COMPLEXATION CATALYSIS - EFFECTIVE CHARGE DEVELOPMENT IN THE AMINOLYSIS OF PHENYL-ESTERS IN CHLOROBENZENE CATALYZED BY CROWN-ETHERS

Kinetics of the butylaminolysis of substituted phenyl acetates in chlo robenzene in the presence of a variety of crown ethers obey the follow ing rate law. Rate = k(b)[BuNH(2)](2)[Ester] + k(c)[BuNH(2)][Crown][Es ter] The individual rate constants fit Bronsted-type relationships: lo g k(b) = -0.75 pK(a) + 4.21 log k(c) = -0.58 pK(a) + 3.41 [18-crown-6] log k(c) = -0.61 pK(a) + 3.18 [12-crown-4] where pK(a) refers to the ionization of the phenol in aqueous solution. The Bronsted beta(1g) va lues for k(b) and k(c) are calibrated with the value of beta(eq) recen tly determined for acetyl transfer between phenolate ions in chloroben zene. The sensitivity, beta(1g), of k(c) is consistent with the rate-l imiting formation of a crown ether-zwitterion adduct with subsequent f ast (non-rate-limiting) ArO-C bond fission. The Bronsted data for k(b) when calibrated by beta(eq) is consistent with rate-limiting proton t ransfer from zwitterion to base. 18-Crown-6 enables proton transfer to occur between phenol and butylamine in chlorobenzene according to the equation: BuNH(2) + ArOH + Crown reversible arrow Bu-NH3+. Crown + Ar O- equilibrium constant (K) for the above reaction with a series of su bstituted phenols has a Bronsted selectivity (beta) of 2.1 compared wi th that for the ionization of phenols in water.