Extraction

INTRODUCTION Anyone who has ever made a cup of coffee or hot tea has performed an extraction.  Extraction is a fundamental technique used to isolate one compound from a mixture.  Becoming familiar with its theory and correct use are essential to successful completion of many organic experiments.  The three most common types of extractions are: liquid/liquid, liquid/solid, and acid/base (also known as a chemically active extraction).  The coffee and tea examples are both of the liquid/solid type in which a compound (caffeine) is isolated from a solid mixture by using a liquid extraction solvent (water). LIQUID/LIQUID EXTRACTION A liquid/liquid extraction involves two immiscible liquids.  Immiscible liquids do not dissolve in each other; they form layers when placed in the same glassware.  Immiscibility is a result of two liquids having different polarity.  The most common pair of extraction solvents used is diethyl ether (often referred to as simply 'ether') and water.  Polarity is a relative term - ether is considered nonpolar and water polar.  The fact that two phases are observed upon adding one to the other is a consequence of their different polarities.  The location (either the top or bottom layer) of an extraction solvent is determined by density.  The density of ether is 0.713 g/cm3 and the density of H2O is 1.0 g/cm3; therefore, ether is always the top phase when the extraction solvent pair is ether and water. HOW TO PERFORM Extractions are performed in a separatory funnel .  When two immiscible liquids are placed in the separatory funnel two phases are observed as discussed above.  As mentioned before, the fundamental reason for carrying out an extraction is to isolate a compound from a mixture.  For example, consider a mixture consisting of 2 polar compounds and 1 nonpolar compound.  After extraction with the  solvent pair of ether and water, the 2 polar compounds would be found in the aqueous layer (a polar solvent dissolves a polar solute) and the nonpolar compound would be found in the nonpolar phase (ether). Note:   the phase consisting of H2O is called the aqueous phase.

As a result, the nonpolar compound is isolated from the mixture.  (See the extraction scheme below.) 'Like dissolves like' is the general phrase used as a reminder that compounds of similar polarity are miscible.  The idea that a compound dissolves more or less readily in a solvent can be quantitated by the distribution coefficient:   which says there exists a ratio of the concentration of the solute between the two phases given the solvent pair and temperature.  This coefficient is discussed further in recitation.

PROCEDURE FOR EXTRACTION The procedure for carrying out an extraction is quite simple:        Add the extraction solvents to the separatory funnel (be certain the Teflon stopcock is closed first!). Two phases should be observed. Place the cap on the separatory funnel. Holding the cap and funnel securely, invert the separatory funnel. Vent the separatory funnel: with the funnel inverted (and cap secured), open the Teflon stopcock to reduce any pressure that has built. Close the stopcock and gently shake the separatory funnel. Repeat the shaking and venting steps several times. After completing the above steps, it is necessary to collect the phase that has dissolved the compound targeted for isolation.  Deciding which phase this actually is requires some knowledge of the polarity of the target compound.  When in doubt, save both phases and consult with the instructor.  Never throw away a phase until absolutely certain it is no longer needed!!! To remove a phase from the separatory funnel, return the funnel to its upright position and rest it on a ring clamp.  Remove the cap from the separatory funnel and drain the two phases into two different beakers. Decide which solvent contains the target compound and place the appropriate beaker in a 'safe place'(probably in a corner of the bench, but always in the hood).  Usually the entire extraction process is repeated several times to insure that the maximum amount of the target molecule has been isolated.  For this reason it is necessary to also save the phase containing the original mixture. Common Source of Error The most common source of confusion for students often occurs when deciding which solvent in the separatory funnel contains the target molecule.  This requires some knowledge of the polarity of the target molecule and the extraction solvents used.  If ether and water are the extraction solvents and the target molecule is nonpolar, then the ether layer would contain most of the target compound ('like dissolves like') after the first extraction.  If this were a real extraction , the bottom layer (aqueous) would be drained and stored in a separate beaker from the ether (which now contains the target compound).  After draining the ether, the aqueous phase would be returned to the separatory funnel and the entire process repeated to insure maximum recovery of the target compound.  The ether layers would then be combined and extraction complete.  Multiple extractions with small amounts of volume are always more efficient than one single extraction with large volume.  This is a direct result of the distribution coefficient and will be discussed in class.

COMMON QUESTIONS 'I think my separatory funnel is broken. I opened the stopcock to drain the layers and nothing happened.'    The separatory funnel may, in fact, be damaged.  Occasionally a contaminate may become lodged in the stopcock.  For this reason it is always good practice to wash the separatory funnel before attempting extraction and checking to be sure it drains properly. The common problem, however, is that the cap has been mistakenly left on the separatory funnel while attempting to drain. This establishes a vacuum within the separatory funnel and will not allow any liquid to drain. 'Why do I have three layers?'  Many times students believe there are three layers when there are just two.  Sometimes an 'apparent' third layer appears between the top and bottom layers.  This third layer is actually an interface between the two layers and generally appears only as the layers are separating.  Leaving the separatory funnel undisturbed for a few minutes until the layers have completely separated usually eliminates the problem.  Alternatively, gently rocking the separatory funnel is another viable solution.  If neither of these suggestions work, check with your TA - there are many other reasons why three layers have appeared. 'I added a second volume of ether to the separatory funnel for my second extraction, but I see only one layer.'  There is only one possible reason: you have added ether to ether!!!  Remember when performing successive extractions to 'keep your eye on the prize.'  If you are trying to move a compound from a water mixture to ether, do the extraction as described above and keep both layers.  Return the aqueous layer to the separatory funnel for a second extraction. This will remove most of the target molecule from the aqueous phase. It does absolutely no good to extract ether with ether. 'I forget which layer is ether and which is H2O.' or 'I can't figure out which layer is aqueous.'  Remember that the position of a layer is determined by density.  If you are using ether and water as the extraction solvents, ether is always the top layer because it is less dense than water.  If you forget that, or are using solvents unfamiliar to you, simply add a little more ether (or water, or whatever solvent you are using) to the separatory funnel and watch which layer becomes larger in volume. 'I'm washing my ether layer with 5% sodium bicarbonate.  What are the 2 layers?'  First, washing is slightly different than extracting. When you wash with 5% sodium bicarbonate, you are usually trying to neutralize an unreacted compound (usually acid) in an ether solution.  Therefore, one layer is usually ether. A 5% sodium bicarbonate solution is 95% water; therefore, the other layer is aqueous.  Further, washing with 5% sodium bicarbonate may produce carbon dioxide gas in the separatory funnel.  It is extremely important to vent the funnel immediately upon inverting and before shaking.  Frequent venting is necessary.