Effect of chain length on the chemical ionisation mass spectra of methyl n-alkanoates

Abstract

The H₂ and CH₄ chemical ionisation (C.I.) mass spectra of a series of methyl n-alkanoates up to methyl stearate have been determined to examine the relative importance of interaction of the reagent gas ions with the carboxy-function compared to interaction with the hydrocarbon chain as a function of hydrocarbon chain length. For both the H₂ and CH₄ systems interaction of the reagent ions with the carboxy-function leads initially to formation of the protonated ester, MH⁺, which sequentially loses CH₃OH and CO. Interaction of the reagent ions with the hydrocarbon chain results primarily in H^– abstraction, leading to (M– H)⁺. The major decomposition mode of the latter ion is CH₃OH elimination, a reaction which requires a C₄ alkyl chain for the H₂ C.I. and a C₅ alkyl chain for the CH₄ C.I. system. This suggests a seven-membered cyclic transition state for H⁺ transfer from the alkyl chain to the OCH₃ group. The relative importance of interaction with the alkyl chain increases rapidly with increasing alkyl chain length for both H₂ and CH₄ reagent gases.