Gas phase substitution of halobenzenes by methylamine and dimethylamine via radical cations

Abstract

The reactions of the radical cations of the halobenzenes C 6H 5X + (X = Cl, Br, I), the isomeric dichlorobenzenes and the isomeric chloroiodobenzenes with CH 3NH 2 as well as the reactions of the radical cations CH 3NH 2 + and (CH 3) 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 R 2NH + (R = H, CH 3) was observed besides charge exchange. The substitution of halobenzene radical cation by CH 3NH 2 is always fast (the reaction efficiency (eff subst) being greater than 28%) compared to the reaction with NH 3 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 CH 3NH 2 + 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 (CH 3) 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.