)
4. Construct a flow chart describing the separation of the mixture and the isolation of each compound in this experiment.
A commonly used method of separating a mixture of organic compounds is known as liquid-liquid extraction. Most reactions of organic compounds require extraction at some stage of product purification. In this experiment you will use extraction techniques to separate a mixture of an organic acid, a base, and a neutral compound. Extraction is a method of separating the components of a mixture by partitioning the components between two immiscible liquids (phases). One of the liquids is aqueous (typically water, acidic, basic or salt solutions) and the other is organic. If a compound is combined with two immiscible solvents and allowed to interact with both liquids, the compound will partition itself in the two liquids according to its solubility in each. The compound will be found in the phase in which it is more soluble. When the compound is much more soluble in one phase than in the other, the combination of phases provides a good separation. A constant can be determined experimentally by determining the amount of compound dissolved in cach phase. This partition (or distribution) constant is the ratio of the solubility of the compound in one solvent over its solubility in the second. Organic acids and bases can be separated from each other and from neutral compounds by extraction using aqueous solutions of different pH values. Most organic carboxylic acids are insoluble or slightly soluble in water, but these compounds are highly soluble in dilute aqueous sodium hydroxide because the acid is deprotonated by the base producing the sodium carboxylate salt. RCO2H(solv) + OH-(aq) +RCO2 (aq) + H2O(aq) The carboxylic acid can be selectively isolated by dissolving the mixture in an organic solvent that is immiscible with water, and then extracting the solution with sodium hydroxide. The basic aqueous solution containing the carboxylate salt is acidified, causing the sodium carboxylate salt to convert back to the carboxylic acid, which is not water soluble. The acid will precipitate from the solution, as shown here. RCO2 (aq) + H'(aq) +RCOH(s) Organic bases (e.g., amines) that are insoluble in water can be separated by extraction with hydrochloric acid. Addition of HCl to the amine produces the corresponding ammonium salt, which is soluble in water but not in organic solvents. RNHz(solv) + H(aq) - RNH3(aq) The amine can be recovered from the aqueous solution by treatment with a base, converting the ammonium salt back to the amine. The amine is not water-soluble and will precipitate, as shown here. RNH3(aq) + OH(aq) - RNHz(s) + H2O(aq) Using your understanding of these properties, separation of a mixture containing a carboxylic acid, an amine, and a neutral compound can be carried out via sequential acid and base extractions. The precipitates will be collected and characterized by melting temperature analysis.
A special piece of glassware called a separatory funnel is used for extraction. The mixture is dissolved in one of the phases and both phases are placed in the separatory funnel. The denser layer will be on the bottom and the less dense layer on the top. Be sure you know which layer is which! Helpful Hints: A better separation may be obtained if the chemist extracts twice with smaller volumes, rather than once with a large volume. It is important to remember that the purpose of shaking and mixing the two layers is to allow the compounds to partition between the two immiscible liquids. It is also important to remember to shake gently to avoid the formation of an emulsion. An emulsion is a suspension of small globules of one liquid in a second liquid that makes it very hard to completely separate the layers again. After the extraction the separated organic phase always contains some water. To remove the last traces of water, a drying agent is added to the organic phase. A drying agent is an anhydrous salt that will react with water to form a hydrate. A small amount of drying agent, enough to cover the bottom of the flask, is added to the organic solution. After swirling the flask, check to see if there is an excess of drying agent when the hydrate is formed, the powder clumps. Therefore if there is excess loose powder, the water has been completely removed. Steps in Extraction 1. Prepare the separatory funnel. Place the funnel in an iron ring attached to a ring stand. Close the stopcock (bar should be horizontal) and place a collection flask underneath. 2. Add the liquids: pour the desired liquids in the top. Make sure it is no more than 4 full to leave room for mixing. 3. Mix the layers: Swirl the layers gently. Then close the top and give it a slight twist. Holding the stopper in the palm of your hand (wearing gloves!) invert the funnel. Immediately open the stopcock to vent any gases. Always aim the funnel into the hood and away from yourself or anyone else. Close the stopcock and shake the funnel back and forth (not around). Vent the stopcock again. Shake and vent again. Repeat. Make sure the stopcock is closed and return to the upright position in the iron ring. Remove the stopper immediately. Separating the layer: The top layer must come out the top and the bottom layer out the bottom. First with the funnel in the iron ring and with the flask below it, open the stopcock. When the lower layer is almost drained, close the stopcock partially to decrease the flow. When the lower layer is removed the upper layer can be poured out the top, or more liquids can be added.
PROCEDURE Part 1 Extraction 1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood. 2. Weigh out approximately 1.0 g of the sample mixture. Record the mass to the nearest 0.001 g. Transfer the mixture to a 100 ml beaker and dissolve it in 15 mL of ethyl acetate. CAUTION: Ethyl acetate is flammable. Be sure that there are no open flames in the room during the experiment. 3. Clamp the support ring onto a ring stand and place the separatory funnel into the ring. Pour the solution into the separatory funnel and add 5 mL of 6.0 M hydrochloric acid. CAUTION: Handle the hydrochloric acid with care. Can cause painful burns if it comes in contact with the skin. 4. Cap the funnel and gently invert several times, venting frequently to avoid pressure buildup. When venting the funnel, point the tip away from your face and open the stopcock to release the pressure. Place the funnel on a support ring with a clamp and allow the solvent and aqueous layer to separate. Leave the funnel uncapped. If an emulsion forms (bubbly layer between the acid and ethyl acetate layers) let the funnel sit for a few extra minutes the proceed to step 5. Organic Chemistry with Vernier Separation of Organic Compounds by Acid-Base Extraction Techniques 5. Drain the lower aqueous layer into a 50 mL Erlenmeyer flask labeled HCl layer. Try to avoid getting any of the emulsion layer into the flask. If necessary leave a tiny amount of acid in the funnel. Repeat the extraction twice more with 5 mL of 6.0 M hydrochloric acid cach, draining the second and third aqueous layers into the same Erlenmeyer flask. Save the solvent layer in the separatory funnel for later use. Continue on to step 6 while your partner continues with the ethyl acetate layer in step 8. 6. Cool the flask containing the acidic aqueous extracts into an ice water bath. Slowly add 6.0 M sodium hydroxide with a pipet until the aqueous layer is basic. Use pH paper to test. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
U.U M U LUUIU WILL a pipet u ue ayucous layer is DASTU. USC pn peper WLSL CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. 7. Collect the solid using vacuum filtration and save the solid for melting temperature analysis in Part II. Note: Be sure to record the mass of the filter paper before placing it in the vacuum funnel. 8. Extract the saved ethyl acetate layer in the separatory funnel with three 5 mL portions of 1.0 M sodium hydroxide. Drain the aqueous layer into a 50 mL Erlenmeyer flask labeled NaOH layer. Save the ethyl acetate layer in the separatory funnel. 9. Cool the flask containing the basic aqueous extract in an ice water bath. Using a pipet, slowly add 6.0 M hydrochloric acid until the aqueous layer is acidic. Use pH paper to test. CAUTION: Handle the hydrochloric acid with care. Can cause painful burns if it comes in contact with the skin. 10. Collect the solid using vacuum filtration and save the solid for melting temperature analysis in Part II. Note: Be sure to record the mass of the filter paper before placing it in the vacuum funnel. 11. Add 10 mL of saturated aqueous sodium chloride solution to the ethyl acetate solution remaining in the separatory funnel and shake gently. Be sure to vent frequently. 12. Allow the layers to separate and discard the lower aqueous sodium chloride layer. Pour the solvent layer into an Erlenmeyer flask containing approximately 1 g of anhydrous sodium sulfate and allow it to stand for about 10 minutes, swirling occasionally. 13. Gravity filter the contents in the flask, including the sodium sulfate, into a clean 100 mL beaker. Evaporate the ethyl acetate in the fume hood using a 75 degree C water bath. CAUTION: Ethyl acetate is flammable. Be sure that there are no open flames in the room during the experiment. Do not leave unattended 14. Weigh the recovered solid and record the mass to the nearest 0.010 g. Save the solid for the melting temperature analysis in Part II. Part II Melting Temperature- to be done next week after samples are dry 15. Obtain a small amount of the isolated solid from the acid extraction. The solid should be in a powdered form. If it is not use a mortar and pestle to carefully grind the solid to a powder. Pack a capillary tube 3-4 mm (-1/8 inch) deep with your sample. 16. Check the control dial on the Melt Station to confirm that it is in the Off position. Connect the Melt Station power supply to a powered electrical outlet 17. Connect the Melt Station to a LabQuest or to a computer interface. Choose New from the File menu of the data collection program.
Separation of Organic Compounds by Acid-Base Extraction Techniques 18. Carefully insert the capillary tube of solid into one of the sample holders of the Melt Station. 19. Begin collecting melting temperature data using the Melt Station. 20. Adjust the control dial in order to determine the approximate melting temperature range for the sample. 21. When finished, stop data collection and turn the dial to the Fan/Cooling setting. Record the melting temperature range in your data table. 22. Store the run and collect a second run, if desired. 23. Repeat the necessary steps to collect melting temperature data for the isolated solids from the base and neutral extractions. 24. At the end of the experiment turn the control dial on the Melt Station to Off. Dispose of the capillary tubes as directed by your instructor.