Reactions of alkyl halides with distonic acylium radical cations

Abstract

Molecular orbital calculations (Becke3LYP/3-21G(d)) indicate that the distonic acylium ions •CH 2CH 2CH 2CO + and •CH 2CH 2CO + are localized σ-radicals with a spin density of 0.95 e and 0.92 e at the terminal methylene carbon, respectively. Most of the positive charge (+0.88) is localized on the carbonyl carbon. The radical site carries a charge that is only slightly more positive than that in the neutral ethyl radical. Based on these values, the distonic acylium ions may be expected to behave like neutral alkyl radicals. However, an examination of the reactivity of gaseous •CH 2CH 2CO + and •CH 2CH 2CH 2CO + toward a series of alkyl halides in a dual-cell Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR) revealed that their reactivity is greatly influenced by the charged group. The ions •CH 2CH 2CO + and •CH 2CH 2CH 2CO + are unreactive toward alkyl chlorides but react with alkyl bromides and iodides by electron, halogen atom, alkyl radical and/or hydrogen atom abstraction. While hydrogen and halogen atom abstraction reactions are common for neutral alkyl radicals, electron and alkyl abstractions are not. A mechanism that involves catalysis by the charge site is proposed for the alkyl abstraction reactions.