Lewis acid-base interactions crown ethers

The metal-ion complexmg properties of crown ethers are clearly evident m their effects on the solubility and reactivity of ionic compounds m nonpolar media Potassium fluoride (KF) is ionic and practically insoluble m benzene alone but dissolves m it when 18 crown 6 is present This happens because of the electron distribution of 18 crown 6 as shown m Figure 16 2a The electrostatic potential surface consists of essentially two regions an electron rich interior associated with the oxygens and a hydrocarbon like exterior associated with the CH2 groups When KF is added to a solution of 18 crown 6 m benzene potassium ion (K ) interacts with the oxygens of the crown ether to form a Lewis acid Lewis base complex As can be seen m the space filling model of this... 

K+ fits into the cavity of the crown ether where it is bound by a Lewis acid/Lewis base interaction with the oxygens. 

The ditopic fluorescent sensors 100 and 101 have been developed by Koskela as reversible AND logic gates with selectivity for potassium fluoride.The binding of fluoride with boronic acids is well documented. In this case the sp hybridised boronic acid, which is a hard Lewis acid, interacts strongly with the fluoride anion, which is a hard Lewis base, to become sp hybridised. The potassium cation is held in situ partly by the crown ether and partly by the electrostatic interaction with the fluoride anion. This co-operative... 

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