A study of gas-phase reactions of radical cations of mono- and dihaloethenes with alcohols by FT-ICR spectrometry and molecular orbital calculations: substitution versus oxidation.

Abstract

The ion-molecule reactions of the radical cations of vinyl chloride (1), vinyl bromide (2), 1,2-dichloroethene (3), 1,2-dibromoethene (4), 1,1-dichloroethene (5), and 1,1-dibromoethene (6) with methanol (MeOH) and ethanol (EtOH) have been studied by FT-ICR spectrometry. In the case of EtOH as reactant the oxidation of the alcohol to protonated acetaldehyde by a formal hydride transfer to the haloethene radical cation is the main process if not only reaction observed with the exception of the 1,2-dibromoethene radical cation which exhibits slow substitution. In secondary reactions the protonated acetaldehyde transfers the proton to EtOH which subsequently undergoes a well known condensation reaction of EtOH to form protonated diethyl ether. With MeOH as reactant, the 1,2-dihaloethene radical cations of 3.+ and 4.+ exhibit no reaction, while the other haloethene radical cations undergo the analogous reaction sequence of oxidation yielding protonated formaldehyde. Generally, bromo derivatives of haloethene radical cations react predominantly by substitution and chloro derivatives by oxidation. This selectivity can be understood by the thermochemistry of the competing processes which favors substitution of Br while the effect of the halogen substituent on the formal hydride transfer is small. However, the bimolecular rate constants and reaction efficiencies of the total reactions of the haloethene radical cations with both alcohols exhibit distinct differences, which do not follow the exothermicity of the reactions. It is suggested that the substitution reaction as well as the oxidation by formal hydride transfer proceeds by mechanisms which include fast and reversible addition of the alcohol to the ionized double bond of the haloethene radical cation which generates a beta-distonic oxonium ion as the crucial intermediate. This intermediate is energetically excited by the exothermic addition and fragments either directly by elimination of a halogen substituent to complete the substitution process or rearranges by hydrogen migration before dissociation into the protonated aldehyde and a beta-haloethyl radical. Reversible addition and hydrogen migrations within a long lived intermediate is proven experimentally by H/D exchange accompanying the reaction of the radical cations of vinyl chloride (1) and 1,1-dichloroethene (5) with CD3OH. The suggested mechanisms are substantiated by ab initio molecular orbital calculations.