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UNIMOLECULAR REACTIONS OF ISOLATED ORGANIC IONS - THE CHEMISTRY OF THE OXONIUM IONS CH3CH2CH2CH2+O = CH2 AND CH3CH2CH2CH = O+CH3

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Abstract

The reactions of the metastable oxonium ions CH3CH2CH2CH2+O=CH2 and CH3CH2CH2CH=O+CH3 are reported and discussed. Both these isomers of C5H11O+ expel predominantly CH2O (75-90% of the metastable ion current), a moderate amount of C3H6 (5-15%), a minor amount of CH3OH (2-8%) and a very small proportion of H2O (0.5-3%). All these processes give rise to Gaussian metastable peaks. The kinetic energy releases associated with fragmentation of these oxonium ions are similar, but slightly larger for dissociation of CH3CH2CH2CH=O+CH3. The behaviour of labelled analogues confirms that the reactions of CH3CH2CH2CH2+O=CH2 and CH3CH2CH2CH=O+CH3 are closely related, but subtly different. Elimination of CH2O and C3H6 is intelligible by means of mechanisms involving CH3CH+CH2CH2OCH3. This open-chain cation is accessible to CH3CH2CH2CH2+O=CH2 by a 1,5-H shift and to CH3CH2CH2CH=O+CH3 by two consecutive 1,2-H shifts (or, possibly, a direct 1,3-H shift). The rates of these 1,2-, 1,3- and 1,5-H shifts are compared with one another and also with the rates of CH2O and C3H6 loss from each of the two oxonium ions. The 1,5-H shift that converts CH3CH+CH2CH2OCH3 formed from CH3CH2CH2CH=O+CH3 into CH3CH2CH2CH2+O=CH2 prior to CH2O elimination is essentially unidirectional. In contrast, the corresponding step converting C5H11O+ ions generated as C3CH2CH2CH2+O=CH2 into CH3CH+CH2CH2OCH3 competes effectively with expulsion of CH2O and C3H6. The implications of the latter finding for the degree of concert in the hydrogen transfer and carbon-carbon bond fission steps in alkene losses from oxonium ions via routes that are formally isoelectronic with the retro 'ene' pericyclic process are emphasized

Topics: QD, QC
Publisher: JOHN WILEY & SONS LTD
OAI identifier: oai:wrap.warwick.ac.uk:20951
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