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Competing Reactions Leading to Propene Loss from the Molecular Ions of Aryl n ‐Propyl Ethers
Author(s) -
Matimba Henri E. K.,
Ingemann Steen,
Nibbering Nico M. M.
Publication year - 1996
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/(sici)1096-9888(199606)31:6<609::aid-jms334>3.0.co;2-0
Subject(s) - chemistry , propene , dissociation (chemistry) , ion , deuterium , polyatomic ion , molecule , isotopic labeling , collision induced dissociation , fragmentation (computing) , tandem mass spectrometry , mass spectrometry , aryl , photochemistry , medicinal chemistry , organic chemistry , alkyl , catalysis , physics , chromatography , quantum mechanics , computer science , operating system
The mechanisms of the elimination of a propene molecule from the molecular ions of a series of 3‐ and 4‐substituted aryl n ‐propyl ethers (YC 6 H 4 OC 3 H 7 , Y = H, CH 3 , CF 3 , NO 2 , CH 3 S and CH 3 O) were examined with the use of deuterium labelling and tandem mass spectrometry. Propene loss dominates in the ion‐source reactions and is the exclusive process observed for the metastable molecular ions of most of the aryl n ‐propyl ethers. This process is concluded to proceed by two distinct pathways on the basis of the relative importances of the losses of C 3 H 4 D 2 and C 3 H 5 D from the ionized ethers labelled with two deuterium atoms at the β‐position of the n ‐propyl group in combination with the results of an analysis of the product ion structures. One pathway involves intermediate formation of an ion–neutral complex composed of a YC 6 H 4 O • radical and a + CH(CH 3 ) 2 carbenium ion. This complex reacts further by proton transfer prior to dissociation with the formation of product ions, which have the same structure as the corresponding ionized and substituted phenols, YC 6 H 4 OH +• . The second pathway involves a reversible 1,5‐H shift from the β‐position of the propyl group to the 2‐ or 6‐position of the aromatic ring with the formation of a distonic ion, which expels propene to afford the molecular ion of a substituted cyclohexa‐2,4‐dienone species. The first pathway prevails for most of the ionized ethers with the exception of the molecular ions of the 3‐methoxy‐ and 3‐methylthio‐substituted ethers, which expel propene largely by the second pathway. In addition, the 1,5‐H shift‐initiated propene loss is particularly pronounced for the metastable molecular ions of these latter two ethers, suggesting that this reaction is associated with a lower critical energy than the reaction involving formation of an ion–neutral complex.