Browsing by Author "Cocks, Alan T."

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    Kinetics of the thermal unimolecular reactions of cyclopropane and cyclobutane behind reflected shock waves
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (09), 1974) Barnard, John A.; Cocks, Alan T.; Lee, Richard K-Y.
    The kinetics of the thermal reactions of cyclopropane and cyclobutane behind reflected shock waves have been studied in a single-pulse chemical shock tube. Comparisons of methods for obtaining reflected-shock temperatures and for estimating the extent of unimolecular fall-off are presented. In the temperature range 950–1653 K, the predominant reaction of cyclopropane is the unimolecular isomerization to propene. Fully-corrected first-order rate constants are in good agreement with extrapolated low-temperature literature data up to ca. 1130 K, but at higher temperatures, previously reported curvature in the Arrhenius plot is confirmed. Between 891 and 1400 K, cyclobutane decomposes unimolecularly giving ethylene as the only observable product. Fully-corrected first-order rate constants for this reaction are also in agreement with extrapolated low-temperature literature data up to ca. 1080 K but pronounced curvature in the Arrhenius plot is evident above this temperature. Possible explanations for these anomalies above ca. 1100 K are discussed.
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    Reactions of Group 3 Metal Alkyls in the Gas Phase*: Part 8.—Homogeneous Thermal Unimolecular Elimination of Ethylene from Triethylaluminium
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1972, 68 (3), 1972) Cocks, Alan T.; Egger, Kurt W.
    The kinetics of the homogeneous gas-phase elimination of ethylene from triethylaluminium have been studied in the presence of excess propene in the temperature range 426-488 K. The observed first-order rate constants based on the loss of ethyl groups fit the Arrhenius relationship, log k'o/s"1 10.93 ±0.13-(30.05 ±0.27)/0, where 0 = 4.58 x 10-3 Tin units of kcal mol"1t [= 10.93 ± 0.13 —(125.7 ±1.1)/19.15 x 10-3 T in units of kJ mol-1]- These activation parameters arc compatible with the 4-centrc polar transition state proposed for olefin eliminations from other Group 3 metal alkyls, and the ctTect of ^-substitution on the rate of elimination is additive. The activation energy for the back-addition of ethylene to dialkylaluminium hydride is estimated to be 4.9 kcal mol-1.