Separating Acids and Neutral Compounds by

Solvent Extraction

Definition

Extraction Washing

• Extraction: a process that selectively dissolves one or more compounds into an appropriate solvent. The resulting solution is called an extract.

• Extraction refers to the transfer of these compounds from one liquid solvent to another one.

• Extraction can be used to separate unwanted impurities or to separate mixtures of compounds.

• The two solvents used for extraction must be immiscible.

• Washing is the removal of impurities from a solvent containing your compound by a second immiscible solvent.

• Choosing the solvents:– Two immiscible solvents: a polar one

(frequently water) and a nonpolar one (an organic solvent)

– Nonpolar hydrocarbons and ethers (polar) are less dense than water, so the water tends to move to the bottom

– Most chlorinated solvents are more dense than water, so the water will be on top.

– Like dissolves like (nonpolar compounds are more soluble in nonpolar solvents…)

• To confirm the identities of the layers, add one or two drops of water to the funnel:

– If water mixes with the top layer, then the top layer is water

– If water falls through the top layer, the bottom layer is water

cyclohexanecyclohexane

petroleum etherpetroleum ether

hexanehexane

toluenetoluene

diethyl etherdiethyl ether

dichloromethanedichloromethane

ethanolethanol

acetoneacetone

methanolmethanol

leastleast polarpolar

most polarmost polar

WaterHydrocarbons/Ethers

Chlorinated solvents

Chemistry of Extraction

Water

Acid-Base Extractions

Analyzing ReactivitypKa = - log Ka

Ka [HA] = [A-] [H+]

O

O

H

+ -OH

O

O

+ H2O

WaterHydrocarbons/Ethers

Chlorinated solventsWater

O

OHOH

t-Bu

HN

O

pKa 4.2 pKa 10.2 pKa 24

pKa = 4.2

pKa = 16

O

O

H

+ H2O

O

O

+ H3O+

pKa = 4.2

The Extraction Setup

Make sure the stopcock of the separatory funnel is closed!

1. Prepare your solution in a beaker. Weigh 1.3 grams and mix with TBME. Transfer the solution into a separatory funnel.

2. Pour a measured amount of extraction liquid.3. Place stopper securely in the funnel.

Experimental Procedure

4. Using your fingers to secure the cover, shake the mixture several times.5. Vent the pressure by inverting and opening the stopcock. (Vent often!)

NOTE: The cover is always secured during the whole operation

Experimental Procedure

6. Place the separatory funnel in the iron ring…7. Remove the cover and allow the layers to separate.8. Identify the layers (solvents). (Don't throw anything away until sure.) 9. Remove bottom solvent by draining through the stopcock. Pour the upper solvent through the top of the funnel.

Experimental Procedure

Use the Microscale Flow Chart on page 135 as a guide (see the handout), but use the Macroscale procedure on page 134.

Step 1. The mixture of the three compounds is already prepared for you. (TA will give you the breakdown of the components!!)

Step 2. Weak base only deprotonates the strong acid, which means your compounds are in both phases.

Step 3. Strong base deprotonates the weak acid, which means you will again have compounds in both phases.

Step 4. Isolated p-toluic acid will need to dry in your drawer. Don't forget to pre-weigh your filter paper.

Step 5. When heating, don't allow the temperature to exceed 60 degrees C. Before adding HCl, cool the solution in an

ice bath. Allow the isolated p-tert-butylphenol to dry in your drawer. (Pre-weigh paper!!) (Heating is done in the

hood.)Step 6. Pre-weigh a round bottomed flask (not a beaker). Perform

distillation by rotary evaporation instead of the evaporation procedure in the lab manual. See 2nd handout for instructions. Use the house vacuum to remove any residual solvent.

Make Note of the Following:

Miscellaneous Notes

• Waste Organic Solvents: a waste bottle will be in the hood that will be used for all of the organic solvents.

• Waste Aqueous Solutions: a waste bottle will be in the hood for the acid/base solutions from your extractions.

• Grading: approximately 2/3 of your grade will be based upon the written lab report (prelab and postlab questions, experimental procedures, conclusions, etc.).