Effects of α-Substituents on Ion Selectivity of Bis(12-crown-4-methyl) Malonates as Neutral Carriers for Sodium Ion-Selective Electrodes

The effects of α-substituents on sodium-ion selectivities of bis(12-crown-4-methyl) malonates, which are highly selective for sodium ions, were investigated by incorporating various substituents with high lipophilicity, bulkiness, polarity and back strain. The incorporation of geminal substituents such as long alkyl chains and benzyl groups into the α-position of the malonate was found to generally enhance the Na+ selectivity, especially against K+, of the bis(12-crown-4) derivatives. The α, α′-dibenzylmalonate derivative gave the best Na+ selectivity of all the bis(crown ether)s designed here.