Effect of reaction exothermicity on the proton transfer chemical ionization mass spectra of isomeric C5 and C6 alkanols

Abstract

The chemical ionization mass spectra of eight C5 alkanols and fourteen C6 alkanols have been obtained using H3+, N2H+, CO2H+, N2OH+, and HCO+ as reactant ions. This choice of reactant ions allows the exothermicity of the protonation reaction to be varied from ∼90 kcal mol−1 (H3+) to ∼50 kcal mol−1 (HCO+). The major fragmentation reaction in all cases was H2O elimination from the protonated alcohol forming the appropriate C5H11+ or C6H13+ alkyl ion. The extent of further fragmentation of the alkyl ions decreased with decreasing exothermicity of the protonation reaction and was greatest for alkyl ions derived from primary alcohols, less for alkyl ions derived from secondary alcohols, and very small for alkyl ions derived from tertiary alcohols. The results indicate that there is negligible rearrangement to more stable alkyl ions prior to attaining the critical configuration which determines the energy partitioning between R+ and H2O in the fragmentation of ROH2+. Other less important reaction modes in the CI spectra involved formation of (M – H)+ ions and formation of oxycarbonium ions by alkane elimination from protonated alcohols.