Separation of Benzoic acid, Ethyl
p-aminobenzoate, and Benzoin
Using Ion-exchange Resins
David. A. Nelson
University of Wyoming
- Dissolve a mixture of 25 mg each of the title compounds in
1 mL of ethanol.
- Pass the solution over a set of two 8 mL
SPE tubes, one loaded with 2ml of a
weak anion exchange resin in the neutral form,and the other
loaded with 2 mL of a strong cation exchange resin in the
H+ form, in any order. Wash with 1 mL of ethanol. Pass
the ethanol solution over the tubes a second time and suck dry
with a vacuum device.
- The acid is retained by the weak anion exchange
- The amine is retained by the cation exchange resin
- The neutral compound passes through into the receiver
- Wash the anion exchange tube with 5 mL of a 1:1 mixture of
EtOH and 1N HCl to remove the acid.
- Wash the cation exchange tube with 5 mL of a 1:1 mixture of
EtOH and 20% NH4OH to remove the
- Solvents may be removed under reduced pressure.
- Determine the percent recovery and analyze the purity of
the separated components.
- Experimental Setup:
- 2 mL of ion-exchange resins will retain over 50 mg of the
acidic and basic compounds used in this experiment.
- Dowex 50-X4 strongly acidic cation exchange resin and
Amberlyst A-21 weakly basic anion exchange resin may be used.
Any other brands should work satisfactorily.
- Benzoate was retained very strongly with a strong anion
exchange resin like Dowex 1-X8 and was difficult to remove
- The HCl and NH4OH
- TLC, HPLC, UV, IR, and NMR may be used for analysis. HPLC
can give qualitative and quantitative results conveniently.
Melting points could be used to retain a conventional approach.
Discussion: This experiment serves as an
alternative to the traditional separatory funnel procedure. A
water-immiscible solvent is not required. The behavior of
ion-exchange supports can be discussed together with the
principles of acid-base chemistry. Basic compounds are retained by
the -SO3H cation resin. The proton is
transferred to the amine forming the conjugate acid of the amine
which thus becomes the retained cation. Upon treatment with
ammonia the conjugate acid is neutralized and replaced with the
ammonium ion. The weak anion exchanger is a tertiary amine which
becomes protonated by the acid compound. The resulting positive
charge then retains the conjugate base anion. Upon treatment with
hydrochloric acid the conjugate base is protonated and released
from the resin, being replaced with chloride ion. The solutions
used to elute the retained components were chosen so that no
residue would contaminate the separated compounds upon
evaporation. Alternatively, the separated components could be left
on the resins. This latter procedure would apply if the resins
were being used to remove an acidic or basic impurity from a
reaction mixture. By incorporating an appropriate chromatographic
column the neutral component could also be retained.
This procedure should be applicable to a wide variety of
separations. Nicotinic acid and 2,4,6-trimethylbenzoic acid may be
substituted in place of benzoic acid. To date no other basic
components have been tried.
David. A. Nelson, mailto:firstname.lastname@example.org
ChemJOULE, WWW, Vol. 1, Manuscript 1,
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© February 1996.
- These experiments are designed to be used in college and
university level chemistry laboratory courses, and assume the
availability of routine supplies, equipment, and instrumentation
usually associated with organic chemistry laboratory courses, as
well as an appropriate modern level of supervision, safety
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