Reaction of radical cations of 1,1-dihaloethenes with ammonia:: substitution and base catalyzed tautomerization in sequence

The reactions of the radical cations of 1,1-dichloroethene (1) and 1,1-dibromoethene (2) with ammonia have been studied by Fourier transform ion cyclotron resonance (FTICR) spectrometry and ab initio molecular orbital calculations. Both radical cations exchange smoothly one of the halogen substituents by an ammonium group (reaction efficiency 60% and 35%, respectively) to yield a primary product ions C2H5NX+ (X = Cl, Br). By using the experimental facilities of the FTICR method it is shown that the primary ions react further with ammonia producing ammonium ions by deprotonation and ions C2H7N+2 by addition of NH3 and elimination of HX. Under the experimental conditions used the ion C2H7N+2 rapidly forms the proton-bound homodimer N2H+7 of ammonia as the final reaction product ion. Ab initio calculations of the minimum energy reaction pathway as well as experimental results show that the primary product ion C2H5NX+ corresponds to a N-protonated 1-halovinylamine (CH2=CX-NH+3) generated by an addition/elimination mechanisms from 1(.+) or 2(.+) and NH3. The proton affinity of the 1-chlorovinyl amine is estimated theoretically and experimentally to be about 15 kJ/mol less than that of NH3, whereas the more stable tautomer acetimidehalogenide CH3-CX=NH is more basic than NH3 by about 15 kJ/mol. This situation leads to a base catalyzed tautomerization of the initially generated N-protonated 1-halovinyl amine into the acetimidehalogenide which subsequently reacts with NH, to yield the proton-bound heterodimer of acetonitrile and NH3, [CH3CN-H+-NH3]. This latter species undergoes fast ligand exchange to produce the proton-bound homodimer of NH3 [H3N-H+-NH3] as the final product ion. (C) 2000 Elsevier Science B.V.