DFT Study of substituent effects of 2‐substituted alkyl ethyl methylcarbonates in homogeneous, unimolecular gas phase elimination kinetics

Abstract

Theoretical studies on the gas phase elimination of 2-substituted alkyl ethyl methylcarbonates were performed at the B3LYP/6-31G * and B3LYP/6-31+G * * level of theory. The results of these calculations provide additional evidence that the mechanism of carbonates with a C β H bond proceeds through a concerted nonsynchronous six-membered cyclic transition state to produce methylcarbonic acid and the corresponding olefin. The unstable intermediate, methylcarbonic acid, rapidly decomposes through a four-membered cyclic transition state to methanol and carbon dioxide. The correlation of the logarithm of theoretical rate coefficients against Hancock's steric parameters E C S gave an approximate straight line (δ = 0.30, r = 0.996 at 400 • C). An additional fact is that when experimental log k re.l is plotted against the theoretical log k re.l. for 2-alkyl ethyl methylcarbonates an approximate straight line (r = 0.997 at 400

Exploring the high-temperature kinetics of diethyl carbonate (DEC) under pyrolysis and flame conditions

“…The dominance of the two-step decomposition in DEC unimolecular decomposition has already been addressed in previous studies [24,25]. In the present work, the pressure and temperature dependent rate coefficients for DEC dissociations were theoretically determined for the first time.…”

“…Previous studies [24,25] have suggested that an intra-molecular hydrogen transfer [DEC → C 2 H 5 C(=O)OH+C 2 H 4 ] is the predominant pathway for DEC unimolecular decomposition. In the previous model [3] the rate coefficient of this important reaction was assigned with the extrapolation of an earlier measurement [26] without considering the pressure-dependence.…”

Elimination Reactions

“…26 The results of these calculations provide additional evidence that the mechanism for carbonates with a C β -H bond proceeds through a concerted non-synchronous six-membered cyclic transition state to produce methylcarbonic acid and the corresponding alkene. The unstable intermediate, methylcarbonic acid, rapidly decomposes through a four-membered cyclic transition state to methanol and carbon dioxide.…”

Theoretical studies of the gas‐phase pyrolysis kinetics of ω‐bromonitriles, ZCH2CH2Br [Z = NC, NCCH2, NCCH2CH2]

“…The fastest reaction is for Z = NCCH 2 CH 2 , suggesting that the acidity of the β‐hydrogen is not a factor for this series of compounds. Entropies of activation suggest a loose TS structure, compared to other types of elimination reactions 16,17. The TS is late in the reaction coordinate in the sense of CBr bond breaking, being more advanced for Z = NCCH 2 CH 2 .…”

“…Theoretical calculations have proved useful in the investigation of the reaction mechanism in a number of reactions 15–17. Particularly, gas‐phase reactions are interesting because of the absence of solvent effects.…”