UNSPECIFIED (1993) UNIMOLECULAR REACTIONS OF ISOLATED ORGANIC IONS - THE CHEMISTRY OF THE OXONIUM IONS CH3CH2CH2CH2+O = CH2 AND CH3CH2CH2CH = O+CH3. ORGANIC MASS SPECTROMETRY, 28 (10). pp. 1197-1209. ISSN 0030-493X

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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.

Item Type:	Journal Article
Subjects:	Q Science > QD Chemistry Q Science > QC Physics
Journal or Publication Title:	ORGANIC MASS SPECTROMETRY
Publisher:	JOHN WILEY & SONS LTD
ISSN:	0030-493X
Date:	October 1993
Volume:	28
Number:	10
Number of Pages:	13
Page Range:	pp. 1197-1209
Publication Status:	Published
URI:	http://wrap.warwick.ac.uk/id/eprint/20951

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