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

The ion-molecule reactions of the radical cations of vinyl chloride (1)? vi nyl bromide (2), 1,2-dichloroethene (3), 1,2-dibromoethene (4), 1,1-dichlor oethene (5), and I,1-dibromoethene (6) with methanol (MeOH) and ethanol (Et OH) have been studied by FT-ICR spectrometry. In the case of EtOH as reacta nt the oxidation of the alcohol to protonated acetaldehyde by a formal hydr ide transfer to the haloethene radical cation is the main process if not on ly reaction observed with the exception of the 1,2-dibromoethene radical ca tion which exhibits slow substitution. In secondary reactions the protonate d 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 formaldeh yde. Generally, bromo derivatives of haloethene radical cations react predo minantly by substitution and chloro derivatives by oxidation. This selectiv ity can be understood by the thermochemistry of the competing processes whi ch favors substitution of Br while the effect of the halogen substituent on the formal hydride transfer is small. However, the bimolecular rate consta nts and reaction efficiencies of the total reactions of the haloethene radi cal cations with both alcohols exhibit distinct differences, which do not f ollow the exothermicity of the reactions. It is suggested that the substitu tion 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 i s energetically excited by the exothermic addition and fragments either dir ectly by elimination of a halogen substituent to complete the substitution process or rearranges by hydrogen migration before dissociation into the pr otonated aldehyde and a beta -haloethyl radical. Reversible addition and hy drogen 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 mechanism s are substantiated by ab initio molecular orbital calculations.