Single-step and multistep mechanisms of aromatic nucleophilic substitution of halobenzenes and halonitrobenzenes with halide anions: Ab initio computational study

The C6H5X + X- (X = Cl-I) gas phase SNAr reactions proceed via a single-step mechanism without the formation of a stable C6H5X2- sigma-complex. The C-2v structures of C6H5X2- (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 SNAr 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/G-31+G(d)). This reaction also follows a concerted pathway without the formation of an intermediate. The stabilization energies for sigma-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/G-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 sigma-complexes 9 and 10 as intermediates. The C6K5F + F- SNAr reaction proceeds via a multistep mechanism with the formation of a discrete C6H5F2- sigma-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.