Chapter Seven Extraction

Copyright © Cengage Learning. All rights reserved.7 | 2 Extraction Depends on being able to selectively dissolve substances to separate them from a mixture Solid/liquid –the substances of interest are contained in a solid Liquid/liquid –the substances of interest are in solution

Copyright © Cengage Learning. All rights reserved.7 | 3 Liquid/Liquid Extraction Often used to separate organic products from aqueous mixtures of inorganic materials Equilibrium process that depends on the substances being separated having different solubilities in two immiscible solvents The concentration of solute in each solvent correlates with the solubility of the solute in that pure solvent

Copyright © Cengage Learning. All rights reserved.7 | 4 Partition Coefficient Ratio of solubilities and is an equilibrium constant with a certain value for a given substance, pair of solvents, and temperature. Also called Distribution Coefficient K d < 1 Substance is mostly in H 2 O K d = 1 Substance is mostly in both K d > 1 Substance is mostly in Organic

Copyright © Cengage Learning. All rights reserved.7 | 5 K d Sample Calculation Methylene chloride/H 2 O Initial mass of unknown0.097 g Solvent Volume1.97 mL Water Volume2.05 mL Mass of unknown in solvent0.059 g Mass of unknown in water0.097 g – g = g

Copyright © Cengage Learning. All rights reserved.7 | 6 Sample Calculation of K d

Copyright © Cengage Learning. All rights reserved.7 | 7 Another Example Suppose we have Compound B whose solubility in water is 2g/100mL and whose solubility in ether is 10 g/100mL. What is the value of K d ? K d = 10g/100mL 2g/100mL K d = 5

Copyright © Cengage Learning. All rights reserved.7 | 8 Example cont’d Suppose we have 1 g of B in 100 mL of water and we carry out the extraction with 100 mL of ether. What amount of B is in water, and what amount is in ether? 5 = X B /100mL (1 g – X B )/100mL X B = g in ether g in water

Copyright © Cengage Learning. All rights reserved.7 | 9 Efficiency It is more efficient to carry out several small-scale extractions instead of one large-scale extraction for a given volume of solvent Usually doing two or three extractions using smaller volumes each time is more efficient than doing one extraction with the same total volume of solvent

Copyright © Cengage Learning. All rights reserved.7 | 10 Multiple Extractions So, what if, for the previous problem, we do four extractions of 25 mL each instead of one 100 mL extraction? Extraction 1: 5 = X B /25mL (1g – X B )/100mL X B = g in ether g in water

Copyright © Cengage Learning. All rights reserved.7 | 11 Multiple Extractions cont’d Extraction 2: 5 = X B /25mL (0.444g – X B )/100mL X B = g in ether g in water Extraction 3: 5 = X B /25mL (0.197g – X B )/100mL X B = g in ether g in water

Copyright © Cengage Learning. All rights reserved.7 | 12 Multiple Extractions cont’d Extraction 4 5 = X B /25 mL (0.088g – X B )/100mL X B = g in ether g in water Total Extracted by four 25 mL: g g g g = g in ether g in water

Copyright © Cengage Learning. All rights reserved.7 | 13 Properties of the Solvent for Liquid/Liquid Extractions Two immiscible layers--solvent not miscible in water –solute should readily dissolve in the solvent at room temperature –solvent should have a low boiling point –solvent should not react with the solute –solvent should not be toxic or highly flammable –solvent should be relatively inexpensive

Copyright © Cengage Learning. All rights reserved.7 | 14 Common Solvents Listed by Density

Copyright © Cengage Learning. All rights reserved.7 | 15 Considerations Good practice to save all layers until the product is identified Solution concentration may affect the relative density –check the layers using the drop test

Copyright © Cengage Learning. All rights reserved.7 | 16 Microscale Mixing and Separating Layers Two layers in a reaction tube Mixed by drawing the two layers up into a Pasteur pipette and then rapidly expelling the liquids Wait for the layers to separate It is usually easier to remove the lower layer View video clip: Extraction with DichloromethaneExtraction with Dichloromethane

Copyright © Cengage Learning. All rights reserved.7 | 17 Drying Chemical drying agents –should not react with substances being extracted –should remove water rapidly –should remove water efficiently –should be easily removed Calcium chloride pellets are often a good choice

Copyright © Cengage Learning. All rights reserved.7 | 18 Drying Agents Clump together and fall rapidly through mixture when wet Settle slowly, remaining separated when water has been removed Add drying agent slowly so this change can be observed before too much is used –wash drying agent with fresh solvent to recover all of the sample

Copyright © Cengage Learning. All rights reserved.7 | 19 Neutral Liquid/Liquid Extraction Workup Four steps –mix the layers –separate the layers –dry the organic layer –remove the solvent

Copyright © Cengage Learning. All rights reserved.7 | 20 Microscale Mixing the Layers A Pasteur pipette can be used also

Copyright © Cengage Learning. All rights reserved.7 | 21 Microscale Separating the Layers--Pasteur Pipette Figure 7.6

Copyright © Cengage Learning. All rights reserved.7 | 22 Extraction with Dichloromethane Return to slide 12