Gas-phase degradation of the aroma compound ethyl butyrate and its methylated derivatives: UV-C photolysis and reactions with the hydroxyl radical

Abstract. The aroma compound ethyl butyrate (EB) and its methylated derivatives ethyl 2-methylbutyrate (EM), ethyl isovalerate (EI), and isopropyl butyrate (IB) are present in many consumer products. To evaluate the environmental and health impacts of these volatile organic compounds, a detailed understanding of their gas-phase photochemical reactivity is required. Here, we performed pulsed laser photolysis/laser-induced fluorescence (PLP-LIF) experiments to investigate the kinetics of their reactions with the hydroxyl radical (OH). Room temperature rate coefficients in units of 10^-12 molec^-1 cm³ s^-1 with 2σ statistical errors were determined as: (5.5±0.2) for EB + OH, (7.0±0.3) for EM + OH, (11.2±0.4) for EI + OH, and (7.5±0.4) for IB + OH. All four reactions exhibited complex kinetics with distinct non-Arrhenius behaviour for temperatures up to about 400 K. This behaviour was attributed to pre-reaction complexes and is consistent with site-specific reactivities as predicted by an established structure-activity-relationship (SAR). In a second series of experiments, quasi-gas-phase UV-vis. spectroscopy and time-dependent density functional theory predictions were used to obtain absorption cross-sections. All four esters displayed an absorption band at around 213 nm (spin-forbidden π*←n transition), but did not absorb appreciably in the visible or UV-A part of the spectrum where light is abundant at ground level. Therefore, the reaction with OH was considered the main loss process, with lifetimes for tropospheric removal ranging from 22 to 45 hours. Photochemical ozone creation potentials were estimated to be in a moderate range between 28 and 34.