Single-pulse shock-tube study on the pyrolysis of small esters (ethyl and propyl propanoate, isopropyl acetate) and methyl isopropyl carbonate

Abstract

Single-pulse shock-tube experiments were used to study the thermal decomposition of selected oxygenated hydrocarbons: Ethyl propanoate (C2H5OC(O)C2H5; EP), propyl propanoate (C3H7OC(O)C2H5; PP), isopropyl acetate ((CH3)2HCOC(O)CH3; IPA), and methyl isopropyl carbonate ((CH3)2HCOC(O)OCH3; MIC) The consumption of reactants and the formation of stable products such as C2H4 and C3H6 were measured with gas chromatography/mass spectrometry (GC/MS). Depending on the considered reactant, the temperatures range from 716–1102 K at pressures between 1.5 and 2.0 bar. Rate-coefficient data were obtained from first-order analysis. All reactants primarily decompose by six-center eliminations: EP → C2H4 + C2H5COOH (propionic acid); PP → C3H6 + C2H5COOH; IPA → C3H6 + CH3COOH (acetic acid); MIC → C3H6 + CH3OC(O)OH (methoxy formic acid). Experimental rate-coefficient data can be well represented by the following Arrhenius expressions: k(EP → products) = 1013.49±0.16 exp(−214.95±3.25 kJ/mol/RT) s−1; k(PP → products) = 1012.21±0.16 exp(–191.21±2.79 kJ/mol/RT) s−1; k(IPA → products) = 1013.10±0.31 exp(–186.38±5.10 kJ/mol/RT) s−1; k(MIC → products) = 1012.43±0.29 exp(–165.25±4.46 kJ/mol/RT) s−1. The determination of rate coefficients was based on the amount of C2H4 or C3H6 formed. The potential energy surface (PES) of the thermal decomposition of these four reactants was determined with the G4 composite method. A master-equation analysis was conducted based on energies and molecular properties from the G4 computations. The results indicate that the length of a linear alkyl substituent does not significantly influence the rate of six-center eliminations, whereas the change from a linear to a branched alkyl substituent results in a significant reactivity increase. The comparison between rate-coefficient data also shows that alkyl carbonates have higher reactivity towards decomposition by six-center elimination than esters. The results are discussed in in the context of reactivity patterns of carbonyl compounds.