Metal Ion Hydrocarbon Bidentate Bonding in Alkyl Acetates, Methyl Alkanoates, Alcohols and 1-Alkenes: A Comparative Study

The relative affinity of the monovalent metal ions Li⁺, Na⁺, Cu⁺ and Ag⁺ towards a series of aliphatic alkyl acetates and some selected 1-alkenes (P) was examined using the kinetic method. A detailed analysis of the dissociation characteristics of a series of mixed metal-bound dimer ions of the type P₁-M⁺-P₂ and the evaluated proton affinities (PAs) of the monomers shows that the affinity of the cation towards long-chain alkyl acetates and alkenes (having a chain length ≤ C₄) is markedly enhanced. In line with recent studies of nitriles, alcohols and methyl alkanoates, this is attributed to a bidentate interaction of the metal ion with the functional group or double bond and the aliphatic chain. In particular, the longer chain alkyl acetates, methyl alkanoates and alcohols show a remarkably similar behaviour with respect to silver ion hydrocarbon bonding. The Ag⁺ adducts of the alkyl acetates dissociate by loss of CH₃COOH. This reaction becomes more pronounced at longer chain lengths, which points to metal ion bidentate formation in [Ag⁺···1-alkene] product ions having a long hydrocarbon chain. In the same vein, the heterodimers [1- hexene···Ag⁺···1-heptene] and [1- heptene···Ag⁺···1-octene] dissociate primarily into [Ag⁺···1-heptene] and [Ag⁺···1-octene] ions, respectively. Hydrocarbon bidentate formation in [Ag⁺···1-octene] also reveals itself by the reluctance of this ion to react with water in an ion trap, as opposed to [Ag⁺···1-hexene] which readily undergoes hydration.