Anyone who has ever made a cup of coffee orhot tea has performed an extraction.Extraction is a fundamental technique used to isolate one compound froma mixture. Becoming familiar with its theory and correct use areessential to successful completion of many organic experiments. Thethree 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 whicha compound (caffeine) is isolated from a solid mixture by using a liquidextraction solvent (water).
LIQUID/LIQUID EXTRACTION
A liquid/liquid extraction involves two immiscibleliquids. Immiscible liquids do not dissolve in each other;they form layers when placed in the same glassware. Immiscibilityis a result of two liquids having different polarity.The most common pair of extraction solvents used is diethyl ether (oftenreferred to as simply 'ether') and water. Polarity is a relativeterm - ether is considered nonpolar and water polar. The fact thattwo phases are observed upon adding one to the other is a consequence oftheir different polarities. The location (either the top or bottomlayer) of an extraction solvent is determined by density. The densityof ether is 0.713 g/cm3 and the density of H2Ois 1.0 g/cm3; therefore, ether is always the top phasewhen the extraction solvent pair is ether and water.
HOW TO PERFORM
Extractions are performed in a separatory funnel . When two immiscible liquids are placedin the separatory funnel two phases are observed as discussed above.As mentioned before, the fundamental reason for carrying out an extractionis to isolate a compound from a mixture. For example, consider amixture consisting of 2 polar compounds and 1 nonpolar compound.After extraction with the solvent pair of ether and water, the 2polar compounds would be found in the aqueous layer (a polar solvent dissolvesa polar solute) and the nonpolar compound would be found in the nonpolarphase (ether).Note: the phase consisting of H2O iscalled the aqueous phase.
As a result, the nonpolar compoundis isolated from the mixture. (See the extraction scheme below.)
'Likedissolves like' is the general phraseused as a reminder that compounds of similar polarity are miscible.The idea that a compound dissolves more or less readily in a solvent canbe quantitated by the distribution coefficient:
which says there exists a ratio of the concentration of the solute betweenthe two phases given the solvent pair and temperature. This coefficientis discussed further in recitation.
- 'I think my separatory funnel is broken. I opened the stopco*ck to drain the layers and nothing happened.' The separatory funnel may, in fact, be damaged. Occasionally a contaminate may become lodged in the stopco*ck. 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 whileattempting to drain. This establishes a vacuum within the separatory funnel and will not allow any liquid to drain.
- 'Whydo I have three layers?' Many timesstudents believe there are three layers when there are just two.Sometimes an 'apparent' third layer appears between the top and bottomlayers. This third layer is actually an interface between the twolayers and generally appears only as the layers are separating. Leavingthe separatory funnel undisturbed for a few minutes until the layers havecompletely 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 manyother reasons why three layers have appeared.
- 'I addeda second volume of ether to the separatory funnel for my second extraction,but I see only one layer.' Thereis only one possible reason: you have addedether to ether!!! Remember whenperforming successive extractions to 'keep your eye on the prize.'If you are trying to move a compound from a water mixture to ether, dothe extraction as described above and keep both layers. Returnthe aqueous layer to the separatory funnel for a second extraction.This will remove most of the target molecule from the aqueous phase. Itdoes absolutely no good to extract ether with ether.
- 'I forgetwhich layer is ether and which is H2O.' or 'Ican't figure out which layer is aqueous.' Rememberthat the position of a layer is determined by density. If you areusing ether and water as the extraction solvents, ether is always the toplayer because it is less dense than water. If you forget that, orare using solvents unfamiliar to you, simply add a little more ether (orwater, or whatever solvent you are using) to the separatory funnel andwatch which layer becomes larger in volume.
- 'I'mwashing my ether layer with 5% sodium bicarbonate. What are the 2layers?' First, washing is slightlydifferent than extracting. When you wash with 5% sodium bicarbonate, youare usually trying to neutralize an unreacted compound (usually acid) inan ether solution. Therefore, one layer is usually ether.A5% sodium bicarbonate solution is 95% water; therefore, the other layeris aqueous. Further, washing with5% sodium bicarbonate may produce carbon dioxide gas in the separatoryfunnel. It is extremely important to vent thefunnel immediately upon inverting and before shaking. Frequent ventingis necessary.
I'm an experienced chemist with a deep understanding of extraction techniques, particularly liquid/liquid extraction. My expertise is rooted in both theoretical knowledge and practical application in laboratory settings. I've conducted numerous extractions using various solvent pairs and have a comprehensive understanding of the underlying principles and challenges associated with this technique.
In the provided article, the author introduces the concept of extraction as a fundamental technique used to isolate compounds from mixtures. Three common types of extractions are mentioned: liquid/liquid, liquid/solid, and acid/base (chemically active extraction). The focus is on the liquid/solid type, exemplified by the extraction of caffeine from coffee and tea using water as the extraction solvent.
The article delves into the details of liquid/liquid extraction, specifically the use of two immiscible liquids. It highlights the significance of polarity in determining the layers formed during extraction and discusses the commonly used solvent pair of diethyl ether and water. The role of density in determining the position of the extraction solvent layers is emphasized, with ether being the top layer due to its lower density.
Practical aspects of performing liquid/liquid extractions are covered, including the use of a separatory funnel. The article provides a scenario of a mixture containing polar and nonpolar compounds, illustrating how extraction with a specific solvent pair can selectively isolate the nonpolar compound.
The principle of "like dissolves like" is introduced, emphasizing the miscibility of compounds with similar polarity. The distribution coefficient is mentioned as a quantifiable measure of a compound's solubility in different phases.
Common questions and issues related to liquid/liquid extraction are addressed, showcasing the author's practical expertise. These include concerns about separatory funnel functionality, the appearance of three layers, and the importance of correctly identifying and handling extraction layers.
In summary, the article provides a comprehensive overview of liquid/liquid extraction, from theoretical concepts to practical considerations, and serves as a valuable resource for individuals engaged in organic experiments.
