Extraction: Separation of Acidic Substances Notes Authors: J. Sevenair, S. L. Weaver

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1 Reminder: These notes are meant to supplement, not replace, the laboratory manual. Extraction: Separation of Acidic Substances Notes Authors: J. Sevenair, S. L. Weaver Application: Acids and Bases are one the most fundamental principles of chemistry. Acidity and basicity are involved in determining chemical reactivity, separation, solubility, and transport of molecules across membranes. Background: Aqueous (water based) solvents are very polar. Organic solvents are much less polar than aqueous solvents. The underlying principle behind acid extractions begins with the fact that many neutral organic compounds are not soluble in water but are soluble in organic solvents. If a neutral organic compound is converted to an ion by reaction with a base, then the polarity of that compound is significantly increased. The ion typically has a high solubility in water and a reduced solubility in the organic solvent. This often causes the ion to migrate to the aqueous phase from the organic phase. One possible experiment begins with two neutral organic compounds (benzoic acid and 2-naphthol) dissolved in an organic solvent, MTBE (methyl t-butyl ether). A base is added which will selectively react with one of the materials and convert it into its conjugate base (an ion). This ion has a high solubility in water and a low solubility in MTBE, so it will migrate (partition) to the aqueous phase. The aqueous phase is removed along with the conjugate base of the first compound. This layer is later acidified, protonating the conjugate base. The solubility of the neutral material in water is low, and the compound precipitates and is recovered by filtration. This process is repeated with the addition of a stronger base to the MTBE which will react with the second compound and that material will deprotonate, form an ion then partition to the aqueous phase. Again the aqueous layer is removed, acidified and the precipitated neutral material is recovered. Safety considerations for this experiment: Concentrated Hydrochloric Acid is a strong acid. It is corrosive and will cause visible destruction of tissue and other materials upon contact. Avoid contact with skin, eyes, and clothing. Clean up all spills immediately. If accidental contact is made, immediately wash with copious amounts of water. Handle HCl in the fume hoods. Sodium Hydroxide Solution (1.5M) is corrosive even though it is not very concentrated. Avoid contact with skin, eyes, and clothing. Clean up all spills immediately. If accidental contact is made, wash with copious amounts of water. Separatory Funnels which are not properly shaken and vented can build up pressure and spew contents onto unsuspecting faces, eyes and neighbors. Wear eyegoggles at all times during this class. Do not point separatory funnel at yourself or others. Vent frequently. er.

2 Methyl t-butyl Ether (MTBE) is an organic solvent with a moderately high vapor pressure. Avoid breathing the fumes. MTBE is highly flammable, have no arcing sparking devices or open flames in close proximity to the solvent. Wear goggles and lab coat at all times during this lab. Be extra cautious when handling concentrated hydrochloric acid. Neutralize and wipe up all spill immediately.. 1. Some terminology related to Acid-base extractions: Acid: In this experiment we are discussing Bronsted-Lowry acids, hence an acid is a material that may lose a proton and form a conjugate base. HA -> H + + A - The stronger the acid, the weaker the conjugate base. Base: In this experiment we are discussing Bronsted-Lowry bases, hence a base is a material that may gain a proton and form a conjugate acid. B - + H + -> HB The stronger the base, the weaker the conjugate acid. Partition i : The distribution of a substance or ions between two immiscible liquids. Extraction ii : Dissolution and removal of one constituent of a mixture in a solvent. Precipitation: If the concentration of a compound in a solvent is greater than the solubility of that compound, the compound will no longer remain dissolved and will form a new solid phase. Equilibruium Constant (K): The numberical value of the concentration of the products divided by the concentration of the reactants. If the value of K is smaller than one, the equilibrium lies in favor of the starting material. The reaction does not proceed greatly in the forward direction. If the value of K is greater than one, the equilibrium lies in favor of the products. The reaction proceeds in the forward direction. pka: The negative log of the acid equilibrium constant. pka= - log K a where the acid equilibrium constanat K a is equal to: pk a the stronger the acid.. [ ][ ] [ ]. The smaller the 2. Useful pka information. (Do not memorize pka values) HA -> H + + A - ACID (HA) pka CONJUGATE BASE (A - ) Hydrochloric Acid (HCl) pka= -7.0 Chloride ion (Cl - ) Benzoic acid (Ph-CO 2 H) pka = 4.17 Benzoate ion (Ph-CO - 2 ) Carbonic acid (H 2 CO 3 ) pka = 6.35 Bicarbonate ion(hco - 3 ) 2-Naphthol pka = Naphthonate ion Water (H 2 O) pka = 15.7 Hydroxide ion (HO-) 3. Predicting if a reaction will proceed in the forward direction or not. To determine if any acid-base reaction will succeed, as shown, HA A 2 - A 1 + HA 2

3 First, identify the acid and the base on both sides of the arrow. Compare the pk a of the acids. Determine if the stronger acid is on the right or the left of the arrow. If the pk a of the acid on the left, HA 1 is a smaller number (that is, the acid is stronger) than the pk a of the acid on the right, HA 2 then the reaction will proceed in the forward direction. If the pk a of the acid on the left, HA 1 is larger (that is, the acid is weaker) than the pk a of the acid on the right, HA 2, then the reaction will not proceed in the forward direction (but it will proceed in the reverse direction). 4. Below are ALL the possible reactions for the lab. Examine each reaction, assign acid and base to each compound, determine if the reaction will proceed in the forward direction or not. Place an X across each arrow which will not proceed in the forward direction. Only write the reactions that WILL proceed in your lab notebook. Possible Reactions of Mixture with Sodium Bicarbonate (NaHCO 3 ) Possible Reactions of Mixture with Sodium Hydroxide (NaOH)

4 Protonation of anions with hydrochloric acid (HCl) O Na+ O O + HCl + OH NaCl(sln) 5. Additional structures and information. MTBE is Methyl t-butyl Ether. This compound is often added to gasoline to increase the oxygen content and improve the burn efficiency. MTBE has a density of g/ml and is less than that of water (density 1.00 g/ml) hence MTBE floats on top of water. or CH 3 OC(CH 3 ) 3 Sodium Bicarbonate or Sodium hydrogen carbonate is NaHCO 3. This is a weak base and is baking soda. We are using sodium bicarbonate dissolved in water. Sodium Hydroxide NaOH, is a strong base. In its pure form it is a white solid. We are using sodium hydroxide as a 1.50 molar (M) solution in water. Concentrated Hydrochloric Acid is 12.0 molar (M). This fumes releasing HCl gas. Avoid breathing the fumes, handle HCl in the hoods. 6. Aqueous solutions are very polar. Organic solvents are less polar than aqueous. Like materials like other materials like themselves. Ions are very polar. Typically ions have a higher solubility in water than organic solvents. Most neutral organic compounds have limited solubility in water but an increased solubility in organic solvents. If a neutral organic compound dissolved in an organic solvent is deprotonated and turned into an ion, the solubility in the organic solvent will be decreased and the ion will want to move or partition into an available aqueous solvent. IF a base is chosen which will selectively deprotante one of a mixture of dissolved organic compounds and turn only into an ion, then mixtures of compounds can be separated. This concept is why this separation experiment is possible. 7. The two compounds are separated as ions dissolved in water. It is desired to reform and isolate benzoic acid and 2-naphthol as pure solids. In order to do this, a very strong acid is added to the aqueous solutions, this will reprotonate the ions and turn them back into neutral organic molecules. Most neutral organic compounds have a low

5 solubility in water. 2-Naphthol and benzoic acid are not an exception. As neutral compounds they have a low solubility in the water, and hence will precipitate as a solid. The solid is isolated by vacuum filtration. A melting point will be taken to assess the purity of these materials. 8. Separatory funnels are specially designed glassware for the separation of immiscible liquids. They have a ground glass access port at the top and a stopcock at the bottom. This is the most expensive piece of glassware in your drawers. A new sep funnel costs approximately $ Handle with care. iii Place separatory funnel in ring clamp. Always place a receiver under stopcock. If the handle of the stopcock is aligned with the funnel, the stopcock is open and contents will drain. If the handle of stopcock is perpendicular to funnel the stopcock is closed. Funnels are stored with the stopper out to prevent the stopper from freezing. The stopper must be removed to allow the funnel to drain properly. Your instructor will show you the proper method of picking up, shaking, venting, and draining your sep funnel. Pay close attention. The funnel must be vented frequently to prevent vapor build up and prevent the stopper or the contents from launching. One reaction produces CO 2 gas which created more vapor pressure. 9. Waste MTBE must be disposed of in the non-halogenated organic liquid waste container. Aqueous neutralized waste in which all of the organics have been removed, may be disposed of down the drain. 10. A flow chart showing the separation which takes place is appended. Make sure you understand each step involved. Topics which may be included in quizzes after the experiment include the following. 11. Given pka, determine if a proposed reaction will proceed in the forward direction or not. 12. Changes to the procedure can make the separation either more or less efficient. More extractions with smaller volumes are more efficient than one extraction with the combined volume (Three 10 ml extractions are more efficient than one 30 ml).

6 If a mixture of dissolved organic compounds is mixed with a strong base to deprotonate both of them, they will both turn into ions, both migrate to the aqueous phase and no separation will occur. Appendix I. Flow sheet of Extraction Experiment. i Hackh s Chemical Dictionary, 4 th Ed, McGraw-Hill, 1972, p 491 ii Ibid p 259 iii Image from Reed College

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