Extraction

 

Read 5.1 & 2.18 & 2.21 in your lab text.

 

††† Extraction is a separation technique based on differences in solubilities of substances in two immiscible solvents (usually water and a water insoluble organic solvent).

 

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††† solubility in solvent 1

Kc = partition coefficient = †††† ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑≠

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††† solubility in solvent 2

 

 

example:Given compound A, K(ether:water) = 4.0, how much of A can be extracted from a solution of 10.0 g of A in 100 mL of water with a single portion of 100 mL of ether?

 

††††††††††††††††† ††††† ††††††††††† X / 100 mL ether

Kc=4.0=††† ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑---------------

†††††††† †† ††††††††††† (10.0 ‑ X) / 100 mL water

 

 

X = 8.0 grams of A extracted into the ether

 

 

‑same as above, but extract two times with 50 mL of ether each time.

 

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††† †† X / 50 mL ether

first extraction:†† Kc=4.0= ††††††††††† ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑≠-------

††††††††††††††††††††††† ††††††††††††††††††††††† †††††† (10.0 ‑ X) / 100 mL water

 

 

†††††††††††††††††††††††† X = 6.67 grams of A extracted

 

 

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††† Y / 50 mL ether

second extraction:Kc =4 0=†† ‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑‑≠-----------

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††† (3.33 ‑ Y) / 100 mL water

 

 

†††††††††††††††††††††††† Y = 2.22 grams of A extracted

 

 

total extracted =X + Y = 6.67 + 2.22 = 8.89 grams

 

 

==>multiple extractions with smaller amounts of solvent are more efficient than a single extraction with the same total amount of solvent. DRYING:an organic liquid that has been in contact with water is "wet" (contains some water).To "dry" it, use an inorganic drying agent.See Table 2.1 in your lab text.

 

 

 

 

EXTRACTION AND WASHING

 

The processes of extraction and washing are mechanically the same.Extraction refers to the recovery of a desired substance from a complex mixture, while washing denotes the removal of unwanted material.


 

In almost all preparative procedures, washing of the crude product is a necessary part of the isolation procedure.Extraction of the product from the reaction mixture is also sometimes necessary as a means of separating it from the other components.Isolation of a substance from animal or plant matter is another application of extraction, either to obtaining the compound for some end use (e.g., medicinal) or as a preliminary step in an analytical procedure.

 

A solvent chosen for extraction usually has a low b.p. so that it can be easily removed if the dissolved material is to be recovered.The solvent must also be inert towards all of the components of the mixture to be extracted.The two most commonly used solvents for extractions of organic substances from aqueous solutions are diethyl ether (CH3CH2-O-CH2CH3, b.p. 35o) and dichloromethane (aka methylene chloride, CH2Cl2, b.p. 41o).

 

EXTRACTION OF PROPANOIC ACID FROM AQUEOUS SOLUTION

 

A simple reaction, followed by isolation of the desired product from the solution, will illustrate a typical application of extraction.

 

Some organic acids are liquid and are soluble in water as indicated.The sodium salts of these acids are ionic compounds that are also very soluble in water.If an aqueous solution of one such salt is acidified with a strong mineral acid, the much weaker organic acid is produced, as shown in the following representative equation:

 

Na+ + CH3CH2CO2- + H3O+ + Cl- ----> CH3CH2CO2H + H2O + Na+ + Cl-

 

††††††† sodium propionate††††††††††††††††††††† hydrochloric††††††††††††††††††††††††††††††† propionic

†††††††† †††††††††††††††††††††††††††††††††††††††††††††††††††acid†††††††††††††††††††††††††††††††††††††††† acid

The weak organic acid is largely un-ionized in the aqueous solution.

 

No apparent change is observed when the reaction is carried out because the solutions of the starting materials and the products are colorless.The organic acid is soluble in water and, thus, does not separate.It is necessary to extract the propionic acid from the aqueous solution with diethyl ether.The ether solution must be dried to remove residual water, following which the ether is separated from the acid by distillation.(Note:Even though there is a wide difference between the b.p.'s of the ether (b.p. 35o) and the propionic acid (b.p. 142o), the latter is sufficiently volatile that it would distill along with the solvent.A fractional distillation would be more efficient, but is also more time-consuming.)

 

 

PROCEDURE

 

†† †††††††† Weigh 15 g of sodium propionate into a 250-mL beaker.

 

†† ††† Dissolve the salt in 60 mL of tap water.Add 30 mL 6 M HCl and stir the mixture with a glass rod.

 

†† ††† Add 12 g solid NaCl to the solution and stir the mixture for about 2 minutes.(This is known as "salting out".The added water-soluble ionic salt reduces the solubility of the less polar organic acid.)

 

†† ††† Support a 250 mL separatory funnel on a ring, making sure that the stopcock is closed and that a clean beaker is placed beneath the exit tube.Transfer the aqueous solution from the beaker to the separatory funnel.Do not transfer any undissolved sodiumchloride in the beaker to the funnel.

 

†† ††† Extract the aqueous solution with diethyl ether in the following manner:

 

††††††††††† a)†††† Add 20 mL of diethyl ether to the solution in the separatory funnel.

 

††††††††††† b)†††† Place the plastic stopper (NOT greased) in the neck of the funnel.Shake ( see pp. 54-56 in lab text)

 

††††††††††† c)†††† Drain the lower aqueous layer through the stopcock into the same 250 mL beaker in which the solution had been prepared in steps 1-3.

 

††††††††††† d)†††† Pour the upper ether layer through the neck of the funnel into a clean 125 mL Erlenmeyer flask.

 

††††††††††† e)†††† Return the aqueous solution from the 250 mL beaker to the separatory funnel.Add another fresh 20 mL of diethyl ether to the funnel and again extract the aqueous solution as you did in b) above.

 

††††††††††† f)†††† Drain the lower aqueous layer into the same 250 mL beaker in which it was contained previously.Pour the upper ether layer through the neck of the funnel into the 125 mL Erlenmeyer flask that already contains the ether solution from the first extraction (step d).

 

††††††††††† g)†††† Pour the aqueous solution from the 250 mL beaker back into the separatory funnel.Add another fresh 20 mL of diethyl ether to the separatory funnel and extract the aqueous solution one more time.

 

††††††††††† h)†††† Drain the lower aqueous layer into the beaker and pour the upper ether layer into the Erlenmeyer flask.Save both liquids.

 

††† ††††††† Add just enough anhydrous magnesium sulfate (a drying agent) to barely cover the bottom of the Erlenmeyer flask.Swirl the contents gently, and allow the flask to stand for at least 15 minutes.

 

 

NOTE:If the solid MgSO4 dissolves in the liquid, you have made a serious error.Magnesium sulfate is insoluble in diethyl ether but is very soluble in water.If the drying agent remains undissolved after 15 minutes, then discard the aqueous solution (still in the 250 mL beaker) down the drain.If you are uncertain, check with the instructor.

 

†† ††† Set up a simple distillation apparatus with a 100 mL r.b. flask as the distillation flask.Use a 125 mL Erlenmeyer flask as the receiver.

 

†† ††† Remove the thermometer and adapter from the assembly and place along-stem funnel in the still pot.Insert a small plug of absorbent cotton at the top of the funnel stem.

 

†† ††† Carefully decant the ether solution from the drying agent through the funnel into the flask.Rinse the Erlenmeyer flask with about 10 mL of ether and pour the ether through the funnel, too.Remove the funnel, add two or three boiling chips and reattach the thermometer and adapter to the still pot.

 

Discard the magnesium sulfate remaining in the Erlenmeyer flask by dissolving it in tap water and pouring the solution down the drain.

 

†† ††† Before beginning the distillation, weigh a clean, dry 1 oz narrow mouth screw cap bottle on a balance.Remove the cap of the bottle, and insert the clean, dry plastic long-stem funnel in the neck of the bottle.Set aside until needed .

 

†† ††† Slowly distill the ether from the solution.(The normal b.p. of diethyl ether is 35oC.The temperature will gradually rise, however, because some propionic acid vaporizes along with the ether.)Eventually, the distillation appears to stop (no liquid is delivered from the end of the condenser).When the still head temperature rises above 100o, discontinue heating.

 

Pour the ether into the RECOVERED ETHER container.

 

†† ††† Allow the crude propionic acid (normal b.p., 142oC) in the still pot to cool.Once cool, decant into an appropriately labeled sample bottle.

 

††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††††††††††††††† ††††††††††††††††††††††† †††††††††††††††††††††††


EXPLANATIONS FOR THE PROCEDURES IN THE EXTRACTION AND WASHING

EXPERIMENT

 

INTRODUCTION:The compound named propionic acid is also called propanoic acid, which is

††††††††††††††††††††††† IUPAC name.The compound will be referred to here by its IUPAC

††††††††††††††††††††††† name, but you will have to know both names.

PROCEDURE:

 

Step 3: By this step the propanoic acid has been formed and you wish to extract as

††††††††††† much of it as possible; therefore sodium chloride is added until the aqueous

††††††††††† solution is saturated because the propanoic acid is much less soluble in a

††††††††††† solution with high ionic strength than in a solution with a very low ionic

††††††††††† strength.The NaCl attracts water to it more strongly than does the

††††††††††† propionic acid because the NaCl is completely ionic; therefore less water

††††††††††† surrounds the propionic acid thereby making the acid less soluble in the

††††††††††† salt solution.This technique is called "salting out".

 

Step 5: The propanoic acid is more soluble in ether than in the salt solution.

 

Step 6:A very small amount of water is soluble in ether and needs to be removed

††††††††††† before the ether and propanoic acid are distilled, otherwise an aqueous

††††††††††† solution would be left after the ether was distilled and the water and

††††††††††† propanoic acid could not be separated.To remove the water anhydrous

magnesium sulfate (magnesium sulfate that contains NO water) is added

to the ether solution and the following reaction occurs

 

MgSO4 + n H2O -----> MgSO4 n H2O (S)

 

††††††††††† The ether containing the propanoic acid can then be filtered off the

††††††††††† magnesium sulfate solid.

 

Step 11:The distillation removes the ether and leaves just the propanoic acid.

††† The propanoic acid is distilled in order to remove any impurities and to

††† obtain a pure compound when the temperature becomes constant.

††† The constant temperature, which is shown by the horizontal line on the

††† graph, providing that the compound is pure.The product should be

††† collected during the time that the temperature is constant.

 

 

 

 

 

Che 317†††††††† Spí2005

 

Lab Report

 

 

 

Title: Extraction

 

Name:

 

Date:

 

 

Reaction

 

CH3CH2COO-Na+ + HCl -> CH3CH2COOH + NaCl

 

 

 

Table of physical Constants

 

 

CH3CH2COO-Na+

HCl

CH3CH2COOH†††††††††††

NaCl

MW

96.1

36.5

74.1

58.5

moles

 

 

 

 

grams

 

 

 

 

mL

 

 

 

 

 

 

 

 

 

 

 

Boiling range for your product††††††††††† ††††††††††††††††††††††† ††††††††††† ††††††††††††††††††† to ††††††††††††††††††††oC

 

 

 

 

 

††† Calculate the % yield by first calculating the moles you actually obtained from

††††††††††††††††††††††† †††† the distillation and then calculating the moles you could have obtained from the

††††††††††††††††††††††† †††† weight of sodium propionate you began with.See the example below.

 

PERCENT YIELD CALCULATION†††††

 

Percent yield actually means percent of theoretical yield, but the latter is too clumsy to say or write

 

††††††††† To begin calculating the % yield, Write the chemical equation of the reaction;

 

CH3CH2COO-Na+ + HCl + -----> CH3CH2COOH + NaCl

 

††††††††† Create a table such as the one below

 

†††††††††††

Data

Org.Salt

†† HC1

†† Acid

†† NaCl

MW(g/mole)

††† 96.0

36.5†††††

††† 74.0

†† 58.5

Grams

 

 

 

 

Moles

 

 

 

 

 

††††††††† Fill in the table.The moles of propanoic acid formed is the theoretical yield.The moles

††††††††††† of propanoic acid formed are equal to the moles of sodium propionate used, (look at the

††††††††††† balanced equation!).

 

††††††††† Now divide the moles of propanoic acid actually obtained from the distillation by the moles

††††††††††† theoretically possible and multiply the quotient by 100.

 

 

 

 

EXTRACTION EXERCISES

 

††††††††††† 1. If, when you are extracting an aqueous solution with an organic solvent, you are

††††††††††† †† uncertain as to which layer in the separatory funnel is aqueous, how would you quickly

††††††††††† †† settle the issue.