Ring-Opening Competes with Peroxidation in Fenchone Low-Temperature Autoignition

Area of Science:

• * Combustion Chemistry
• * Chemical Kinetics
• * Organic Chemistry

Background:

• * Understanding the oxidation mechanisms of cyclic ketones is crucial for predicting combustion behavior and byproduct formation.
• * Fenchyl radicals are key intermediates in the oxidation of bicyclic ketones, but their reaction pathways, particularly at low temperatures, remain complex.
• * Previous studies have focused on stirred reactor experiments, necessitating further investigation under different conditions.

Purpose of the Study:

• * To investigate the competition between β-scission and peroxidation pathways for fenchyl radicals at low temperatures.
• * To elucidate the influence of ring strain and substituent location on these reaction pathways.
• * To develop and validate a kinetic model for fenchyl hydroperoxide oxidation.

Main Methods:

• * Utilized Rice-Ramsperger-Kassel-Marcus (RRKM) modeling to simulate reaction kinetics.
• * Employed rapid compression machine (RCM) experiments to study oxidation under high-pressure conditions.
• * Analyzed the effects of dimethyl substituents and radical position on reaction rates and product selectivity.

Main Results:

• * RRKM modeling indicated that radicals on secondary carbons undergo the fastest β-scission, maximizing local ring relief.
• * Dimethyl substituents increase local strain, hindering bridge scission and favoring cyclopentene and isoprene formation.
• * The kinetic model accurately predicted experimental ignition delay times (IDT) but suggested a minor pressure inflection point not observed experimentally.

Conclusions:

• * Strain energy and substituent location significantly impact the balance between β-scission and peroxidation in fenchyl radicals.
• * Radical position dictates selectivity, with β-scission dominating for F1-F4 radicals and peroxidation for F5-F6 radicals.
• * The study provides valuable insights into the ring-opening and closing mechanisms of fenchones during high-pressure oxidation, applicable to other polycyclic ketones.