Extraction exp - Chemistry

ACID-BASE EXTRACTION: SEPARATION OF AN ORGANIC ACID, AN ORGANIC BASE,AND A NEUTRAL COMPOUND (7/20/04)Extraction is one of the oldest chemical operations, and it is, undoubtedly, the most frequentlyperformed chemical operation. One or more extractions are performed during the isolation of theproducts from practically every organic chemical reaction. Whenever you make a pot of coffee,a cup of tea, or pot of vegetable soup you are performing an extraction (more precisely, a solidliquidextraction). The isolation of trimyristin from nutmeg which you performed earlier isanother example of a solid-liquid extraction. Solid-liquid extraction is commonly used byorganic chemists to isolate natural products. However, organic chemists employ liquid-liquidextractions even more often than solid-liquid extractions. There are actually two types of liquidliquidextractions: chemically passive and chemically active. In chemically passive extractions,distribution of a compound between two immiscible solvents occurs because of differentsolubilities in the two solvents, whereas in chemically active extractions, a compound ischemically altered to change its distribution between the two different solvents. This chemicalalteration of a compound to change its distribution between the two solvents is most commonlydone through an acid-base reaction. Sometimes, this type of extraction is referred to as an acidbaseextraction.For an example of how a chemically active extraction works, let us consider the separation of amixture of a neutral organic compound and a carboxylic acid. Let us assume that bothcompounds are soluble in diethyl ether but insoluble in water. A passive extraction using theseto solvents obviously will not work. However, if the carboxylic acid were converted into itsconjugate base, an anion which would be soluble in water but insoluble in diethyl ether, then aclean separation of the carboxylic acid (as its salt) from the neutral compound could be effected.Consider a mixture of a hydrocarbon and a carboxylic acid, both of which are soluble in etherbut insoluble in water. If an ether solution of the mixture is treated with an aqueous solution of abase,Ether-soluble, water-insoluble mixture:OC OHa hydrocarbon(neutral)a carboxylic acid(acidic)such as sodium hydroxide or sodium carbonate, the carboxylic acid will react to form its watersoluble carboxylate anion, but the neutral hydrocarbon will not react. The hydrocarbon willremain ether-soluble and water-insoluble. By an acid-base reaction, we have converted thecarboxylic acid (ether-soluble, water-insoluble) into its carboxylate anion (ether-insoluble,water-soluble).

Formation of a water-soluble salt:OOCOH+ OHCO+ H 2 Oether-solublewater-insolubleether-insolublewater-solubleBy extraction of an ether solution containing a neutral compound and a carboxylic acid with anaqueous solution of sodium hydroxide, we can separate the two compounds. The neutralcompound remains in the ether layer, while the carboxylic acid reacts with hydroxide ion to formthe carboxylate anion, which dissolves in the water layer. The two layers are separated using aseparatory funnel, and the aqueous layer is then acidified to regenerate the water-insolublecarboxylic acid.Recovery of the carboxylic acid:OC O+ HOCOHcarboxylate anioncarboxylic acidUsing a similar acid-base reaction, amines (organic bases) can be separated from neutral oracidic compounds by conversion into their conjugate acids, water-soluble cations. Thustreatment of an ether solution of a mixture containing an amine with an aqueous acid (e.g.,hydrochloric acid) produces a water-soluble cation that can be separated from water-insolublecompounds.Formation of a water-soluble salt:RNH 2 + H RNH 3ether-solublewater-insolubleether-insolublewater-solubleAfter separation of the ether and water layers, the water layer is then treated with aqueous baseto regenerate the water-insoluble amine.

Recovery of the amine:RNH 3 + OH RNH 2 + H 2 Oammonium cationamineBecause different conditions are used to convert acidic and basic organic compounds into watersolublesalts, the acid-base reactions we have discussed can be used to effect a number of typesof chemically active extractions. The flow chart that follows outlines the steps involved in theseparation of an organic acid (RCO 2 H), an organic base (RNH 2 ), and a neutral compound (RH)by acid-base extractions.

In this experiment, a mixture comprised of 0.5 g each of benzoic acid, p-nitroaniline, andanthracene will be separated into the individual components by acid-base extractions. All threecomponents are soluble in dichloromethane (CH 2 Cl 2 ) but only slightly soluble in cold water.ProcedureCO 2 H NH 2O 2 Nbenzoic acid p-nitroaniline anthraceneObtain 1.5 g of a 1:1:1 mixture of benzoic acid, p-nitroaniline, and anthracene and dissolve it in40 mL of dichloromethane in an Erlenmeyer flask. Pour this solution into a separatory funnelwhich has a teflon stopcock. (Use the addition funnel from your organic kit if necessary.) Ifsmall amounts of solid material do not dissolve, the solution should be gravity-filtered through asmall cotton plug into the separatory funnel. Extract the organic solution with 25 mL of 2Msodium hydroxide solution. Separate the layers and extract the organic layer with a second 25mL of 2M sodium hydroxide. Combine the basic extracts in an Erlenmeyer flask and label theflask "basic extract".Now similarly extract the organic layer twice with 25 mL portions of 6M hydrochloric acid.Combine the acidic extracts in a second Erlenmeyer flask and label the flask "acidic extract".Transfer the organic layer into a third Erlenmeyer flask and label the flask "neutral fraction".Add approximately 2-3 g of anhydrous sodium sulfate to this neutral fraction, swirl and let standfor at least 15 minutes with occasional swirling to hasten the drying.Cool the flasks containing the "acidic extract" and the "basic extract" in an ice-water bath.Neutralize the "acidic extract" by carefully adding 6M sodium hydroxide until the solution isdistinctly basic to pH paper. Similarly, neutralize the "basic extract" by carefully addingconcentrated (12M) hydrochloric acid until the solution is distinctly acidic to pH paper.Precipitates should form in each flask upon neutralization. The neutralizations will liberateconsiderable amounts of heat, so let the flasks stand in the ice-water bath until their contents areat, or below, room temperature.Collect each precipitate by vacuum filtration using a Buchner funnel and wash the filter cakeswith a small amount of cold distilled water. Allow the collected solids to air-dry until the nextlaboratory period.Decant the "neutral fraction" from the sodium sulfate into a tared round-bottomed flask (the flaskshould be of sufficient size such that it will be no more than half-filled by the solution). Rinsethe Erlenmeyer flask and the sodium sulfate with a small amount of dichloromethane, decantingit into the round-bottomed flask. Remove the solvent on a rotary evaporator. Reweigh the flask

with its contents, determine the weight of the neutral material, and calculate a percent recovery.Transfer this neutral material to a labelled vial and store it until the next laboratory period.After the base (p-nitroaniline) is dry, transfer it to labelled and tared vial. Reweigh the vial todetermine the weight of the solid and calculate it’s percent recovery. After benzoic acid is dry,transfer it to a tared flask. Reweigh the flask to determine the weight of the solid and calculateit’s percent recovery.Recrystallize benzoic acid from a minimum amount of hot water. (Use the solubility data tocalculate the approximate amount below, based on the grams you recovered.) Add an extra 2-5mLs of hot water if there is still solid present after adding the minimum amount of solvent. Ifthis does not dissolve the solid, you will need to perform a hot filtration (perform over a steambath, use a small plug of cotton). Before cooling, heat on a hot plate to remove the extra solventadded prior to the hot filtration (if performed).Dry and weigh the crystals. Determine melting points for benzoic acid and p-nitroaniline andcompare them to literature values. Separately, dissolve a small amount of each of the threecompounds in chloroform and spot a silica gel TLC plate with these solutions. Develop the TLCplate using chloroform. Visualize the spots under both short and long wavelength UV light andrecord your observations. Please help conserve ice. Share with your bench partner when possible! Flush aqueous solutions down the sink w/ plenty of running water. Sodium hydroxide,hydrochloric acid are both aqueous. Write out the reactions for the extraction (e.g., C 6 H 5 -COOH + NaOH C 6 H 5 -COO - Na + +H 2 O) and think about the difference in solubility between the acid and base compounds vs.their salt forms. Note: the pre lab does not need to include these reactions. Before lab, calculate the moles of benzoic acid and 2M NaOH you will use to extract it with.Then calculate the amount of concentrated (12M) HCl it will take to neutralize it. Includethese amounts in the reagent table of your pre lab. Do likewise for p-nitroaniline! The melting points of benzoic acid and p-nitroaniline are important physical constants forthis experiment (and therefore belong in the reagent table of your prelab)! Read all applicable techniques in your lab text (Zubrick) and otherwise prepare well for thislab experiment!

SOLUBILITY DATACOMPOUND TEMPERATURE SOLUBILITY (g/100 mL)0 o C 0.17Benzoic Acid20 o C 0.2990 o C 4.55p-Nitroaniline0 o C 0.08100 o C 2.22Anthracene 78 o C 1.49

Name:________________________________________Section:_______Date:________POSTLAB EXERCISEACID-BASE EXTRACTION: SEPARATION OF AN ORGANIC ACID, AN ORGANIC BASE,AND A NEUTRAL COMPOUND (7/20/04)>> Due no later than the 2 nd Friday following the scheduled experiment; no exceptions willbe made! Please answer questions on this form. (30pts TOTAL)

2. (2pts) Explain how benzoic acid was recoved from extracted solution:How was p-nitroaniline recovered?3. (2pts) The percent recovery for benzoic acid is usually lower than the percent recovery for p-nitroaniline. Comparing percent recoveries before purification, explain why this is true:Which compound had the highest recovery?4. (2pts) How was benzoic acid purified?5. (2pts) Which compound showed the largest R f value?Which compound showed the smallest R f value?Which compound fluoresced under longwave UV light?6. (2pt) On the basis of what you have learned in this experiment, plus the information thatphenol is soluble in aqueous NaOH but not in aqueous NaHCO 3 , whereas benzoic acid issoluble in both, draw a flow diagram showing how you would separate a mixture of benzoicacid, phenol, and aniline and attach it to this exercise.

7. (3pts) The pK a of benzoic acid is 4.19. Explain whether or not precipitation of benzoic acidfrom an aqueous solution of benzoate anion would be complete if hydrochloric acid is addeduntil a pH of 7 is obtained.