Steric effects as a basis for improved monovalent cation extraction selectivity in crown ethers

Incorporation into a 20-crown-6 of a bulky substituent capable of impeding cation/anion access to one face of the crown ether cavity is shown to afford compounds exhibiting good extraction selectivity for potassium ion over both alkaline earth cations (Ca2+, Sr2+) and other alkali metal ions (Na+, Cs+), an apparent result of diminished flexibility of the crown ether cavity, inhibition of the formation of extractable "sandwich" complexes with large cations, and the destabilizing effect of forcing charge-neutralizing counter anions to approach from one face of the crown cavity.