Ion/molecule reactions of isomeric bromobutene radical cations with ammonia

The ion/molecule reaction of the radical cations of three isomeric bromobutenes (2-bromobut-2-ene 1, 1-bromobut-2-ene 2, 4-bromobut-1-ene 3) with ammonia were studied by Fourier transform ion cyclotron resonance spectrometry to reveal the effect of a different position of the bromo substituent relative to the C-C double bond. Further, the reaction pathways of the ion/molecule reactions were analyzed by theoretical calculations at the level B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d). All three bromobutene radical cations 1(.+) to 3(.+) react efficiently with NH(3). The reactions of 1(.+) carrying the halogen substituent at the double bond follow the pattern observed earlier for other ionized vinylic halogenoalkenes. The major reaction corresponds to proton transfer to NH(3) as to be expected from the high acidity of but-2-ene radical cations exposing six acidic H atoms at allylic positions. The other, still important, reaction of 1(.+) is substitution of the Br substituent by NH(3). Although the radical cations 2(.+) and 3(.+) are expected to be as acidic as 1(.+), proton transfer is the minor reaction pathway of these radical cations. Instead, 2(.+) displays bomo substitution as the major reaction. It is suggested that the mechanism of this reaction is analogous to S(N)2' of nucleophilic allylic substitution. Substitution of Br is not efficient for the reactions of 3(.+)-the two major reactions correspond to C-C bond cleavage of the two possible beta-distonic ammonium ions which are generated by the addition of NH(3) to the ionized double bond of 3. This observation, as well as the results obtained for 1(.+) and 2(.+), emphasize the role of the fast and very exothermic addition of a nucleophile to the ionized double bond for the ion/molecule reactions of alkene radical cations. Clearly the energetically-excited distonic ion arising from the addition fragments unimolecularly by energetically accessible pathways. In the case of a halogene subsituent (except F) at the vinylic or allylic position, this is loss of thesubsituent. In the case of remote halogeno substituents, this is C-C bond cleavage adjacent to the radical site of the distonic ion.