The origin of high ortho:para-reactivity ratios in the reactions of fluoronitrobenzenes with potassium t-butoxide in t-butyl alcohol

Abstract

Kinetic data for fluorine substitution of 1-fluoro-2- or 1-fluoro-4-nitrobenzene by potassium methoxide in methyl alcohol in the presence or absence of added dicyclohexyl-18-crown-6 or by potassium t-butoxide in t-butyl alcohol in the presence of the crown ether are reported. The crown ether has a negligible influence on the kinetics of the reactions of potassium methoxide where the ortho:para-activation ratio is around unity. In sharp contrast, comparison with previous data for the reactions of the same substrates with potassium t-butoxide in t-butyl alcohol shows that on addition of the crown ether (in equimolar quantities to the nucleophile, with consequent nearly total removal of potassium cation from the bulk system), the rate of reaction of the o-nitro-substituted substrate is increased by only a factor of three while that of the p-nitro-substituted substrate is increased by a factor of nearly 2000. Consequently, the ortho:para-ratio changes from 3·6 × 102 in the absence to ca. 1 in the presence of the crown ether. These results show that the high value of the ortho:para-ratio obtained in t-butyl alcohol in the absence of addenda can be attributed mainly to specific stabilization of the transition state of the reaction of the o-nitro-substituted substrate by potassium cation bridging between the nucleophile and nitro-group oxygen atoms. This intramolecular assistance is a consequence of the low polarity and low dissociating ability of the medium used, t-butyl alcohol, and does not appear when the medium is more polar and dissociating, like methyl alcohol.