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The kinetics of the gas phase intramolecular elimination of isobutene from 2‐d 1 ‐triisobutylaluminum and the deuterium isotope effect in reactions involving cyclic transition states
Author(s) -
Egger Kurt W.
Publication year - 1969
Publication title -
international journal of chemical kinetics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.341
H-Index - 68
eISSN - 1097-4601
pISSN - 0538-8066
DOI - 10.1002/kin.550010505
Subject(s) - chemistry , kinetic isotope effect , deuterium , intramolecular force , arrhenius equation , isobutane , transition state , kinetics , reaction rate constant , thermodynamics , computational chemistry , photochemistry , activation energy , stereochemistry , catalysis , organic chemistry , atomic physics , physics , quantum mechanics
The intramolecular elimination of isobutene from 2‐d 1 ‐triisobutylaluminum has been studied in the gas phase for temperatures ranging between 102.4 and 184.6°C. The reaction is apparently homogeneous and obeys the first order rate law, yielding the following Arrhenius relationship:\documentclass{article}\pagestyle{empty}\begin{document}$$ \log \,k_{{\rm el}im} \left( {\sec ^{ - 1} } \right) = 11.1 - {{27.2} \mathord{\left/ {\vphantom {{27.2} {\theta \,{\rm where}\,\theta \,{\rm equals}\,4.58 \times 10^{ - 3} }}} \right. \kern-\nulldelimiterspace} {\theta \,{\rm where}\,\theta \,{\rm equals}\,4.58 \times 10^{ - 3} }}T\left( {{}^ \circ {\rm K}} \right)\,{\rm in}\,{\rm units}\,{\rm of}\,{{{\rm kcal}} \mathord{\left/ {\vphantom {{{\rm kcal}} {{\rm mole}{\rm .}}}} \right. \kern-\nulldelimiterspace} {{\rm mole}{\rm .}}} $$\end{document}Excess ethylene was added to the starting material in order to avoid complications from the backreaction. The cyclic 4‐center nature of the transition state proposed earlier has been unequivocally demonstrated by deuterium labelling. Mass‐spectral analyses show that the isobutene formed contains no deuterium. The hydrolyses products of the mixed trialkylaluminum formed during the reaction consist of monodeuteroethane and 2‐d 1 ‐isobutane. The observed negative entropy of activation of ∼12 cal/°‐mole agrees with prediction and implies a reasonably tight transition state structure. Combined with the corresponding data for the non deuterized Al(i‐bu) 3 reported earlier, these data result in a primary kinetic deuterium isotope effect of k H / k D = 1.3 × 10 0.6/θ corresponding to a ratio of the isotopic rate constants of 3.7 at 25°C. This result is in excellent agreement with a predicted value of 1.4 × 10 0.7/θ and it is in line with literature data on similar reactions involving cyclic transition state complexes.