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Abstract

The reactions of the radical cations of the halobenzenes C6H5X.+ (X = Cl, Br, I), the isomeric dichlorobenzenes and the isomeric chloroiodobenzenes with CH3NH2 as well as the reactions of the radical cations CH3NH2.+ and (CH3)2NH.+ with the above neutral halobenzenes were studied by Fourier transform ion cyclotron resonance spectrometry. In all cases the substitution of a halogen substituent by R2NH+ (R = H, CH3) was observed besides charge exchange. The substitution of halobenzene radical cation by CH3NH2 is always fast (the reaction efficiency (eff(subst)) being greater than 28%) compared to the reaction with NH3 studied before, but exhibits principally the same dependence on the nature and position of the halogen substituent as the latter reaction. The substitution of neutral halobenzenes, where the ionization energy of the halobenzene is greater than the ionization energy of methylamine, by CH3NH2.+ is also fast (eff(subst) > 50%) and is not much influenced by the structure of the halobenzene. In contrast, the rate of substitution of all halobenzenes by (CH3)2NH.+ is small (eff(subst) < 14%), increases with the halogen lost in the series Cl < Br < I, and exhibits a distinct orientation effect for dihalobenzenes, the meta isomer being least reactive. These experimental results are discussed using thermochemical data from the literature and the curve crossing model of Shaik and Pross. The results are consistently rationalized for the reaction of ionized halobenzenes with neutral amines as well as the reaction of neutral halobenzenes with ionized amines by a mechanism involving addition to the aromatic system, rearrangement of the intermediate distonic ion to an ipso-substituted isomer, and dissociation of the intermediate by loss of halogen. However, the rate-determining step of this mechanism depends on the substrates involved, and this structural dependence of the reactions can be understood by the curve crossing model.

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