Heptyl ions

Ring-protonated alkylcyclopropanes, well-known in solution , were suggested to explain the most unusual behaviour of heptyl ions. Extensive C- and D-labelling as well as a computer simulation and the analysis of CA data indicate that the major fragmentation reactions of CvHj s ions, generated via loss of I from 1-heptyl iodide, are preceded by extensive skeletal reorganization which via cyclopropane-like structures (140, 142, 144) eventually isomerize to tertiary carbocations (145 and 146, Scheme 23 where only the carbon skeleton is shown). The latter serve as actual precursors for the loss of CsH. ... 

Beside Bu, heptyl ions are the alkyl ions that were studied the most, in particular by labelling with D and When trying to explain the fragmentation mechanism of this ion, two questions come to mind ... 

The most often used method is to start with a functional group in a defined position that is easily lost upon ionization. Halogen atoms are mostly used for the production of heptyl ions, the iodides are particularly useful since the intensity of the molecular ion is weak, the heptyl parent ion is relatively abundant and the ions with iodine or fragments thereof contribute to less than 1% of the total ion current They can be considered a clean source of heptyl (or any alkyl) ions. Another possibility are ion/molecule reactions occurring in chemical ionization, either by adding a proton to an olefin, by hydride (or another anion) abstraction with, e.g., Et, or by proton transfer, e.g., by to an... 

Reaction 54 is of a more complex nature, since the intermediate heptonium is not known. For lower alkanes Hiraoka and Kebarle were able to demonstrate the existence of two species, one corresponding probably to a complex between an alkyl ion and a H2 molecule, the other to a C—C protonated alkane. Different mechanisms have been discussed by Houriet, Parisod and Gaumann. In trying to rationalize the observations, the authors were led to propose two paths for the formation of heptyl ion. Reaction 56 is particularly intriguing, since it is the only case where they observed a clean C—C bond split without a prior positional scrambling vide supra). Unfortunately, it cannot be decided if this elimination of an H2 and an olefin is so fast that there is no time for a skeletal randomization, or if the loss of an alkane neutral moiety is a rather clean, unknown elimination. 

The spectrum of the heptyl ion is given in Table 18. The results for the source spectra are obtained from the spectrum of 1-iodo-n-heptane after correction for the fragments containing iodine. Bu" is by far the most abundant ion. The formation of propyl ion in the source might be astonishing, since it does not correspond to Stevenson s rule it could be formed by successive eliminations of two ethylenes, but we will show later... 

TABLE 19. The probability (in %) to lose a given label to the neutral for 1-heptyl ions... 

FIGURE 4. The probability of the different positions in the n-heptyl ion to be lost to the neutral C3H6 (after Stahl and Gaumann )... 

It has been shown above that Bu ions, at least on a long time scale, scramble the carbon and hydrogen atoms randomly, whereas heptyl ions seem to follow some unknown distribution law directed by the way the lowest-energy structure(s) is attained. Practically only olefins are eliminated. The metastable fragmentation of the dodecyl ion, formed in the fragmentation of n-tetradecane (6% C7H14, 17% 30% CgHio 30%... 

---