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

Theoretical studies on the gas phase elimination of 2-substituted alkyl ethyl methyl carbonates 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-beta-H bond proceeds through a concerted nonsynchronous six-membered cyclic transition state to produce methylcarbonic acid and the corresponding olehn. 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-S(C) gave an approximate straight line (delta = 0.30, r = 0.996 at 400 degrees 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 degrees C) is obtained. (c) 2006 Wiley Periodicals, Inc.