Rate and mechanism of alkyl ion formation from alkyl halides as studied by field ionization kinetics

Abstract

It is shown by field ionization kinetics (FIK) that the differences in stability of the isomeric butyl carbenium ions upon generation from the corresponding chloride and bromide molecular ions are reflected in their formation time, i.e. the position of the maximum of their normalized rate curve. Thus, t-butyl ions are formed faster than s-butyl ions, which are formed faster than i-butyl and n-butyl ions. Other observations made are that the C-Br bond cleavage is faster than the C-Cl bond cleavage, i.e. the rate of direct bond cleavage is inversely proportional to the strength of the bond being broken, and that the rate of formation is inversely proportional to the number of degrees of freedom in the molecular ions. These three observations are in line with the rate equation as formulated in QET-RRKM theory. Furthermore, two maxima are observed in the normalized rate curve for gas-phase bromine loss from the molecular ions of n-alkyl and i-alkyl bromides. The first maximum may be attributed to the “real” direct C-Br bond cleavage, whereas the second maximum indicates a lower energetic and entropically more complicated process, presumably a 1,2-hydride shift assisted C-Br bond cleavage. Finally, it has been possible to perform, in conjunction with FIK, collisional activation (CA) experiments on the fragment butyl ions formed, which show in the corresponding spectra differences for ions formed at different molecular ion lifetimes.