Single-Step and Multistep Mechanisms of Aromatic Nucleophilic Substitution of Halobenzenes and Halonitrobenzenes with Halide Anions:  Ab Initio Computational Study

The C 6 H 5 X + X - (X = Cl−I) gas phase S N Ar reactions proceed via a single-step mechanism without the formation of a stable C 6 H 5 X 2 - σ-complex. The C 2 v structures of C 6 H 5 X 2 - (X = Cl−I), which are transition structures in these reactions, are predicted to be considerably higher in energy (by 102.3 (Cl), 100.3 (Br), and 103.7 (I) kJ mol - 1 at the MP2/6-31+G(d) + ZPE(HF/6-31+G(d)) level) than their isolated reactants. These high overall barriers suggest that it would be hardly possible to observe the S N Ar reactions of monosubstituted halobenzenes with halide anions in the gas phase. The introduction of a nitro group into the benzene ring, however, leads to a significant decrease in the overall barrier and for 1-chloro-4-nitrobenzene ( 5 ) in its reaction with Cl - the overall barrier is only 18.6 kJ mol - 1 (B3LYP/6-31+G(d)+ ZPE(B3LYP/6-31+G(d)). This reaction also follows a concerted pathway without the formation of an intermediate. The stabilization energies for σ-complexes 8 − 10 formed in the reactions of 1-chloro-4-nitrobenzene ( 5 ), 1-chloro-2,4-dinitrobenzene ( 6 ), and picryl chloride ( 7 ) with chloride anion demonstrate that the introduction of the first nitro group leads to a stabilization energy of 94.1 kJ mol - 1 and the second and third nitro groups result in an additional stabilization by 70.0 and 128.9 kJ mol - 1, respectively (at the B3LYP/6-31+G(d) + ZPE(B3LYP/6-31+G(d)) level). As a consequence, the gas phase reactions of 1-chloro-2,4-dinitrobenzene ( 6 ) and picryl chloride ( 7 ) with chloride anion follow a multistep mechanism with the formation of the σ-complexes 9 and 10 as intermediates. The C 6 H 5 F + F - S N Ar reaction proceeds via a multistep mechanism with the formation of a discrete C 6 H 5 F 2 - σ-complex as an intermediate, the energy of which is 15.5 kJ mol - 1 lower than that of the separated reactants. The activation barrier for the elimination of the fluoride anion from this complex is 6.3 kJ mol - 1 at the MP2/6-31+G(d) + ZPE(HF/6-31+G(d)) levels. The negative overall barrier of −9.2 kJ mol - 1 for reaction (1) with X = F indicates that this reaction may be feasible in the gas phase provided that competing reactions do not dominate.