Acid and Base Extraction - Fall 2002 | CHEM 217, Lab Reports of Organic Chemistry

Download Acid and Base Extraction - Fall 2002 | CHEM 217 and more Lab Reports Organic Chemistry in PDF only on Docsity! Updated October 10 th 2002 Acid and Base Extraction Click here to print in pdf Objective The purpose of this laboratory exercise is to introduce the concept of separating organic compounds by utilizing the concept of acid/base chemistry in order to effect the separation. Grading You will be assessed on: Completion of separation of low organic compounds● Write-up in your Lab Notebook (see Lab Notebook Guidelines) and answers to the Post-Lab Questions ● TA evaluation of lab procedure● Introduction Let us first begin with an introduction about extractions. As you learned so long ago, like dissolves like. Sodium chloride will dissolve in water (as an ionic species) but will not disolve in a nonpolar solvent. Nitrobenezene is insoluble in water but soluble in organic solvents such as methylene chloride. This premise is very useful when you are separating salt from an organic compound but this lab will not be as straight forward as that. AcidBase.html http://www.owlnet.rice.edu/~chem217/AcidBase.html (1 of 7) [10/10/2002 8:14:23 PM] In this lab you will be separating a mixture of four organic compounds. All are soluble in an organic solvent. In order to separate these compounds you will have to use another concept, acid/base chemistry. As you learned in general chemistry a Bronsted-Lowry acid is a proton source and a base is an acceptor and more importantly so is the resulting conjugate base and acid. You now have charged species (as seen below for HCl reacting with a generic organic base OrgB). HCl + OrgB ‡ Cl- + OrgH+ + B This ionic species is very soluble in water and poorly soluble in a typical organic solvent. You will use this principle and extraction technique to isolate and identify your four compounds. As mentioned before this will not be a mundane task. You will receive a mixture containing one strong acid, one weak acid, one base, and one neutral compound. You will then be asked to separate and identify these four compounds based on their solubility properties at various acidities. The compounds that you have been given can be identified by their pH dependent colors. When a separation has been achieved, you should have no trouble identifying the mixture. Your compounds will be limited to one of each of the following possibilities: Bases: Strong Acids: AcidBase.html http://www.owlnet.rice.edu/~chem217/AcidBase.html (2 of 7) [10/10/2002 8:14:23 PM] In the case shown above, where the acid is bromcresol green, the result is a dark blue salt that is readily soluble in water. When you add aqueous sodium bicarbonate (better known as baking soda) two layers, an aqueous and an organic layer, will form. The salt formed by the weak base and the strong acid will dissolve in the aqueous layer, and you will see a dark blue layer form on top. By removing this top aqueous layer and the compounds dissolved in it, you can remove all of the strong acid that was in the original mixture. The same general principle applies in removing the weak acid and the basic compound. The weak acid is weak enough that it will not react to form a salt with NaHCO3 but will react with the stronger base, NaOH. Therefore, adding NaHCO3 will not extract the weak acid while NaOH will. To remove the base, we will add an acid, HCl, which will also initiate an acid-base reaction to form a water soluble salt. Once the base, the strong acid, and the weak acid have been removed from your mixture, you will ideally be left with a pure sample of your neutral compound dissolved in methylene chloride. Remember that AcidBase.html http://www.owlnet.rice.edu/~chem217/AcidBase.html (5 of 7) [10/10/2002 8:14:23 PM] this neutral compound has no tendency to react with either acids or bases and will not form salts that allow it to dissolve in water. It will remain permanently in the organic layer, where you will be able to identify it by its color. In reality, some of the acids and bases you extracted will probably be left in the mixture as well, since the reactions may not have gone to completion. As a result you should expect a darker color in the neutral compound than would normally occur. Fortunately, the two compounds you will be given have very strong and bright colors in low pHs, and it is easy to tell the difference between the two by adding a small amount of concentrated acid to these neutral compounds. Remember to note all the colors you see in this lab, or you’ll be unable to identify your mixture. Experimental 1) Obtain 2 ml of your unknown in a 10x 100 mm test tube. 2) Obtain 5 ml of 1M HCl, 5 ml of saturated NaHCO3 and 5 ml of 1M NaOH in separate 10x 100 mm test tubes. Label each. 3) Using a Pasteur pipet, add about 1 ml of 1M HCl to your unknown 4) Mix the sample by pipetting up and down repeatedly 5) Let the tube sit undisturbed until two distinct layers form. 6) Note the color of the aqueous layer. This (upper) layer should indicate which base was in your mixture. Use a CLEAN Pasteur pipette to remove all of the aqueous layer and transfer to a clean, labeled test tube. 7) Repeat steps 3-6 two more times. By the third extraction the aqueous layer should be nearly clear. If it is not, continue to repeat the extraction steps until very little color remains in the top layer. 8) Make sure that all of the aqueous layer has been removed and add 1-2 ml of saturated NaHCO3. Mix the sample as before, repeating steps 4-6. Be sure to note the color of the aqueous layer. This will indicate your strong acid. Repeat two more times. 9) Once the strong acid has been removed, use the same procedure with NaOH to extract the weak acid. In this step some of you may have an orange layer on top and an orange layer on the bottom. These are still two different compounds, they just happen to be the same color. You may have to hold the test tube up to the light or view it against a solid background to see the interface of two phases, but it will be visible if you are careful. Again, be sure to note the color of the aqueous layer, as it will tell you the identity of the weak acid. 10) You are now left with just the neutral compound. In order to positively identify it, you may want to place about 0.5 ml in a small test tube and add 5-10 drops of concentrated HCl. AcidBase.html http://www.owlnet.rice.edu/~chem217/AcidBase.html (6 of 7) [10/10/2002 8:14:23 PM] This will lower the pH enough so that there will be no ambiguity between the distinct red-orange of Sudan 1 and the light yellow of Quinoline yellow. Caution: Concentrated hydrochloric acid is corrosive. Do not let it come in contact with your skin or clothing. If it does, wash it extensively with running water. 11) Using the colors you noted in the aqueous layers, identify the base, strong acid, and weak acid in your solution. The color of your acidified organic layer will allow you to identify the neutral compound. Check to see that you obtained the correct results, and if not explain why this might have happened. ● AcidBase.html http://www.owlnet.rice.edu/~chem217/AcidBase.html (7 of 7) [10/10/2002 8:14:23 PM]