Gas-phase reactions of chloro- and bromoanisoles with methyl- and dimethylamine via radical cations

Abstract

Ionic gas-phase reactions of chloro- and bromoanisoles with CH 3NH 2 and (CH 3) 2NH via radical cations were investigated by FT-ICR spectrometry using an external ion source. Protonated N-methylanisidines are formed quantitatively from bromoanisole radical cations with CH 3NH 2 by ipso substitution of the bromo atom. The analogous reaction of chloroanisole radical cations produce a mixture of protonated and radical cationic N-methylanisidines, the latter ions arising by loss of HCl from the addition complex and corresponding also to ipso substitution products. The branching ratio of product ions and the reaction efficiencies depend on the structure of the haloanisole radical cations, the efficiencies ranging from 1.3% to 24%. The observed dependence of the reactivity on the substitution pattern is in good agreement with earlier results of the reactions of dihalobenzene radical cations with NH 3 and shows that all substitution reactions proceed by the same multistep mechanism in which the addition of the amine to the aromatic radical cation in the collision complex is rate determining. The reactions of (CH 3) 2NH ·+ with neutral bromoanisoles in the ICR cell produce protonated, N,N-dimethylanisidines besides bromoanisole radical cations by charge exchange. Chloroanisoles produce with (CH 3) 2NH ·+ additionally N,N-dimethylanisidine radical cations by loss of HCl. Deuterium labelling experiments reveal that the H atom eliminated with HCl originates from the amino group. The kinetic behaviour of both substitution processes indicates that branching between loss of Cl and HCl occurs after the rate determining addition step in a chemically activated intermediate. Competition of loss of Cl and by elimination of HCl is observed only for reactions of low reaction efficiencies and is apparently controlled by the excess energy of the excited intermediate adduct.