Reactions of ionized dibutyl ether

Abstract

Abstract 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 (CH 3 CH 2 CH 2 CH 2 ) 2 O +. is C 2 H 5 ⋅ loss (˜85%), but minor amounts (2–4%) of CH 3 ⋅, C 4 H 7 ⋅, C 4 H 9 ⋅, C 4 H 10 and C 4 H 10 O are also eliminated. In contrast, C 2 H 5 ⋅ elimination is of much lower abundance (20 and 4%, respectively) from metastable CH 3 CH 2 CH 2 CH 2 OCH(CH 3 )CH 2 CH 3 +. and [CH 3 CH 2 (CH 3 )CH] 2 O +. , which expel mainly C 2 H 6 and CH 3 ⋅ (35–55%). Studies on collisional activation spectra of the C 6 H 13 O + oxonium ions reveal that C 2 H 5 ⋅ loss from (CH 3 CH 2 CH 2 CH 2 ) 2 O +. gives the same product, (CH 3 CH 2 CH 2 CH 2 + OCHCH 3 ) as that formed by direct cleavage of CH 3 CH 2 CH 2 CH 2 OCH(CH 3 )CH 2 CH 3 +. . Elimination of C 2 H 5 ⋅ from (CH 3 CH 2 CH 2 CH 2 ) 2 O +. is interpreted by means of a mechanism in which a 1,4‐H shift to the oxygen atom initiates a unidirectional skeletal rearrangement to CH 3 CH 2 CH 2 CH 2 OCH(CH 3 )CH 2 CH 3 +. , which then undergoes cleavage to CH 3 CH 2 CH 2 CH 2 + OCHCH 3 and C 2 H 5 ⋅. Further support for this mechanism is obtained from considering the collisional activation and neutralization‐reionization mass spectra of the (C 4 H 9 ) 2 O +. species and the behaviour of labelled analogues of (CH 3 CH 2 CH 2 CH 2 ) 2 O +. . 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, C 2 H 5 ⋅ loss via skeletal rearrangement and fragmentation of the unlabelled butyl group in CH 3 CH 2 CH 2 CH 2 OCH 2 CH 2 CD 2 CH 3 +. occurs approximately five times more rapidly than C 2 H 4 D⋅ 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.