Investigation of cyclopentene + OH and cyclopentene thermal decomposition reactions

Abstract

This study presents the first high-temperature measurements of cyclopentene+OH→k1Products reaction over 856 – 1258 K and 1.03 – 4.33 bar. The determined rate coefficients of k1 exhibited Arrhenius temperature dependence with an activation energy ∼ 3.3 kcal mol−1 and negligible pressure dependence. Rate coefficients of OH + cyclopentene are faster than OH + 1-pentene but comparable to OH + 2-pentene, highlighting the significance of allylic CH bonds in these reactions. At 1258 K, our rate coefficient is ∼ 30 % slower than the average room-temperature measurement, indicating the role of OH-addition pathways at low temperatures. Site-specific rate coefficients and branching ratios are determined, with H-abstraction at the allylic CH site accounting for ∼68 % of the reaction and alkylic CH bond contributing ∼22 %. Our measured k1 may be given as (unit: cm3molecule−1s−1):k1=1.64×10−10e(−1660T)Cyclopentene decomposition reaction (cyclopentene→k2cyclopentadiene+H2) is studied over 1186–1472 K and 1.17 – 1.38 bar. Time-resolved yields of the product, cyclopentadiene, are monitored using a sensitive UV laser diagnostic near 220 nm. Absorption cross-sections of cyclopentadiene, measured in two separate mixtures, showed weak positive temperature dependence (8.7 – 9.8 × 10−18 cm2molecule−1) over 961 – 1326 K. Our rate coefficients of k2 exhibited a positive Arrhenius temperature dependence with an activation energy of ∼ 54.7 kcal mol−1. At 1300 K, the decomposition of cyclopentene proceeds 5.5 and 28 times faster than that of cyclopentadiene and cyclopentane, respectively. Our measured k2 may be expressed as (unit: s−1):k2=2.55×1012e(−27,527.6T)The pentagonal structure of cyclopentene influences its H-abstraction and decomposition reactions distinctively. While H-abstraction by OH follows a mechanism akin to cyclohexene and linear alkenes, with comparable rate coefficients at allylic CH bonds, the penta-ring structure significantly impacts the decomposition mechanism. Cyclopentene decomposition predominantly produces cyclopentadiene and H2 in a ring-conserving manner, differing from the cleavage of allylic-alkylic CC bonds in the decompositions of 1-alkenes and cyclohexene.