Extraction What is the purpose of extraction? to separate a mixture into its different components How do acid-base reactions work? separates strong organic acids, weak organic acids, neutral organic compounds, and basic organic substances Flowchart on how to separate strong acids, weak acids, bases, and neutral compounds. Why are extractions carried out in a particular sequence (weak base then strong base)? because otherwise there will be unwanted reactions between the strong acid and weak base What are the pKa requirements for conjugate acid of bases to extract organic acids? need to be at least equivalent or less than the organic acid pKa Be able to recognize and draw species in aqueous and organic layers before and after acid base extractions and after acid or base regeneration. What is the reaction of acid-base extractions? HCl + NaHCO3 ( NaCl + H2O + CO2 Acid + base ( salt + water + CO2 What are some requirements for extraction solvents? readily dissolve the substance to be extracted at room temperature low boiling point so it can be removed easily should not react with solute or other solvent should not be highly flammable or toxic inexpensive How do you mix and separate layers? allow the layers to separate and put the pipette to the bottom layer and take out the aqueous layer allow the layers to form again before each time you pipette What are emulsions? suspension of one liquid as droplets in another liquid How are they caused? by shaking the mixture excessively What is a remedy for it? add a highly ionic substance such as NaCl to the aqueous layer to separate it Besides drying agents, what other method can be used to remove water from an ether layer? by shaking it with saturated NaCl solution Why is a drying agent used? to remove all water from the organic layer into the aqueous layer What observations indicate that an organic layer is dry? What did we use as a drying agent? the organic layer will appear to turn clear we used calcium chloride pellets What is the criteria for choosing a drying agent? will not react with substance being extracted must remove water quickly efficient easy to recover solute from drying agent What is the advantage of using calcium chloride? clumps together then excess water is present which makes it easy to determine how much to add do not integrate into powder fast and effective How do you ensure all product is transferred from the drying agent? reaction starts to snow What are the advantages of t-butylmethyl ether? forms an azeotrope with water does not need drying cheap What are the disadvantages of diethyl ether? it is flammable and explosive easily forms peroxides Why don?t you get 100% recovery? because there is always some solute left that has not been transferred to the organic layer What are some sources of error? incomplete acid-base reactions removing the wrong layer not completely transferring the aqueous layer recrystallization errors as well Prelab/Postlab Questions Compound X dissolves in water and needs to be extracted with an organic solvent. Its solubility in toluene is 0.65 g/mL, in ethanol is 20g/mL and in methylene is 12.5g/mL. Ethanol forms a homogeneous later when mixed with water. Toluene and methylene chloride do not. Why would ethanol and toluene not be chosen as the organic solvents for the extraction discussed above? ethanol not chosen because it is miscible and soluble in water toluene not chosen because its does not readily dissolve Compound X After the methylene chloride and water layers are mixed and separated, is it the upper or lower later that would be the organic layer? the lower layer You have to separate 4-toluic acid from 4-bromoaniline. Assume the two substances don?t react with each other. The mixture of 4-toluic acid and 4-bromoaniline is dissolved in ether. You may need the following information. The pKa of HCl is ?7; pKa of carbonic acid is 6.52 and the pKa of water is 15. Why would using either sodium bicarbonate or sodium hydroxide be able to extract 4-toluic acid into the aqueous layer? sodium bicarbonate neutralizes the acid while forming carbon dioxide and high pressure build up sodium hydroxide because it reacts with the acid to form a water soluble ionic compound What would be the name of the species in the aqueous layer when the mixture mentioned about is extracted with either sodium hydroxide or sodium bicarbonate? If you could react the mixture above with aqueous HCl first, why would it only extract the 4-bromoaniline into the aqueous layer? HCl would remove the lower aqueous layer because the conjugated base being used has to be weaker than the base being extracted The distribution coefficient between methylene chloride and water for solute Y is 14. An amount of 67.0 g of Y is dissolved in 200 mL of water. What weight of Y would be removed from water with a single extraction with 150 mL of methylene chloride? X = amount of solute in organic solvent (X/150mL) / [(67.0g ? X) / 200mL] = 14 What total weight of Y would be removed from water with two successive extractions with 75 mL portions each of methylene chloride? Y = amount of solute in organic solvent (Y/75mL) / [(67.0g ? Y)/200mL] (Y2/75mL) / [(Y ? Y2)/200mL] Partition Coefficient of Extraction K = (solute concentration in organic layer) / (solute concentration in aqueous layer) Apparatus Percent Recovery Already covered as Percent yield Same calculation as that Reactions Sodium bicarbonate added to Tube 1 Na+ + HCO32- + ( + H2O + Na+ + CO2 Sodium hydroxide added Na+ + OH- + ( + Na+ + H20 Hydrogen chloride added to tube 3 made 4-tert-butylphenol + H+ + Cl- ( + Na+ + Cl- Solvent removed from both tubes ? precipitate was extracted HCl + NaHCO3 ( Na+Cl- + CO2 + H2O
Want to see the other 5 page(s) in Extraction?JOIN TODAY FOR FREE!