A Gas-Phase E2 Reaction: Methoxide Ion and Bromopropane

Article abstract of DOI:10.1021/ja00187a017

We describe a procedure for studying gas-phase ion-molecule chemistry in which the reaction pathway is elucidated by direct detection of the neutral products.Our experiment uses a flowing afterglow device configured with a novel cold finger trap coupled to a GC/MS.Material collected by the trap is separated by capillary gas chromatography and the individual components identified by their retention times and electron impact mass spectra.We have used this device to study the reaction of methoxide ion with 1-bromopropane.CH3O(-) + CH3CH2CH2Br --> Br(-) + CH3OH + CH3CH=CH2 (a) <*> Br(-) + CH3CH2CH2OCH3 (b) We find that the reaction produces only products resulting from elimination a; there is no evidence for the species resulting from displacement b.The E2 product, propylene, is detected while the S_N2 product, 1-methoxypropane, is not.Our gas-phase results contrast sharply with solution studies, which show an overwhelming preference for the displacement channel b.