Separation of Benzoic acid, Ethyl p-aminobenzoate, and Benzoin
Using Ion-exchange Resins
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.
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