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Abstract The investigation of several 13carbon and deuterium labelled n-butyl and n-pentyl benzenes demonstrate that chemical ionization (reagent gas: methane) induces specific carbon-carbon bond cleavages of the alkyl group. The extent of competing reaction channels as for instance direct alkene elimination versus dealkylation/reprotonation is analyzed. Partial hydrogen exchange processes between reagent ions and substrate molecules are restricted to the phenyl ring. Intramolecular exchange reactions between the side chain and the aromatic ring which are typical for the open shell molecular ions of alkyl benzenes are not observed for analogous closed shell cations.

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Thus the stabilization energy due to the second cyclopropane in quadricyclyl should be no greater than 0.11/0.52 = 0.21 of the initial stabilization In nortrlcyclyl. This fraction Is reduced still further since | --jJ < | AEi-sl (I.e., the methyl cation LUMO is lower than the cyclopropyl carbinyl LUMO). This simple perturbation theory result is to be compared with the experimental relative rates (12) This species is stabilized yet further, of course, because of relief of angle strain at the cationic center. It is the intention of this paper to focus on only electronic effects, however. (13) Using the procedure of ref 11, (HL_h)2 " 0.47 for the interaction of cyclopropane with the already strongly stabilized cyclopropyl allyl cation; this represents 90% of the stabilization due to interaction of cyclopropane with methyl cation. ( 14) M.

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