These two Case Studies come from a National Center on Case Studies. I think that a case study approach is very useful in applying knowledge and this is what makes….
Acid-base extraction is the process of purifying of an organic acid and an organic base of an organic mixture. It is the procedure using sequential liquid-liquid extraction to purify the acid and the base from the mixture based on their chemical properties and solubility of the molecular and ionic forms. Acid- base extraction is commonly performed during the work-up of a chemical reaction. The acid-base extraction procedure can also be used to separate very-weak acids from stronger acids and very-weak bases from stronger bases as long as the difference of their pKa (or pKb) constants is large enough.
The major limitation of this reaction is that it is not possible to separate chemically similar acids or similar base with a large different form of ions solubility between their charged and their uncharged form. The main purposes of this experiment are as follows: i. To extract benzoic acid, 4-nitroaniline and biphenyl from organic compound or from the mixture in order to purify them.
ii. To develop the TLC plate and visualize the obtained TLC plate. iii. Learn to use a separatory funnel properly in lab while doing any other similar or different experiments.
iv. Learn to use and see how vacuum filtration and rotory evaporation/vacuum works. Reactions; i. ii. NaOH 4-nitroaniline 4nitroaniline hydrochloride (salt of PNA) iii. O Remains unchanged in theCH3 C O CH2 CH3 Biphenyl Ethyl-acetate Experimental procedure: DAY ONE: Obtained tube with labeled as sample#7 of 2. 12gm of a dry mixture containing benzoic acid, 4-nitroaniline and biphenyl place it into the 100ml beaker, 30ml of ethyl acetate were measured and poured into the same beaker and stirred until all solids were dissolved.
A small amount (1-2) drops of this mixture was separated into a separate test tube, covered and label it a “M” as a mixture and set aside to use for next day experiment. The mixture that remains is poured into a separatory funnel, and set it to the ring stand. 10ml of 1M of HCL were added to separatory funnel which is tightly secured. The separatory funnel was taken to the hood where it was shook gently for about a minute and, allows the gas to escape that build up from reaction. Vent after 5 seconds, then vent again every 10 seconds.
After ~1 minute of shaking, allow the two layers to separate. Collect the aqueous layer in 50ml Erlenmeyer flask. Label this flask as A for “Acid Extract”. Repeat this process in same manner by adding another 10ml of 1 M of HCL again. And collect the aqueous layer again in same flask. (20ml total) Add 10ml of 1M NaOH to the remaining organic layer of separatory funnel. With the stopper firmly secured take funnel back to hood and shake it gently for ~ 1 Minute and then allow the gas to escape in hood. Vent after about 5 second, then vent again every 10 second.
After gently shaking allow the two layers to separate and collect the aqueous layer into separate 50ml Erlenmeyer flask. Label this flask as B for “Base Extract”. Repeat the same process in a same manner one more time by adding 10ml of 1M NaOH. And collect the aqueous layer in same flask by combining the extract. Poured the organic layer into another 50ml Erlenmeyer flask and add 1 or 2 spatulas of anhydrous Na2SO4 to the flask. Swirl gently and, stir for 5 minutes. Add 10ml of ethyl acetate to the mixture. Filter it into a 100ml of round bottom flask.
Ethyl acetate is then removed under a vacuum with a rotary evaporator. The beakers with aqueous acid “A” and Aqueous base “B” are cooled by placing them in an ice bath. Add 5-10ml of 6 M NaOH to the flask “A” and add 5-10ml of 6 M HCL to the “B”. Allow the flasks to sit in ice bath for around 5 minute to complete the crystallization. Isolate that crystal by using vacuum filtration using a Buchner funnel. Separate the crystal and the compound of the round bottom flask into the pre weight filter paper and watch glass. Leave those crystals uncovered but secured for next period.
DAY TWO: Take the crystals out and use a capillary tube to tap some of those crystals into the capillary tube, settle those crystals to down of the capillary tube for about 3-4mm by using a long tube. To move the crystals to the end of the tube, invert the capillary tube so that the sealed end is down and drop it into the piece of long glass tubing so that the capillary tube hits the bench top with enough momentum to force those crystals to slide down to the bottom of capillary tube. Make sure to note which tube is acid, base and neutral.
Place the three tubes in Mel-Temps slots that can be seen through magnifying glass of your melting point apparatus. Adjust the Mel-Temps heating so that rate of heating is around 2 Degree Celsius per minute. Watch carefully to determine the melting point for all three compounds and compare them to the melting point of the pure substance from the Aldrich catalog. DAY THREE: Take a TLC plate and capillary tubes, since there are four compounds need to be tested. Take four capillary tubes, and four test tube to dissolve the compound for TLC plate.
Dissolve a crystals that was obtained from day two period, spatula full will work with 2-3 drops of 20% ethyl acetate/hexane into the test tube. Label all the test tube for each mixture, Acid (A), Base extract (B), Mixture (M), Organic extract (O). On TLC plate, using pencil spot the following four compounds as well and record it into separate paper which spot represent which compound. Then use capillary tube to put all four mixtures of the test tube to put into TLC plate, just a dot of mixture in TLC plate will work.
Make 20ml of 20% ethyl acetate/hexane mixture and pour that into 250ml beaker, cover a beaker with watch glass. Put TLC plate into the beaker and cover it with watch glass. Watch the TLC plate to develop, let the solvent travel TLC plate just below the 1cm from the top of the plate. Once it reach the site took the plate out and let it air dry for a while then visualize the plate under UV-light, circle the spots gently with a pencil. The main purpose of this experiment was to separate a prepared mixture of benzoic acid, 4-nitroaniline and biphenyl by using acid-base extraction technique.
The fundamental theory behind this technique is that salts, which are ionic, tend to be water-soluble while neutral molecules tend not to be. A sample comprised of mixture of benzoic acid, 4-nitroaniline and biphenyl was obtained which was pre-weighed of 2. 12gm. Out of 2. 12gm of mixture we obtained 0. 32gm of benzoic acid, 0. 98gm of biphenyl and 0. 40gm of 4-nitroaniline. The benzoic acid which is base extract appears white in color has 64% of mass recovery whereas, biphenyl has 82% and 4-nitroaniline got highest percentage recovery of 92%. Furthermore, the melting points of all these three isolated crystals were determined.
The results of the melting point were slightly different than what the reference book (Aldrich catalog) told us they should be, for instance, benzoic acid melting point range went from 121°C-125°C slightly higher than what was expected melting point, which give us 1. 24% of an error. Biphenyl melting point range was from 68°C-70°C which fits the reference book (Aldrich catalog) explanation of melting point for biphenyl and finally for 4-nitroaniline melting point went from 146°C-149°C which also shows us some error as it only starts melting from 146°C and ends at 149°C, giving us 2.
4% an error. These results indicate us there were some impurities mixed with these crystals that we obtained in our experiment. This error could have occurred while we were separating the layers from the separatory funnel as it could have picked up some slight part of other layer. Since, benzoic acid and the biphenyl were extracted after the 4-nitroaniline, it is possible that the layers could have mixed and contaminated the extracted layer from the funnel. The precision required to pipette a specific layer out of a reaction tube becomes more accurate with experience.
Such contamination could have fluctuated melting point ranges from the expected temperature readings as well. The final procedure of collecting the crystals yielded from the re-crystallization for analysis of melting ranges could have resulted unwanted contaminates that affected the melting ranges. Possible contaminates could have originated from the laboratory equipment or while separating the layer from the separatory funnel. Either situation could potentially result in a lower than expected melting range.
The last thing is preparing the TLC plate and calculating the Rf of each isolated crystal from the experiment as well as the mixture that was separated of the organic layer base on their movement in TLC plate. Lab Questions: 1) Draw the TLC plate in the space below. Use the exact dimensions. Label each compound and label the distance traveled by each spot and the solvent on your drawing. Calculate Rf of each compound and record the values in the data table. 2) For each compound on the TLC plate, explain the observed Rf.
Discuss the correlation between Rf and the relative polarity of each compound. On the TLC plate the most polar compound travel the farthest and the most non-polar travel the least distance. Retention factor is a function of how far the compound migrates down the plate, which occurs due to the attraction of polar molecules to polar compound. On TLC plate silica is very polar and our isolated crystals like biphenyl traveled only 2. 4cm which indicate that biphenyl is very polar and that’s why it couldn’t travel much in TLC plate.
Also 4-nitroaniline and mixture were also polar which didn’t traveled much on TLC plate giving very low Rf. Whereas benzoic acid traveled 3. 6cm showing that it is non-polar compound in compare to other compounds. Reference: Clayden, J; Greeves, N; Warren, S; Wothers, P. Acidity, basicity, and pKa. In Organic Chemistry, 2nd ed. ; Oxford: New York, 2009; pp 181-207. University of Texas chemistry department. http://sbrs. cm. utexas. edu/WS/commonmistakes. pdf (accessed on 09/23/2013) Encyclopedia website; Acid-base extraction. https://en. wikipedia. org/wiki/Acid-base_extraction (accessed on 09/23/2013)