A Comparison between the Stereoselective Thermal-Induced and Ionization-Induced Elimination of Acetic Acid from 2-Butyl Acetate

Photoelectron photoion coincidence spectrometry shows that the ionized acetate ester of sec-butyl alcohol eliminates acetic acid just above the ionization potential but cleaves to form C₂H₃O⁺ at energies greater than 2 eV above this point. Experiments with the C-3 deuterium-labeled diastereomers of 2-butyl acetate show the elimination reaction to be stereoselective at all energies 0.1 eV greater than the ionization potential. These measurements were made by threshold photoelectron photoion coincidence spectrometry (TPEPICO). The TPEPICO stereoselective channel ratios were fit well by RRKM calculations in which the threshold energy difference for the stereoisomeric pathways was 600–800 cal/mol. These numbers may be minimum values if a C-3 epimerization, i.e., stereochemical scrambling, mechanism intervenes in these ions. Such a process is probably responsible for the absence of stereoselection in the metastable ions formed in an electron-impact mass spectrometer. Published data on the thermal decomposition of 2-butyl acetate imply a difference in threshold energy for the stereochemical paths of 690–830 cal/mol. Existing mechanistic proposals for the loss of acetic acid from the ion (McLafferty rearrangement) and from the neutral suggest similar threshold energy differences for the stereoselective pathways. Integration of the microscopic stereoselective channel ratios over the PEPICO-derived energy distribution function of those molecular ions which undergo elimination of acetic acid predicts the photoionization source temperature dependence of the stereoselectivity should be less than that experimentally observed in an electron-impact mass spectrometer.