Separation of Benzoic acid, Ethyl p-aminobenzoate, and Benzoin

Using Ion-exchange Resins

Procedure:
  1. Dissolve a mixture of 25 mg each of the title compounds in 1 mL of ethanol.
  2. 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.
    1. The acid is retained by the weak anion exchange resin
    2. The amine is retained by the cation exchange resin
    3. The neutral compound passes through into the receiver
  3. Wash the anion exchange tube with 5 mL of a 1:1 mixture of EtOH and 1N HCl to remove the acid.
  4. Wash the cation exchange tube with 5 mL of a 1:1 mixture of EtOH and 20% NH4OH to remove the amine.
  5. Solvents may be removed under reduced pressure.
  6. Determine the percent recovery and analyze the purity of the separated components.


Experimental Setup:

Notes:
  1. 2 mL of ion-exchange resins will retain over 50 mg of the acidic and basic compounds used in this experiment.
  2. 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.
  3. Benzoate was retained very strongly with a strong anion exchange resin like Dowex 1-X8 and was difficult to remove quantitatively.
  4. The HCl and NH4OH evaporate completely.
  5. 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.

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© February 1996.


DISCLAIMER:

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 training, personal protective equipment, and other safety facilities. Any users of these procedures assume all responsibility for the safe handling of hazardous chemicals and procedures. In any event the authors, webmasters, The University of Wyoming , Rochester Institute of Technology, or Dartmouth College shall not be liable for incidental or consequential damages in connection with, or arising out of, the furnishing, performance, or use of these procedures.