ACID-BASE EXTRACTION: SEPARATION OF AN ORGANIC ACID, AN ORGANIC BASE,
AND A NEUTRAL COMPOUND (5/27/09)
Extraction is one of the oldest chemical operations, and it is, undoubtedly, the most frequently
performed chemical operation. One or more extractions are performed during the isolation of the
products from practically every organic chemical reaction. Whenever you make a pot of coffee,
a cup of tea, or pot of vegetable soup you are performing an extraction (more precisely, a solidliquid extraction). The isolation of trimyristin from nutmeg which you performed earlier is
another example of a solid-liquid extraction. Solid-liquid extraction is commonly used by
organic chemists to isolate natural products. However, organic chemists employ liquid-liquid
extractions even more often than solid-liquid extractions. There are actually two types of liquidliquid extractions: chemically passive and chemically active. In chemically passive extractions,
distribution of a compound between two immiscible solvents occurs because of different
solubilities in the two solvents, whereas in chemically active extractions, a compound is
chemically altered to change its distribution between the two different solvents. This chemical
alteration of a compound to change its distribution between the two solvents is most commonly
done through an acid-base reaction. Sometimes, this type of extraction is referred to as an acidbase extraction.
For an example of how a chemically active extraction works, let us consider the separation of a
mixture of a neutral organic compound and a carboxylic acid. Let us assume that both
compounds are soluble in diethyl ether but insoluble in water. A passive extraction using these
to solvents obviously will not work. However, if the carboxylic acid were converted into its
conjugate base, an anion which would be soluble in water but insoluble in diethyl ether, then a
clean separation of the carboxylic acid (as its salt) from the neutral compound could be effected.
Consider a mixture of a hydrocarbon and a carboxylic acid, both of which are soluble in ether
but insoluble in water. If an ether solution of the mixture is treated with an aqueous solution of a
base,
Ether-soluble, water-insoluble mixture:
O
C
a hydrocarbon
(neutral)
OH
a carboxylic acid
(acidic)
such as sodium hydroxide or sodium carbonate, the carboxylic acid will react to form its water
soluble carboxylate anion, but the neutral hydrocarbon will not react. The hydrocarbon will
remain ether-soluble and water-insoluble. By an acid-base reaction, we have converted the
carboxylic acid (ether-soluble, water-insoluble) into its carboxylate anion (ether-insoluble,
water-soluble).
Formation of a water-soluble salt:
O
O
C OH
+
C O
OH
ether-soluble
water-insoluble
+
H2O
ether-insoluble
water-soluble
By extraction of an ether solution containing a neutral compound and a carboxylic acid with an
aqueous solution of sodium hydroxide, we can separate the two compounds. The neutral
compound remains in the ether layer, while the carboxylic acid reacts with hydroxide ion to form
the carboxylate anion, which dissolves in the water layer. The two layers are separated using a
separatory funnel, and the aqueous layer is then acidified to regenerate the water-insoluble
carboxylic acid.
Recovery of the carboxylic acid:
O
O
C
C
O
+
H
carboxylate anion
OH
carboxylic acid
Using a similar acid-base reaction, amines (organic bases) can be separated from neutral or
acidic compounds by conversion into their conjugate acids, water-soluble cations. Thus
treatment of an ether solution of a mixture containing an amine with an aqueous acid (e.g.,
hydrochloric acid) produces a water-soluble cation that can be separated from water-insoluble
compounds.
Formation of a water-soluble salt:
RNH2
+
ether-soluble
water-insoluble
H
RNH3
ether-insoluble
water-soluble
After separation of the ether and water layers, the water layer is then treated with aqueous base
to regenerate the water-insoluble amine.
Recovery of the amine:
RNH3
+
ammonium cation
OH
RNH2
+
H2O
amine
Because different conditions are used to convert acidic and basic organic compounds into watersoluble salts, the acid-base reactions we have discussed can be used to effect a number of types
of chemically active extractions. The flow chart that follows outlines the steps involved in the
separation of an organic acid (RCO2H), an organic base (RNH2), and a neutral compound (RH)
by acid-base extractions.
Solution of RCO 2 H, RNH2 , and RH in Organic Solvent
Extract with
dilute NaOH
Aqueous layer
RCO2 Organic Layer
RNH2 and RH
Make acidic, filter or
extract with solvent
Extract
with dilute
HCL
Solid or
solution of
RCO2 H
Aqueous
layer
RNH3 +
Make basic,
filter or extract
with solvent
Remove
solvent
Organic
layer RH
RCO2 H
Aqueous
layer
discard
Solid or
solution of
RNH2
Remove
solvent
RNH2
Remove
Solvent
RH
Aqueous
layer
discard
In this experiment, a mixture comprised of 0.5 g each of benzoic acid, p-nitroaniline, and
anthracene will be separated into the individual components by acid-base extractions. All three
components are soluble in dichloromethane (CH2Cl2) but only slightly soluble in cold water.
NH2
CO2H
O2N
benzoic acid
p-nitroaniline
anthracene
Procedure
Obtain 1.5 g of a 1:1:1 mixture of benzoic acid, p-nitroaniline, and anthracene and dissolve it in
40 mL of dichloromethane in an Erlenmeyer flask. Pour this solution into a separatory funnel
which has a teflon stopcock. (Use the addition funnel from your organic kit if necessary.) If
small amounts of solid material do not dissolve, the solution should be gravity-filtered through a
small cotton plug into the separatory funnel. Extract the organic solution with 25 mL of 2M
sodium hydroxide solution. Separate the layers and extract the organic layer with a second 25
mL of 2M sodium hydroxide. Combine the basic extracts in an Erlenmeyer flask and label the
flask "basic extract".
Now similarly extract the organic layer twice with 25 mL portions of 6M hydrochloric acid.
Combine the acidic extracts in a second Erlenmeyer flask and label the flask "acidic extract".
Transfer the organic layer into a third Erlenmeyer flask and label the flask "neutral fraction".
Add approximately 2-3 g of anhydrous sodium sulfate to this neutral fraction, swirl and let stand
for at least 15 minutes with occasional swirling to hasten the drying.
Cool the flasks containing the "acidic extract" and the "basic extract" in an ice-water bath.
Neutralize the "acidic extract" by carefully adding 6M sodium hydroxide until the solution is
distinctly basic to pH paper. Similarly, neutralize the "basic extract" by carefully adding
concentrated (12M) hydrochloric acid until the solution is distinctly acidic to pH paper.
Precipitates should form in each flask upon neutralization. The neutralizations will liberate
considerable amounts of heat, so let the flasks stand in the ice-water bath until their contents are
at, or below, room temperature.
Collect each precipitate by vacuum filtration using a Buchner funnel and wash the filter cakes
with a small amount of cold distilled water. Allow the collected solids to air-dry until the next
laboratory period.
Decant the "neutral fraction" from the sodium sulfate into a tared round-bottomed flask (the flask
should be of sufficient size such that it will be no more than half-filled by the solution). Rinse
the Erlenmeyer flask and the sodium sulfate with a small amount of dichloromethane, decanting
it into the round-bottomed flask. Remove the solvent on a rotary evaporator. Reweigh the flask
with its contents, determine the weight of the neutral material, and calculate a percent recovery.
Transfer this neutral material to a labelled vial and store it until the next laboratory period.
NEXT LAB
After the base (p-nitroaniline) is dry, transfer it to labelled and tared vial. Reweigh the vial to
determine the weight of the solid and calculate it’s percent recovery. After benzoic acid is dry,
transfer it to a tared flask. Reweigh the flask to determine the weight of the solid and calculate
it’s percent recovery.
Recrystallize benzoic acid from a minimum amount of hot water. (Use the solubility data to
calculate the approximate amount below, based on the grams you recovered.) Add an extra 2-5
mLs of hot water if there is still solid present after adding the minimum amount of solvent. If
this does not dissolve the solid, you will need to perform a hot filtration (perform over a steam
bath, use a small plug of cotton). Before cooling, heat on a hot plate to remove the extra solvent
added prior to the hot filtration (if performed).
Dry and weigh the crystals. Determine melting points for benzoic acid and p-nitroaniline and
compare them to literature values. Separately, dissolve a small amount of each of the three
compounds in chloroform and spot a silica gel TLC plate with these solutions. Develop the TLC
plate using chloroform. Visualize the spots under both short and long wavelength UV light and
record your observations.
Please help conserve ice. Share with your bench partner when possible!
Flush aqueous solutions down the sink w/ plenty of running water. Sodium hydroxide,
hydrochloric acid are both aqueous.
¡ Write out the reactions for the extraction (e.g., C6H5-COOH + NaOH Æ C6H5-COO-Na+ +
H2O) and think about the difference in solubility between the acid and base compounds vs.
their salt forms.
¡ Before lab, calculate the moles of benzoic acid and 2M NaOH you will use to extract it with.
Then calculate the amount of concentrated (12M) HCl it will take to neutralize it. Include
these amounts in the reagent table of your pre lab. Do likewise for p-nitroaniline!
¡ The melting points of benzoic acid and p-nitroaniline are important physical constants for
this experiment (and therefore belong in the reagent table of your prelab)!
¡ Read all applicable techniques in your lab text (Zubrick) and otherwise prepare well for this
lab experiment!
SOLUBILITY DATA
COMPOUND
Benzoic Acid
p-Nitroaniline
Anthracene
TEMPERATURE
SOLUBILITY IN WATER
(g/100mL)
0 ºC
0.17
20 ºC
0.29
90 ºC
4.55
0 ºC
0.08
100 ºC
2.22
78 ºC
1.49 (IN ETHANOL)
Name:_____________________________________________Section:___________Date:___________
POSTLAB EXERCISE
ACID-BASE EXTRACTION: SEPARATION OF AN ORGANIC ACID, AN ORGANIC BASE,
AND A NEUTRAL COMPOUND (6/5/09)
>> Due the lab following the completion of the experimental protion of the notebook (30pts.
TOTAL). Please answer questions on this form.<<
A. PRODUCT INFORMATION (15pts)
%Recovery
(relative to
the amount
in initial
mixture
(6pts):
Benzoic Acid (5pts)
p-Nitroaniline (5pts)
Anthracene (4pts)
(2pts)
(2pts)
(2pts)
BEFORE Purification:
AFTER Purification:
Melting
(1pt)
Point (2pts):
(1pt)
Appearance
(describe)
(4pts):
(2pts)
(2pts)
X (0pts – n/a)
(2pts)
(1pt) All Vials Properly Labeled
B. SUMMARY QUESTIONS (15pts)
1. (2pts) Which of the three compounds you separated in this experiment dissolved in aqueous
NaOH?
Which compound dissolved in aqueous HCl?
Which compound was left in the dichloromethane?
2. (2pts) Explain how benzoic acid was recoved from extracted solution:
How was p-nitroaniline recovered?
3. (2pts) The percent recovery for benzoic acid is usually lower than the percent recovery for pnitroaniline. Comparing percent recoveries before purification, explain why this is true:
Which compound had the highest recovery?
4. (2pts) How was benzoic acid purified?
5. (2pts) Which compound showed the largest Rf value?
Which compound showed the smallest Rf value?
Which compound fluoresced under longwave UV light?
6. (2pt) On the basis of what you have learned in this experiment, plus the information that
phenol is soluble in aqueous NaOH but not in aqueous NaHCO3, whereas benzoic acid is
soluble in both, draw a flow diagram showing how you would separate a mixture of benzoic
acid, phenol, and aniline and attach it to this exercise.
7. (3pts) The pKa of benzoic acid is 4.19. Explain whether or not precipitation of benzoic acid
from an aqueous solution of benzoate anion would be complete if hydrochloric acid is added
until a pH of 7 is obtained.