The influence of crown ethers on the activation parameters of the β-elimination reactions of some p-chlorophenylethanes with alkoxide bases

Abstract

The addition of crown ethers 1,4,7,10,13-pentaoxacyclopentadecane and 1,4,7,10,13,16-hexaoxacyclo-octadecane in quantities equimolar to that of the base, to the dehydrochlorination reactions of the p-chlorophenylethanes, 2,2-bis-(4-chlorophenyl)-1-chloro-, -1,1-dichloro-, and 1,1,1-trichloro-ethane, promoted by sodium ethoxide and sodium t-butoxide, has been investigated. The crown ethers cause no change in the kinetic order of the reaction, but the second-order rate constants and activation parameters are significantly changed to a greater extent than can be accounted for merely by the assumption that the crown ethers bring about complete dissociation of the alkoxide ion-pairs. The nature of the changes are such that different transition state structures within the E2 mechanism must exist in the presence of crown ethers, in addition to different initial state solvation conditions.