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Ten crown ethers with pendant carboxylic acid groups are synthesized from corresponding hydroxy crown ethers. Within this series of crown ether carboxylic acids there is systematic variation of the following: (a) the crown ether cavity size, while holding the pendant carboxylic acid group constant, (b) the length of the linkage which joins the carboxylic acid group to a common polyether ring, and (c) the lipophilicity, while keeping the polyether ring and linkage which joins the carboxylic acid and polyether ring portions invariant. Dissociation constants of the crown ether carboxylic acids in water are determined.

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Competltive solvent extraction of alkali-metal cations from aqueous solutions into chloroform by a series of lipophilic dlbenzocrown ethers wlth pendant carboxylic acld groups has been Investigated. Both the extraction selectivity and efflciency are Influenced by variations of the crown ether ring size and the attachment site of the ilpophillc group. Preorganlzation of the binding site by proper positioning of the ilpophlilc group enhances extraction seiectlvity. Very high Na' extraction selectivity (Na+/K+ = 32, Na+/LI+ = 66 and no detectable extraction of Rb' or Cs') was obtained with sym-decyldbenzo-16-crown-5-oxyacetic acld. INTRODUCTIONIn his recent review of crown ethers as solvent extraction agents for metal ions, McDowell points out that when a proton-ionizable group is attached to the cyclic polyether framework, the molecule is both a cation exchanger and a coordinator ( I ) . This arrangement has the potential for providing an extraction system with greater selectivity and efficiency than one in which an organophilic acid is simply mixed with a crown ether.In earlier work (2,3), we examined competitive alkali-metal cation extraction from aqueous solutions into chloroform by dibenzocrown ether carboxylic acids 1-3. I t was found that these proton-ionizable ionophores were of insufficient lipophilicity to remain completely in the organic phase during extraction of alkali-metal cations from alkaline aqueous phases. To avoid such complications in extraction behavior, a lipophilic group was attached either to each benzene ring or to the carboxylic acid containing sidearm of the dibenzo-16-crown-5 compound 2 to produce the lipophilic dibenzo-16-crown-5-carboxylic acids 4 and 5, respectively (3-5). Compounds 4 and 5 were found to be sufficiently lipophilic to remain completely in the chloroform phases even when the contacting aqueous solutions of alkali-metal cations were highly alkaline (3,5). Although the overall alkali-metal cation extraction behavior was similar for structural isomers 4 and 5, the former gave enhanced Na+/K+ selectivity and excluded Li+ from the chloroform phase (5). Hence the lipophilic group attachment site was found to exert some influence upon extraction behavior.The efficiencies and selectivities of competitive alkali-metal cation extraction into toluene (6) and of alkali-metal cation transport across chloroform and toluene liquid membranes