Reactions of Ionized Dibutyl Ether

Article abstract of DOI:10.1002/oms.1210291211

The 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)O^+. is C2H5^. loss (ca. 85percent), but minor amounts (2-4percent of CH3^., C4H7^., C4H9^., C4H10 and C4H10O are also eliminated.In contrast, C2H5^. elimination is of much lower abundance (20 and 4percent, respectively) from metastable CH3CH2CH2CH2OCH(CH3)CH2CH3^+. and 2O^+., which expel mainly C2H6 and CH3^. (35percent-55percent).Studies on collisional activation spectra of the C6H13O⁺ oxonium ions reveal that C2H5^. loss from (CH3CH2CH2CH2)2O^+. gives the same product, (CH3CH2CH2CH2⁺O=CHCH3) as that formed by direct cleavage of CH3CH2CH2CH2OCH(CH3)CH2CH3^+..Elimination of C2H5^. from (CH3CH2CH2CH2O^+. 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 than undergoes cleavage to CH3CH2CH2CH2⁺O=CHCH3 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 the labelled analogues of the (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.