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Chapter 5. Chemical Extraction

1. Solid-liquid extraction2. Liquid-liquid extraction

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Introduction

- Extraction is a physical process by which a compound istransferred from one phase to another :

- Water–soluble components in the tea leaves or in the coffeebeans are transferred from a solid phase in the leaves orbeans into a liquid phase the boiling water. This process isknown as solid-liquid extraction.

- Liquid-liquid extraction is a basic operation that should bemeasured in the organic chemistry laboratory :

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Soxhlet Extraction

- If a component in a solid mixture has a lowsolubility a Soxhlet extraction can be usedto isolate it.

- The solid mixture is placed in a porous‘thimble’ which is repeatedly washed withfresh condensing solvent in a specializedpiece of glassware know as Soxhletextractor

Compound to be extracted

Insoluble material

Extraction Thimble

Condenser

Vapor By-pass Tube

Syphon Tube

Soxhlet Extraction

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Compound to be extractedInsoluble material

- The condensing solvent falls into the chamber in which the thimble is placed

- Eventually the chamber fills and the liquid siphons back into the boiling flask

- The process is repeated and the solid is eventually extracted into the boiling flask

Soxhlet Extraction

http://www.chem‐ilp.net/labTechniques/SoxhletExtractionVideo.htm

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Chapter 5. Chemical Extraction

1. Solid-liquid extraction2. Liquid-liquid extraction

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Introduction- Liquid-liquid extraction is a basic operation that should be

measured in the organic chemistry laboratory :

- By liquid-liquid extraction we can isolate single components from amixture. The physical process that rules liquid-liquid extraction isknown as solvent-solvent partitioning or the distribution of solutesbetween pair of solvents(immiscible solvent) .

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- Solvents such as diethyl ether, tert-butyl methyl ether, methylenechloride and toluene have verylimited solubility in water. Thesolubility of water in such solvents isalso very low. We say that thesesolvents are immiscible with water.

- Two layer system is obtained :(lower layer) dense solvent(upper layer) less dense solvent

Introduction

- The organic solvent layer is called the organic layer or organic phaseand the water layer the aqueous layer or aqueous phase.

- Inorganic and water soluble materials will stay in the aqueous(water)layer and more organic molecules will remain in the organic layer

Toluene(d=0.867)

Water(d=1.000)

Immiscibility & Density of sovlent

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Common extraction solvents listed by density.

- The densities of the solvents will predict which solvent is the top or bottom layer.

- In general, the density of nonhalogenated organic solvents are less than 1.0 g/mL and halogenated solvents are greater than 1.0 g/mL.

- One common solvent pair is dichloromethane and water(aqueous layer).

(=Methylene chloride)

(ex. diethyl ether, methyl tert‐butyl ether)

(=trichloromethane)

Solvent-Solvent Partitioning

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ImmiscibleMiscible

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Solvent-Solvent Partitioning

Organic phase Aqueous phase

Solvent Nonpolar solvent Water

Solute Nonpolar solutes dissolve here Ionic and polar solutes dissolve here

Density

The organic phase does not always float on top. Chloroform, for example, sinks below the aqueous phase.

Water is usually denser than other solvents, but some organic solvents are even denser.

d=1.492g/ml

d=1.000g/ml

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- Lets recall the golden rule of solubility: like dissolves like; Highlypolar and ionic compounds are readily soluble in water but havevery low solubility in most organic solvents, which are polar thanwater.

- On the other hand organic compounds of medium and low polarityare more soluble in organic solvents than water.

- This selective solubility behavior can be advantageously used toseparate compounds by liquid-liquid extraction.

Solvent-Solvent Partitioning

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- ex1. Due to their high solubility in water and low solubility inorganic solvents, very polar solutes such as ionic compounds andsugars, display very low partition coefficients between mostorganic solvents and water. Therefore these solutes remain in thewater phase even after several extractions with organic solvents.

- ex2. Conversely organic compounds of medium and low polarityfor example, cholesterol, DDT(Dichloro‐Diphenyl‐Trichloroethane),and aspirin have larger partition coefficients and thus are easilyextracted into organic solvents.

Solvent-Solvent Partitioning

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Criteria for selecting an extracting solvent

The organic solvent used for extraction must meet a few criteria:

- Should readily dissolve substance to be extracted. - Should not react with the substance to be extracted. - Should not react with or be miscible with water (the usual

second solvent). - Should have a low boiling point so it can be easily removed from

the product. - Should be reasonably safe to work with and relatively

inexpensive.

Common extraction solvents are diethyl ether and methylene chloride.

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Criteria for selecting an extracting solvent

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Nernst Distribution Law

- In the simplest extraction case, the distribution ratio is constant in accordance with the classical Nernst distribution law;

- At constant temperature, a solute distributes itself between two immiscible solvents only in a particular ratio.

Nobel Prize in Chemistry 1920

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- Lets suppose that we add a solute A to a mixtureof water and toluene, shake the systemthoroughly to attain equilibrium, and allow it tosettle.

- Solute A will be present in both layers but weexpect to find it in a larger amount in the solventsis directed by the Distribution coefficients of Abetween the solvents.

Distribution Ratio (Partition Coefficient)

- The Distribution ratio is the ratio of the total analyticalconcentration of a solute in the extract (regardless of its chemicalform) to its total analytical concentration in the other phase

‐ The phrase “Partition coefficient" is now considered obsolete byIUPAC, and “Distribution ratio" is more appropriate terms thatshould be used

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- The distribution of a solute between two immiscible solvents can be described by the Distribution Ratio “D”.

where, [solute] represents the stoichiometric or formal concentration of a solute and the subscripts 1 and 2 refer to the two phases.

- Since in most cases, two-phase system is of analytical interest, an organic(Org) solvent and aqueous(Aq) are involved, D will be understood to be;

- ex.

D =

D =

[solute]1

[solute] 2

[solute] Org

[solute] Aq

Distribution Ratio

DToluene/Water = [AToluene] / [AWater],

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- If the ionized species has an appreciable solubility only in the aqueous phase, then the distribution coefficient, D, can be defined as

where, [solute]Org : the conc. of the unionized species in organic phase[solute]Aq : the conc. of the unionized species in aqueous phase[solute]Aq : the conc. of the ionized species in aqueous phase

[soluteu]Org

[soluteu] Aq + [solutei]Aq

D =

Distribution Ratio

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% Extraction

- The more commonly used term for expressing the extraction efficiency by analytical chemist is the percent extraction “E”, which is related to “D” as

where V represent solvent volume

- The percent extraction may be seen to vary with the volume ratio of the two phases as well as with D.

- At extreme values of “D”, “E” becomes less sensitive to changes in “D”. For example, at a phase volume ratio of unity(Vaq=Vorg),for D values from 500 to 1000, the value of “E” changes only

from 99.5 to 99.9%.

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- Stopper the funnel and invert it. Slowly open the stopcock to release any built up pressure, then close the stopcock.

- Gently shake the separatory funnel to allow intimate mixing of the solutions (Caution: When shaken, the mixture may develop pressure; be sure to vent it periodically).

- Clamp the separatory funnel to a retort stand and allow the mixture to separate into two layers.

- Remove the stopper and collect the layer containing the compound what we want to obtain. → Vacuum rotary evaporation

Extraction procedureThe total capacity of the separatory funnel should be at least twice the volume of the solution

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- One of the most common and frustrating mistakesmade by beginners to liquid-liquid extraction is tokeep the wrong phase and discard the one withthe desired product.

- In principle the identity of the layers can bepredicted by the density of the solvents.

- However a high solute concentration may changethe density of the solution to the point that thelayer with the less-dense solvent becomes thedenser and lower layer.ex. Saturated sodium chloride solution : 1.2 g/ml

Conc. Sulfuric acid : 1.8g/ml

Extraction Techniques: Layer identification

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- If there is any doubt concerning the identity of the layers the followingtest should be performed: Place about 1 mL of water in a test tube andadd a few drops of the upper layer with a Pasteur pipet.

→ If the drops dissolve in the water, it means they come from theaqueous phase.

→ If the drops are immiscible with the water in the test tube, they belongto the organic phase.

Extraction Techniques: Layer identification

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Extraction Techniques: Multiple extraction

- Since the distribution coefficient is a ratio, unless K is very large, not all of a solute will reside in the organic layer in a single extraction.

- Usually two, three, or four extractions of the aqueous layer with an organic solvent are carried out in sequence in order to remove as much of the desired product from the aqueous layer as possible.

- The effectiveness of multiple small volume extractions : if one extraction can recover 90% of the compound. A second extraction with the same solvent may be able to pull out 90% of the remaining material. Effectively 99% of the compound was recovered with two extractions.

- Many smaller extractions are more efficient than one large extraction. This phenomenon can be proved mathematically, but in short follows the equation:

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Extraction Techniques: Multiple extraction

- Many smaller extractions are more efficient than one large extraction : Fraction of material extracted by solvent B is

where, n : the number of extractions performed, K : the distribution coefficient, VA, VB : the volume of solvent A and B

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Extraction Techniques: Multiple extraction

Example Prob. Give at 20o C only 0.24 g of an organic acid “A” dissolves in 100 ml of water, but 2.70 g of the same acid dissolves in 100 ml of ether.

a) calculate the value of partition coefficient. : 11.25

b) calculate the percentage of extraction if 0.12 g of acid extracted in 100 ml of aqueous solution: ≈ 92 %

c) calculate the volume of ether required to extract 85% of a 3.00 g of acid “A” in 100 ml solution : ≈ 51 ml

d) calculate the total amount of acid extracted by a double extraction of 50 ml ether in each extraction in part ( c).

First extraction (first 50 ml ether) : Xo1 = 2.57 g acid extracted in first extraction Xw1 = 3.00 g – 2.57 g = 0.43g acid remained in aqueous solution

after first extraction Second extraction (second 50 ml ether)

Xo2 = 0.38 g acid extracted in first extraction Xo (total) = Xo1 + Xo2 = 2.57 g + 0.38 g = 2.93 g

%E = (2.93 /3.00) x100 = 97.7 %

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Breaking Emulsions- After shaking the two phases may fail to separate sharply and

may instead form a suspension of one liquid in the other. This iscalled emulsion and it is unwelcome in the laboratory.

- In order to break the emulsion and get a clean separationbetween layers, Swirl the funnel very gently. If the emulsionpersists add a small amount of a saturated solution of sodiumchloride in water: The sodium chloride increases the ionicstrength of the water phase and decreases its miscibility with theorganic solvent. This process is called salting out and may helpbreak the emulsion.

- Swirl the flask occasionally over a period of 5-10 minutes andthen gravity-filter the system using flutted filter paper. Rinse themagnesium sulfate on the filter with a few milliliters of the sameorganic solvent. Discard the magnesium sulfate. The solutionshould be clear now.

- Another way of breaking the emulsions is to centrifuge the layersfor a few minutes. Glass tubes are preffered for this operationbecause some organic solvents may dissolve plastic centrifugetubes.

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Microscale Liquid liquid Extraction- When small volumes (0.5-10 mL) are to be extracted, a

conical vial or a screw capped test tube are the glassware of choice. Conical vials are available in differentsizes (0.5-5mL), test tubes are suitable when thevolume to be extracted is between 2 and 10 mL.

- The solution of the desired compound and the extractingsolvent are placed in the vial, the total volume shouldnot be larger than three-fourths of the maximumcapacity of the vial.

- The vial is tightly capped and shaken vigorously for afew seconds. The pressure is released by carefullyunscrewing the cap. This process is repeated four to sixtimes.

- The lower layer is withdrawn with the aid of a Pasteurpipet and placed in a clean test tube. In doing thisoperation squeeze the rubber bulb before immersing thePasteur pipet in the extraction vial. This will prevent airbubbles from disrupting the interface between thephases.

- Very volatile solvents such as diethyl ether andmethylene chloride have a tendency to squirt out the tipof the pipet. One way is to saturate the inside of thepipet with the solvents vapors by drawing fresh solvent

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Drying the Organic Layer

- One significant problem with liquid/liquid extraction is that no solvent is COMPLETELY insoluble in another solvent.

- After contact with an aqueous phase any organic solvent will be saturated with water, and this poses a problem in the recovery of the solute from the organic layer: evaporation of the solvent renders a product mixed with water.

- Also, many reactions themselves are performed in an aqueous solution. This water must be removed before the required compound can be properly characterized.

- To avoid these wet residues water should be removed before evaporation. Two methods of drying solutions are commonly used: saturated aqueous sodium chloride and solid drying agents.

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Saturated aqueous sodium chloride

- The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride (otherwise known as brine).

- The salt water works to pull the water from the organic layer to the water layer: the concentrated salt solution wants to become more dilute and salts have a stronger attraction to water than to organic solvents.

Drying the Organic Layer:

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Drying the Organic Layer Saturated aqueous sodium chloride

- To dry your organic product by this method, place the organic solution (methylene chloride, d=1.3) in a separatory funnel.

- Add an amount of saturated aqueous sodium chloride(d=1.2 g/ml), less than or equal to the amount of organic solution you have.

- Stopper the funnel and shake as in an extraction. Allow the layers to separate.

- Drain off the lower layer: in this case, this is the organic layer and the layer you want to save. Dispose of the aqueous layer in the aqueous waste carboy.

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Drying the Organic Layer

- Final traces of water are removed by treating the organic solution with a drying agent. A drying agent is an inorganic salt which readily takes up water to become hydrated: drying over anhydrous sodium sulfate or other drying agent.

- chemists remove the water from the organic layer by adding an insoluble inorganic solid to the solution which will absorb the water, thus “drying” it.

- Granular anhydrous sodium sulfate is the drying agent most often used: all of the inorganic solids work by reacting with the water to form hydrates, if water is available.

Solid drying agents

Sodium sulfateCalcium chloride Magnesium sulfate

ethyl‐acetate was dried with a little Na2SO4 (the solid at the bottom). 

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Drying the Organic Layer

Hydrated complexes for some common drying agents.

Because these compounds will deliquesce (absorb moisture from the air), you MUST cap bottles of drying agents between uses! Otherwise, they will end up looking like this!

Solid drying agents

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Drying the Organic Layer

- Add a small amount (but enough to ensure that all the water is removed) of the solid drying agent directly to the organic solution.

- Swirl the solution and Observe the drying agent; if it is all clumped together, add more.

- The pictures below show how a solution with drying agent looks when it is clumped and when it is free-flowing.

- In most cases, drying is as complete as it will get in 20 minutes. When drying is complete, you need to remove the dried organic solution from the drying agent.

Solid drying agents

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Drying the Organic Layer

- If the powder is quite fine (as when using magnesium sulfate) or if the volume is large, gravity filtration is the method of choice.

- If the drying agent is of larger particle size (as when using sodium sulfate or calcium chloride), decanting is the method of choice.

- An alternative to decanting is removing the liquid from the drying agent simply by drawing it off with a Pasteur pipet.

Solid drying agents

decanting

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Drying the Organic Layer

Sodium: A very powerful drying agent, removes traces of water fromsolvents such as toluene, benzene and diethyl ether and aliphatic aminesbefore distillation. It reacts violently with water. It should be handled withextreme care by trained people only.

Molecular sieves: Small spheres made of porous aluminosilicates. Theaverage pore size is 3 and 4Å respectively.

- There are other choices for drying agents including molecular sieves and sodium metal. There are benefits and disadvantages to each one.

Solid drying agents