The Mechanism(S) of the Addition of Alcohols to Protonated Acetone in the Gas-Phase

Abstract

The gas phase reactions of the systems Me₂C=⁺OH/ROH (R = Me and Prⁱ ) have been investigated by using a combination of experimental [ion cyclotron resonance ( i.c.r .) and flowing afterglow ( f.a .)] and ab initio (4-31G, GAUSSIAN 82) methods. A stabilized adduct, thought to correspond to Me₂C=⁺OH...O(H)R, is observed when R = Prⁱ but not for Me. Decomposing forms of adducts eliminate water; ¹⁸O labelling shows the major (overall) reaction to be

Me₂C=⁺OH+R¹⁸OH → Me₂C=⁺(¹⁸O)R+H₂O

Ab initio calculations indicate ( i ) that the product ion in this reaction is produced by two channels, both proceeding through Me₂(HO)C-⁺O(H)R; one directly, the other indirectly through Me₂C=⁺OH...O(H)R, and (ii) that the transition state for the key proton transfer reaction

Me₂(HO)C-⁺O(H)Me → Me₂( MeO )C-⁺OH₂ is of higher energy than reactants. The latter result suggests that precursor ion Me₂C=⁺O needs to be vibrationally hot for the elimination of water to occur: a comparison of i.c.r . and f.a . experiments confirms this to be the case.