Reactions of ionized dibutyl ether
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AbstractThe reactions of ionized di‐n‐butyl ether are reported and compared with those of ionized n‐butyl sec‐butyl and di‐sec‐butyl ether. The main fragmentation of metastable (CH3CH2CH2CH2)2O+. is C2H5⋅ loss (˜85%), but minor amounts (2–4%) of CH3⋅, C4H7⋅, C4H9⋅, C4H10 and C4H10O are also eliminated. In contrast, C2H5⋅ elimination is of much lower abundance (20 and 4%, respectively) from metastable CH3CH2CH2CH2OCH(CH3)CH2CH3+. and [CH3CH2(CH3)CH]2O+., which expel mainly C2H6 and CH3⋅ (35–55%). Studies on collisional activation spectra of the C6H13O+ oxonium ions reveal that C2H5⋅ loss from (CH3CH2CH2CH2)2O+. gives the same product, (CH3CH2CH2CH2+OCHCH3) as that formed by direct cleavage of CH3CH2CH2CH2OCH(CH3)CH2CH3+.. Elimination of C2H5⋅ from (CH3CH2CH2CH2)2O+. is interpreted by means of a mechanism in which a 1,4‐H shift to the oxygen atom initiates a unidirectional skeletal rearrangement to CH3CH2CH2CH2OCH(CH3)CH2CH3+., which then undergoes cleavage to CH3CH2CH2CH2+OCHCH3 and C2H5⋅. Further support for this mechanism is obtained from considering the collisional activation and neutralization‐reionization mass spectra of the (C4H9)2O+. species and the behaviour of labelled analogues of (CH3CH2CH2CH2)2O+.. The rate of ethyl radical loss is suppressed relative to those of alternative dissociations by deuteriation at the γ‐position of either or both butyl substituents. Moreover, C2H5⋅ loss via skeletal rearrangement and fragmentation of the unlabelled butyl group in CH3CH2CH2CH2OCH2CH2CD2CH3+. occurs approximately five times more rapidly than C2H4D⋅ expulsion via isomerization and fission of the labelled butyl substituent. These findings indicate that the initial 1,4‐hydrogen shift is influenced by a significant isotope effect, as would be expected if this step is rate limiting in ethyl radical loss.
