Selectivity of nitro versus fluoro substitution in arenes in their reactions with charged O‐ and S‐nucleophiles

The relative mobility of nitro and fluoro substituents in 1,3‐dinitro‐ and 1‐fluoro‐3‐nitrobenzenes, 3,5‐dinitro‐ and 3‐fluoro‐5‐nitrobenzotrifluorides under the action of the nucleophiles (2ArYH)·K₂CO₃ and ArY^?K⁺ in solution and the nucleophiles ArYH·K₂CO₃ (Y = O, S) under heterogeneous conditions was studied by a competitive method in DMF at 40–140 °C. The unique dependences of ΔΔH^≠ on ΔΔS^≠ and ΔΔH^≠ on ΔΔG^≠ were determined for all the substrates and nucleophiles. The dependence of the mechanistic pathway on the nucleophile is discussed. Two results are relevant to the reactions studied: (i) substituent effects in the nucleophiles (2ArYH)·K₂CO₃ and ArYH·K₂CO₃ on the activation entropies suggest that the entropy favours the displacement of nitro group; (ii) the negative signs of ΔΔH^≠ and ΔΔS^≠ for the reactions of the nucleophiles ArY^?K⁺ indicate that the enthalpy determines the displacement of nitro group. It is concluded that the selectivity of the reactions with aryloxide and arylthioxide ions cannot be explained by the hard–soft acid–base principle only. Copyright © 2007 John Wiley & Sons, Ltd.