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Using acid base chemistry in the separation of substances can be a quite useful technique in organic chemistry. A basic extraction can be carried out by applying a hot solvent such as water to a dry …

Using acid base chemistry in the separation of substances can be a quite useful technique in organic chemistry. A basic extraction can be carried out by applying a hot solvent such as water to a dry substance like coffee. This will allow for the extraction of caffeine and flavor from the coffee beans. Acid base chemistry can be useful in separating neutral, basic, and acidic organic substances by controlling the pH of the aqueous layer. In acid base extraction a solvent such as ether is mixed with an aqueous solution of a different pH. This will aid in ionizing the intended compound and separate it from the mixture. Once the molecule is ionized it will precipitate out of the ether solvent and dissolve into the aqueous layer. The aqueous layer can then be removed and the pure substance can be obtained through crystallization.

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  • 1. Separation of Acidic SubstancesAuthor: Dr. Robert D. Craig, Ph.D.
  • 2. Separation of Acidic SubstancesAuthor: Dr. Robert D. Craig, Ph.D.• 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.
  • 3. Background• Aqueous (water based) solvents are very polar. Organic solvents are much less polar than aqueous solvents.
  • 4. Such that
  • 5. Background• 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.
  • 6. Background• . 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.
  • 7. Get to the point Rob . .• Benzoic acid –not soluble in water• But the “sodium salt is”
  • 8. Get to the point . . ..• In order to separate these compounds, we will convert the water insoluble acid into a watersoluble salt:
  • 9. Subsequently . . .• Can use a strong acid to take the salt back to organic layer soluble acid form
  • 10. Background• One possible experiment begins with two neutral organic compounds (benzoic acid and 2-naphthol) dissolved in an organic solvent, MTBE (methyl t-butyl ether). ####This is changed (diethyl ether)
  • 11. Diethyl ether instead A base is added which will selectively react withone of the materials and convert it into itsconjugate base (an ion). These ions have a highsolubility in water and a low solubility inether, so they will migrate (partition) to theaqueous phase.
  • 12. Like dissolves like• 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.
  • 13. Not MTBE• 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.
  • 14. Oil floats on water . . .
  • 15. Acetanilde and Acetanilde and p-Tolic Acid
  • 16. Acetanilde and p-Tolic Acid
  • 17. Some terminology• 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.
  • 18. Some Terminology• 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.
  • 19. Some Terminolgy• Partitioning: The distribution of a substance or ions between two immiscible liquids.• Extractioning: 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
  • 20. Some terminology• 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
  • 21. Want to discuss• For octanol and water-partition and distribution
  • 22. Two phase system, hydrophobic (top) and hydrophilic (bottom) for measuring the partition coefficient of compounds
  • 23. Some terminology• pKa: The negative log of the acid equilibrium constant. pKa= - log Ka where the acid equilibrium constant Ka is equal to:The smaller the pKa the stronger the acid
  • 24. Smaller -pKa the stronger the acid.• 2. Useful pKa information. (Do not memorize pKa values)
  • 25. Smaller-pKa the stronger the acid. .ACID (HA) pKa CONJUGATE BASE (A-)Hydrochloric Acid (HCl) pKa= -7.0 Chloride ion (Cl-)Benzoic acid (Ph-CO2H) pKa = 4.17 Benzoate ion (Ph-CO2-)Carbonic acid (H2CO3) pKa = 6.35 Bicarbonate ion(HCO3-)2-Naphthol pKa = 9.5 2-Naphthonate ionWater (H2O) pKa = 15.7 Hydroxide ion (HO-)
  • 26. smaller-pKa the stronger the acid. .• First, identify the acid and the base on both sides of the arrow.• Compare the pKa of the acids. Determine if the stronger acid is on the right or the left of the arrow.• If the pKa of the acid on the left, HA1 is a smaller number (that is, the acid is stronger) than the pKa of the acid on the right, HA2 then the reaction will proceed in the forward direction
  • 27. .which way??????• If the pKa of the acid on the left, HA1 is larger (that is, the acid is weaker) than the pKa of the acid on the right, HA2, then the reaction willnot proceed in the forward direction (but it willproceed in the reverse direction).
  • 28. .which way??????•HA1 + -A2 →• -A1 + HA2
  • 29. All possible reactions• 4. Below are ALL the possible reactions for the lab. @@@@the handout@@@@@• Examine each reaction, assign acid and base to each compound, determine if the reactionwill proceed in the forwarddirection or not.Place an X across each arrow which will not proceed inthe forwarddirection. Only write the reactions that WILL proceed inyour lab notebook.
  • 30. Possible Reactions of Mixture with Sodium Bicarbonate (NaHCO3• Goto handout now!!
  • 31. Possible Reactions of Mixture with Sodium Hydroxide (NaOH)• See handout-show pdf
  • 32. Protonation of anions with hydrochloric acid (HCl)• Show pdf
  • 33. @@@@@@@@@@@@@• 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 0.7404 g/mL and is less than that of water (density 1.00g/mL) hence MTBE floats on top of water.
  • 34. Diethyl ether-goto portal-Now Rob
  • 36. • 3. Predicting if a reaction will proceed in the forward direction or not. To determine if any acid-base reaction will succeed, as shown,• HA1 + -A2 → -A1 + HA2
  • 38. • 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.
  • 39. • 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.
  • 40. 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.
  • 41. Slide 44• 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
  • 42. • 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
  • 43. • 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 $110.00. Handle with care
  • 44. • Prelab. *Before lab begins, read the following: Zubrick’s Chapters on Extracting and Washing (Ch 15 and 16). Zubrick’s Chapter on Drying Agents (Ch 10). *Draw the balanced acid-base reactions including the structures of benzoic acid, 2- naphthol, and 1,4-dimethoxybenzene and the products.
  • 45. • *Draw a flow chart of the extraction procedure. *Make a table of physical constants and safety information. Include only relevant data for this experiment.
  • 46. • A-Carboxylic-Acid-A/193808•• Grading. Completion 5 pts (includes lab and a report turned in on time) Prelab 3 pts Experimental 2 pts Results 5 pts Conclusion 5 pts