Expt 2. Acid/Base Extraction, Recrystallization, Sublimation, and C-13 NMR.

Part 1 - Acid/Base Extraction

Reading Assignment - Mohrig Chapter 10, 11, 12

Background

In this experiment you will learn how to use a separatory funnel for the purpose of carrying out liquid/liquid extraction, a useful separation method commonly referred to as just "extraction". The process of extraction involves intimately mixing a solution with an immiscible extraction solvent so as to allow compounds in the solution to partition between the two liquid layers that form after mixing is discontinued. Compounds that are more soluble in the extraction solvent than in  the original solvent will end up predominantly in that layer, whereas compounds that are less soluble will stay in the original solvent. Water is almost always one of the solvents with the other solvent being an organic compound such as diethylether (CH₃CH₂OCH₂CH₃) or dichloromethane (CH₂Cl₂), so the two layers can be referred to simply as the aqueous layer and the organic layer.  Physical separation of the layers then accomplishes the separation of the compounds in the solution based on their solubility characteristics.

Acid/base extraction is a process that allows the separation of organic acids, organic bases, and organic neutral compounds (not an acid or base) from each other based on the solubility differences of the organic acid (or base) and its conjugate base (or conjugate acid). Organic acids such as carboxylic acids (RCOOH), phenols (PhOH), and thiols (RSH), all have an acidic proton that can be deprotonated by aqueous base (usually NaOH) to form a salt form of the acid, which is much more soluble in aqueous solution than in organic solvents, as illustrated by equation 1 for a carboxylic acid. Hence, the carboxylic acid can be extracted from an organic solvent by aqueous NaOH.

                                                           RCOOH  +   NaOH(aq)  →   RCOO^-Na⁺(aq)  +  HOH   (1)

The original carboxylic acid can be retrieved from the aqueous layer by simply neutralizing the base with HCl(aq) and reforming the carboxylic acid (eq 2). The relatively insoluble carboxylic acid often precipitates at this point and can be collected by vacuum filtration.

                                             RCOO^-Na⁺(aq)  +   HCl(aq)  →   RCOOH(s)   +  NaCl(aq)   (2)

Conversely, organic bases (e.g., amines, RNH₂, R₂NH, or R₃N) are protonated by aqueous acid (usually HCl) to form salts that are much more soluble in the aqueous layer (eq 3). Hence, amines can be extracted from an organic solvent by aqueous HCl.

                                                                 RNH₂  +   HCl(aq)   →   RNH₃⁺Cl^-(aq)    (3)

The original amine is retrieved by treating the aqueous layer with aqueous base (NaOH) so as to deprotonate the salt (eq 4), which often precipitates and can be collected by vacuum filtration.

                                                    RNH₃⁺Cl^-(aq)  +   NaOH(aq)   →    RNH₂(s)   +  HOH  + NaCl(aq) (4)

Organic compounds that are neither acids or bases do not react with either NaOH or HCl and, therefore remain more soluble in the organic solvent and are not extracted.

Overview

You will use an acid/base extraction to separate a mixture of ibuprofen, caffeine, and the mosquito repellant, DEET. We will use the second week of the lab to purify the separated compounds using recrystallization or sublimation and then test their purities by mp determination.

Pre-Lab

The mixture given includes an organic acid, an organic base, and a (non-acid non-base) neutral compound. Identify which is which and write equations for the reactions that will occur when the mixture is extracted with HCl and then later with NaOH.  Also, make sure to include a literature pKa value for the acid and pKb for the base in your table of reactants and products.   Your planned procedures should explicitly state which compounds are obtained in the respective steps 11, 13, and 15.

Experimental Procedures

1. Dissolve 1.5 g of the provided mixture in 20 mL dichloromethane (DCM) and transfer the contents to a separatory funnel.
2. For steps 3-7 pay special attention to the information in chap 11.2 of Mohrig and to the instructions provided by the instructor on the proper use of the sep funnel. Also see https://www.youtube.com/watch?v=2A98YEKzsMI for a nice video tutorial. It is strongly recommended that you label all containers used to contain the various layers separated and solids obtained.
   
3. Add 15 mL 1M HCl. Cap the funnel and shake gently at first with frequent venting.  As it becomes clear that excessive pressure is not building up, end by shaking vigorously for 30 s or more and then allow the layers to separate. Before you go onto step 4 make sure you know which layer is organic and which is aqueous.
   
4. Into separate Erlenmeyer flasks, drain off the bottom layer through the stopcock and then pour out the top layer through the top of the sep funnel.
5. Return the organic layer to the sep funnel and and extract it with another 15 mL of 1M HCl using the same procedures as before. Combine the aqueous layers from the first two extractions and return the organic layer to the sep funnel.
6. Extract once more with 1M HCl, combining the obtained aqueous layer with that obtained previously.
7. Return the organic layer to the separatory funnel and extract it with 15 mL of 1M NaOH. Separate the layers as before and return the organic to the funnel.
   
8. Extract the organic layer twice more each time with 15 mL of 1M NaOH combining the obtained aqueous layers with that obtained from the first bicarbonate extraction.
9. Dry the organic layer over anhydrous Na₂SO₄. Remove the drying agent by simply decanting the liquid into a dry round bottom flask. See Chap 12 in Mohrig.
10. Remove the DCM solvent on the rotary evaporator (Mohrig, Fig 12.7), the instructor or TA will assist with this.
    
11. Use a Pasteur pipet to transfer as much of the obtained oily liquid as possible into a vial. Weigh and save for week 2.
12. Carefully add 1M NaOH to the aqueous layer from the HCl extractions so as to neutralize the pH. You can use pH paper to make sure the solution is no longer acidic and/or you can use the formation of precipitate as a gauge. The organic base is not soluble in water so it precipitates as it is reformed from the salt by deprotonation. Thus, maximum precipitate formation indicates complete neutralization.
13. Collect the formed solid by vacuum filtration on a Buchner funnel (Mohrig Chap 10.4). Weigh it and allow it to air dry until week 2.
14. Use 1M HCl to neutralize the aqueous layer from the NaOH extractions. Do the addition carefully and slowly.
    
15. Also collect this solid by vacuum filtration, weigh it and and allow to air dry until next week.

Assigned Questions

1. The original mixture was prepared  by using equal masses of the three compounds. Use this information to calculate the percent recovery of each compound. (The proper word here is "recovery" not "yield" because no net chemical reaction occurred. Instead we merely are recovering the unchanged components of the mixture.)
2. Use the pKa's of the acids on both sides of your equations for the acid/base extraction reactions (see prelab assignment) to calculate the equilibrium constants for these reactions. Hint 1: the pKb of a base and it's conjugate acid are related by the equation, pKa + pKb = 14. Hint 2: You will need to look up values for the pKa of water and of HCl. Hint 3: You can use this equation to calculate pKeq from pKa's: pKeq = pKa(reactant) - pKa(product).
3. Biochemists like to use the equation pKa = pH + log([HA]/[A^-]) to qualitatively predict whether an acid (HA) is mostly deprotonated (A^-)  or mostly in its protonated form (HA). The idea is that if pH < pKa (by at least one unit) then the log term is greater than 1.0 meaning that [HA]/[A^-] is greater than 10. Thus, under these conditions the acid is at least 90% in its protonated form. Conversely, if pH > pKa then [HA]/[A^-] is less than 10 and the acid is at least 90% deprotonated. In very general terms, if the solution is more acidic than the acid is (i.e., pH < pKa) then the acid is forced to stay protonated, but if the solution is less acidic than the acid (pH > pKa) then the acid will be fully deprotonated. Question: Calculate the pH of 1M NaOH(aq) and use it to determine if the organic acid was fully deprotonated in steps 7 and 8 of the extraction procedure.
   
4. What do your answers to questions 2 and 3 tell us about how much of the carboxylic acid and amine should theoretically have reacted when reacted with NaOH(aq) and HCl(aq) respectively?