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The mechanisms of the homogeneous, unimolecular, elimination kinetics of several β-substituted diethyl acetals in the gas-phase

Abstract:

The rates of gas-phase elimination of several β-substituted diethyl acetals have been determined in a static system and seasoned with allyl bromide. The reactions, inhibited with toluene, are homogeneous, unimolecular, and follow first-order law kinetics. These elimination processes involve two parallel reactions. The first parallel reaction yields ethanol and the corresponding ethyl vinyl ether. The latter product is an unstable intermediate and further decomposes to ethylene and the corresponding substituted aldehyde. The second parallel reaction gives ethane and the corresponding ethyl ester. The kinetics has been measured over the temperature range of 370-441 °C and pressure range of 23-160 torr. The rate coefficients are given by the following Arrhenius equations: For 1;1-diethoxyethane log kI (Etoh)(s -1) - (13:60 ± 0:16) (219:0 ± 2:3)kJ mol-1 (2:303 RT)-1; r - 0:9996 For 1;1-diethoxypropane log k I (EtOH) (s-1) - (14:19 ± 0:31) (230:4 ± 4:1)kJ mol-1 (2:303 RT)-1; r = 0:9995 log k II(EtH) (s-1) = (13:51 ±0:31) (230:4 ±4:1)kJ mol-1 (2:303 RT)-1; r = 0:9994 For 1-(2;2diethoxyethyl) piperidine log kI (EtOH) (s-1) - (13:78 ± 0:23) (215:1 ± 2:9ikJ mol-1 (2:303 RT)-1; r = 0:9995 log kII(EtH)(s-1) = (13:76 ±0:20) (215:9 ±3:1)kJ mol-1 (2:303 RT)-1; r = 0:9994 For 2-chloro-1; 1-diethoxyethane log kI (EtOH) (s-1) - (13:59 ± 0:32) (211:8 ± 4:4)kJ mol-1 (2:303 RT)-1; r = 0:9995 The differences in the rates of ethanol formation may be attributed to electronic transmission of the β-substituent. The comparative kinetic and thermodynamic parameters of the parallel reactions suggest two different concerted polar four-membered cyclic transition state types of mechanisms. Copyright © 2010 John Wiley & Sons, Ltd.