* Please note that the naphthalene has been replaced by biphenyl
Separation of Organic Compounds by Chemically Active Extraction
In this experiment we will separate an organic mixture (biphenyl and benzoic acid) by a chemically
active extraction. Biphenyl and benzoic acid are neutral aromatic compounds that are soluble in
most organic solvents (like diethyl ether) and insoluble in water (benzoic acid is slightly soluble in
water). They cannot be separated from each other by a passive extraction process like the one
you used for caffeine. However, we can change the solubility of one of the components (benzoic
acid) by a simple acid base reaction. When this mixture is treated with an aqueous base like
sodium hydroxide the carboxylic acid will be converted to the carboxylate, which is insoluble in
diethyl ether but soluble in water. The biphenyl will remain unchanged. Therefore we can move
the benzoic acid into the aqueous layer by extracting an ether solution of the above mixture with
an aqueous sodium hydroxide.
CO2H
RCO2H
RH
1
Bold numbers indicate the
corresponding step of the
procedure
Ether solution of
RCO2H and RH
2 1. Extract with 5% NaOH
2. Separate layers
ORGANIC LAYER
RH
4 1. Dry with Na 2SO4
2. Filter and Evaporate the solvent
RH
AQUEOUS LAYER
_ +
RCO2Na
3
1. Acidify with dil. HCl
2. Cool solution and filter soild
RCO2H
Procedure
1.
Dissolve 1 g of the biphenyl/benzoic acid mixture in 20 mL of diethyl ether.
2.
Pour this solution into a 125 mL separatory funnel and extract with 10 mL of 5% sodium
hydroxide. Drain the aqueous layer carefully into a 125 mL erlenmeyer flask. Repeat this
step and combine the aqueous layers.
You have now separated the two compounds. The biphenyl is in the organic layer and the
benzoic acid (in the form of the benzoate anion) is in the aqueous layer.
3.
Neutralize the aqueous layer with dil. hydrochloric acid to regenerate benzoic acid from
the benzoate anion. The hydrochloric acid should be added carefully using a plastic
transfer pipette while swirling gently. The pH of the solution should be checked
occasionally using litmus paper. Benzoic acid will precipitate when the aqueous layer
becomes acidic. Cool the flask in an ice bath for 15 min. Filter the benzoic acid using a
vacuum filtration. Allow the benzoic acid to dry on the buchner funnel for 20 min. and
store it in your locker till the next lab. Weigh the benzoic acid and record its melting point.
4.
Drain the organic layer into a 50 mL erlenmeyer flask and add a small amount (sufficient to
form a 1/8" layer) of anhydrous sodium sulfate. This will remove traces of water from the
organic layer. Allow the flask to stand for 10 min with occasional swirling. Decant the
organic later carefully into a 50 mL beaker. Add a few boiling chips and evaporate the
solvent on a hot plate (controller setting about 1.5).
Remove the beaker when the
solvent has evaporated. *Do not let the flask sit on the hot plate for too long after the
solvent has evaporated. Biphenyl will crystallize upon cooling to RT. Weigh the biphenyl
and record its melting point.
Find the melting points of biphenyl and benzoic acid from the Aldrich catalog. Compare
your melting points with the literature values.