The SN mechanism in aromatic compounds. Part XXXIV. Comparative nucleophilic strength and halogen mobility in reactions of some nucleophiles with some fluoro- and iodo-nitrobenzenes

Abstract

The reactivity pattern of four reactive nucleophiles of the first and second rows of the Periodic Table, viz., azide, methoxide, thiomethoxide, and thiophenoxide ions with p-fluoronitro-, p-iodonitro-, 1-fluoro-2,4-dinitro-, and 1-iodo-2,4-dinitro-benzenes, is discussed, together with the halogen-mobility pattern in the sixteen reactions.

Individual reactivities and the irregular patterns are shown to be quantitatively predictable by Miller's method of calculation based on the use of fundamental thermochemical data, and the two-stage addition–elimination mechanism of activated aromatic nucleophilic substitution.

Changes in halogen mobility are a consequence of a shift from formation of the first to formation of the second transition state as the rate-limiting step. This leads to loss of an electronegativity (α-substituent effect) favouring high fluorine mobility, and thus to marked changes in halogen mobility with reagent and the number of activating nitro-groups. As a further result the pattern of reagent reactivity is largely obscured.

There is no support from these data for the suggestion that a polarisability effect enhances reactivity, when atoms of high polarisability are brought together in the transition state. On the contrary, when in the relationship of entering and replaced group there is an adverse kinetic effect (heavy nucleophile interaction).