Extraction: Separation of Acidic Substances Notes

Transcription

1 Reminder: These notes are meant to supplement, not replace, the laboratory manual. Extraction: Separation of Acidic Substances Notes 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 have a low solubility in water but have a high solubility in organic solvents. If a neutral organic compound undergoes an acid-base reaction and is converted into its conjugate base, that conjugate base is an ion. Ions have separate negative and positive charges. Ions have a high solubility in polar water and a very low solubility in non-polar organic solvents. If water is present, this causes the ion to migrate or partition to the aqueous phase from the organic phase. In CHEM 1020 Module 11A, you learned how the presence of polar groups determine if a compound will be soluble in water or not. Overview of the Experiment: The experiment begins with two neutral organic compounds (benzoic acid and 2- naphthol) dissolved in an organic solvent, MTBE (methyl t-butyl ether). An aqueous 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 the ion 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 and transforming the ion back into a neutral organic compound. The solubility of the neutral material in water is low, and the compound precipitates. The solid 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.

2 Separatory Funnels which are not properly shaken and vented can build up pressure and spew contents onto unsuspecting faces, eyes and neighbors. Wear eye goggles at all times during this class. Do not point separatory funnel at yourself or others. Vent frequently. 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. Proper Waste Disposal is very important in this experiment. All excess sodium bicarbonate solution, excess hydrochloric acid and the waste from the vacuum flask is aqueous and not organic. Aqueous waste needs to be disposed of down the drain with copious amounts of water. Do not place excess sodium bicarbonate solution, excess hydrochloric acid or the waste from the vacuum flask in the Non-halogenated liquid organic waste. If sodium bicarbonate and acid are mixed in a bottle and sealed, gasses will form and the container can rupture. If the container is glass, as organic waste containers typically are, this can cause dangerous broken glass projectiles. Be very careful about proper waste segregation. Wear goggles and lab coat at all times during this lab. Be extra cautious when handling concentrated hydrochloric acid. Neutralize and wipe up all spills 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. Solubility: The amount of a solute which will dissolve in a given amount of solvent. This is typically given as grams of solute per 100 gram 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. Density: The mass per unit volume of a material. This is commonly listed as grams per ml (g/ml) or grams per cubic centimeter (g/cc). Equilibrium Constant (K): The numerical 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.

3 pka: The negative log of the acid equilibrium constant. pka= - log K a where the acid equilibrium constant K a is equal to: K a = [H 3O + ][A ]. The smaller the [HA] pk a the stronger the acid. Polarity: An unequal distribution of positive and negative charges. Ions with a positive and negative charge are exceedingly polar. The polarity of a molecule (microscopic) is often quantified by dipole moment. The polarity of a solvent (macroscopic) can be described by dielectric constant. Dipole Moment: The vector sum of all the individual bond moments within a molecule. Dielectric Constant: A measure of a materials ability to transmit an electric field. 2. Hydrocarbons typically have a low solubility in water. As the carbon number increases, the solubility in water decreases. As a general rule of thumb, an organic molecule with 4 or less carbons will be soluble in water. A molecule with 7 or more carbons will not be soluble in water. If a molecule has 5 or 6 carbons it may or may not be soluble in water. As more polar groups are added to a carbon skeleton, the solubility in water increases. Ethanol (2 carbons) is completely miscible in water. 1-Hexanol (6 carbons) is only slightly soluble in water (0.59 grams will dissolve in 100 g of water iii ), while Glucose (6 carbons and 5 OH groups) is highly soluble in water (133 g/100ml) iv. 3. Ionic materials, with separate positive and negative charges (Na +, K +, B - ), are incredibly polar. Hydrocarbons are very non-polar and exist at the other end of the polarity spectrum. Intermediate polarity is determined by the polarity of the bonds within the molecule and the intermolecular forces between molecules. Less More Polar Low Solubility in water High solubility in hexane Polarity of compounds by class: Hydrocarbons < ethers < esters <ketones < alcohols< carboxylic acids< water< ions Examples: Polar High Solubility in water Low solubility in hexane Polarity of bonds covalent < polar covalent < ionic Polarity of intermolecular forces London dispersion forces < dipole-dipole < hydrogen bonding < ionic 4. Two solvents will be used; water and Methyl t-butyl ether (MTBE). Water has a dielectric constant of 78 and MTBE has a dielectric constant of 47. (Hexane for comparison has a dielectric constant of 1.0) v Water is much more polar than MTBE. 5. Compounds dissolve best in solvents with a similar polarity. Like dissolves like. Compounds of high polarity have a high solubility in high polarity solvents and a low solubility in low polarity solvents. Ions are very polar. Ions have a high solubility in water

4 and a low solubility in organic solvents. Sodium benzoate and sodium naphthoxide are ions. They have a low solubility in MTBE and a high solubility in water. Most neutral organic compounds have a low solubility in water but an increased solubility in organic solvents. Neutral benzoic acid and neutral 2-naphthol have a low solubility in water and a high solubility in MTBE. 6. 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 (C 10 H 7-0H) pka = Naphthonate ion ((C 10 H ) Water (H 2 O) pka = 15.7 Hydroxide ion (HO-) 7. 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 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). Below are ALL the possible reactions for the lab. Examine each reaction, determine if each compound is acting as an acid or a base. Compare the two acids and determine which acid is stronger. Next 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.

5 Possible Reactions of Organic Mixture with Sodium Bicarbonate (NaHCO 3 ) Possible Reactions of Organic Mixture with Sodium Hydroxide (NaOH) Protonation of anions with hydrochloric acid (HCl) 8. Additional structures and information. The organic solvent is MTBE or Methyl t-butyl Ether. The IUPAC name, which is rarely used for this material is 2-methyl-2-methoxypropane. 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.

6 MTBE= Methyl t-butyl Ether= or CH 3 OC(CH 3 ) 3 Sodium bicarbonate or sodium hydrogen carbonate is NaHCO 3 or. This is baking soda and a weak base. We are using sodium bicarbonate dissolved in water. Sodium hydroxide Na + - OH, is a strong base. This is a primary ingredient in Draino Liquid Clog Remover vi. We are using sodium hydroxide as a 1.50 molar (M) solution in water. Concentrated hydrochloric acid is 12.0 molar (M) HCl dissolved in water. 9. If a neutral organic compound dissolved in an organic solvent is deprotonated and turned into an ion, the solubility of that material 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 deprotonate one of a mixture of dissolved organic compounds and turn only one of the compounds into an ion, then that compound can be separated from the other materials. This concept enables this separation experiment to be successful. The ion in water is subsequently treated with hydrochloric acid, reforming a neutral organic material. If no low polarity solvent is present into which it can partition, it will precipitate as a solid. The solid is isolated by vacuum filtration. 10. The concentration of benzoic acid and 2-naphthol in the stating MTBE solution is 2.50 x 10-2 g/ml ( g/ml). To determine the mass of starting benzoic acid and 2-

7 naphthol, the starting volume of solution that you used in milliliters (ml) needs to be multiplied by this concentration (g/ml). This is the concentration of each material. Starting mass = starting volume (ml) x starting concentration (g/ml) = 40.0 ml x (2.50 x 10-2 g/ml) = 1.00 g If a different starting volume is used, a different starting mass of each will result. Note proper use of significant figures. 11. The percent recovery of benzoic acid, 2-naphthol and the total will be determined. Percent recovery is equal to the mass recovered divided by the starting mass times 100 percent. The mass recovered is the mass of product collected from the Buchner funnel. The starting mass is determined from the starting volume and the starting concentration. mass benzoic acid recovered Percent Benzoic Acid recovered = x 100 % starting mass benzoic acid The total is the combined mass recovered over combined starting mass. Total Percent Recovered mass benzoic acid recovered + mass 2 naphthol recovered = x 100 % starting mass benzoic acid + starting mass 2 naphthol 12. The purity of the recovered solids will be checked by melting point. Remember what you learned previously about the melting point range compared to literature values. The literature melting point vii for benzoic acid is o C, and 2-napthol is 123 o C. 13. 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 separatory funnel costs approximately $ Handle with care. 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 separatory 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 creates pressure.

8 14. Vacuum filtration is a method to isolate solids from liquids. The solid precipitate from flasks 1 and 2 will be recovered using vacuum filtration with a Buchner funnel, a dilter adaptor (gasket) and a 500 ml Filter flask. The set up will look like this. Your instructor will demonstrate. Ensure the filter paper covers all the holes in the bottom of the Buchner funnel. Turn the vacuum on all the way. Wet the filter paper with solvent (in this case water) and allow the paper and funnel to seat before adding product mixture. Allow to pull vacuum for a few minutes. Break vacuum seal, remove Buchner, remove filter paper with product from funnel. Replace with new filter paper. Replace funnel on flask. Wet paper, add second product. Wait a few minutes, then remove second filter paper and product. 15. 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, should be disposed of down the drain. Do not place aqueous waste into organic waste container. Never combine sodium bicarbonate and acid in a sealed container. Gasses will develop and the waste container may shatter. 16. Two flow chart showing the separation which takes place is appended. Make sure you understand each step involved. 17. Procedural changes may improve or diminish the success of the separation. The reaction with sodium bicarbonate is not very efficient. If all of the benzoic acid does not react with the weak base sodium bicarbonate, the residual benzoic acid will remain in the MTBE and further react with the second base, sodium hydroxide. If this happens it is common to have high purity but low recovery of the benzoic acid, and a low purity but high (over 100%) recovery of the the 2-naphthol. Two sodium bicarbonate extractions are performed to try to avoid or reduce this from happening. 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). 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.

9 Appendix 1. Flow sheet of Extraction Experiment.

10 Appendix 2. Step-wise flow sheet of Acidic Extraction Experiment. Separation and Extraction Recovery of Solids

11 References i Hackh s Chemical Dictionary, 4 th Ed, McGraw-Hill, 1972, p 491 ii Ibid p 259 iii International Programme on Chemical Safety, ICSC (March 6, 2013) iv Sigma-Aldrich, D(+)-Glucose Product Information Sheet, ile.tmp/g5400pis.pdf, (March 6, 2013) v Carey, F.A., Sundberg, R. J., Advanced Organic Chemistry Part A: Structure and Mechanisms, 4 th Ed, Kluwer Academic, New York,2000, p240 vi What s inside S.C. Johnson, (October 2, 2017) vii CRC Handbook of Chemistry and Physics, 65 th Ed, 1985, p C-149, C-390 Revised October 2, 2017 S.L. Weaver