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

We report an atypical competition between fenchyl radical
β-scission
and peroxidation at low temperatures and unravel the impacts of strain
energy and ring substituent location on their respective contributions.
Our RRKM modeling reveals that radicals positioned on secondary carbons
are the fastest-scission ones, exhibiting maximum local ring relief.
Dimethyl substituents contribute to increased local strain compared
to norbornane, hindering bridge scission and leading to cyclopentene
and isoprene products. The dimethyl corset generates extra torsional
strain during HO2 elimination from QOOH, while ether formation
is favored by electron donation from the carbonyl group. The falloff
extent is also affected by steric hindrance, insofar as it increases
bridge stiffness, leading to a lower vibrational partition function
and low-pressure rate constant. Furthermore, methyl-induced restrictions
on reactant reorganization are found to modulate an enthalpy–entropy
compensation in the Korcek reaction of fenchyl hydroperoxide. Unlike
in our previous stirred reactor experiments, the impact of fenchyl
peroxidation on reactivity is notable under our new rapid compression
machine (RCM) experiments. The present model predicts contrasted fenchyl
selectivities with radical position, β-scission and peroxidation
prevailing respectively for F1/F2/F3/F4 and F5/F6 radicals. The kinetic
mechanism accurately predicts the experimental IDT but indicates a
slight first-stage pressure inflection point at the lower experimental
temperature, which could not be confirmed experimentally. This new
insight into fenchone ring-opening and -closing mechanisms under high-pressure
oxidation can be useful for other polycyclic ketones.