Isotoluenes, radical cations

Thus, the secondary C3HeO radical cations created by the McLafferty rearrangement of the molecular ions of hexan-2-one (Scheme 7) correspond to the ionized enols of acetone and not acetone radical cations, and further fragmentation may require re-ketonization . Similarly, elimination of an alkene fragment from the molecular ions of alkylbenzenes gives rise primarily to an isotoluene radical cation (Scheme 8) which differs in reactivity from the toluene molecular ion. In most cases these differences alter the details of the fragmentation mechanisms, but not the principal routes of a further fragmentation. 

The intermediacy of the radical cations of isotoluenes (methylenecyclohexadienes) and their derivatives is a common feature in organic mass spectrometry however, it is widely ignored because of the rather difficult experimental access to neutral isotoluenes. Again, the reader is referred to the discussion on methylenecyclohexadienes in the 1990 review on ionized alkylbenzenes7. An early paper by Lifshitz and Bauer150 on mass spectrometry of bicyclo[3.2.0]hepta-2,6-diene, another C7H8 isomer, as well as of bicyclo[3.2.0]hept-6-ene and one of its isomers, cyclohepta-1,3-diene, may also be mentioned in this context. 

Cycloheptatriene, Norbomadiene, Methylenecyclohexadienes (Isotoluenes) and Bicy-clof3.2.0/heptadienes. The gas-phase ion chemistry of ionized 1,3,5-cycloheptatriene is closely related to that of ionized toluene, in particular, and to that of norbomadiene and other non-aromatic ( yllx isomers. This extensive body of work will not be discussed here since a detailed review on this topic has been published by one of these authors in the context of the gas-phase chemistry of the alkylbenzene radical cations This chemistry pertains also to the well-known isomerization of the even-electron CvHv ions and to their formation from the respective parents, e.g. CyHs" " . A related, albeit chemically different held concerns protonated cycloheptatriene, i.e. the even-electron C7H9+ ions , and alkylcycloheptatrienes, which are closely related to protonated toluene and higher alkylbenzenium ions. A parallel review by one of these authors on protonated alkylbenzenes has been pubUshed, and recent investigations on protonated alkylcycloheptatrienes have highUghtened the complexity of this gas-phase ion chemistry 42 jp a minor extent, ionized and protonated fulvenes have also been investigated with respect to their interconversion to their (mainly arene-derived) isomers. 

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