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Kinetics of the gas‐phase elimination of isopropyl α‐halo acetates and the taft correlation
Author(s) -
Chuchani Gabriel,
Hernandez Jose A.,
Yepez Magdalia,
Alonso Miguel E.
Publication year - 1977
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.550090511
Subject(s) - chemistry , isopropyl , arrhenius equation , gas phase , kinetics , atmospheric temperature range , thermodynamics , reaction rate constant , fluoroacetate , pyrolysis , medicinal chemistry , computational chemistry , organic chemistry , activation energy , biochemistry , physics , quantum mechanics
The gas‐phase pyrolysis of three secondary acetates in the temperature range of 290‐350°C and the pressure range of 40‐236 mm Hg reveals a homogeneous first‐order law and unimolecular elimination. The rate constants are given by the following Arrhenius equations: isopropyl fluoroacetate:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log}\,k_{\rm 1} ({\rm sec}^{- 1}) = (12.83 \pm 0.53) - (43,700 \pm 1400){\rm cal}/{\rm mol}/2.303RT $$\end{document}isopropyl chloroacetate:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log}\,k_{\rm 1} ({\rm sec}^{- 1}) = (12.63 \pm 0.33) - (42,800 \pm 900){\rm cal}/{\rm mol}/2.303RT $$\end{document}isopropyl iodoacetate:\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm log} \,k_1 ({\rm sec}^{- 1}) = (13.09 \pm 0.23) - (43,300 \pm 600){\rm cal}/{\rm mol}/2.303 RT $$\end{document}The differences in rates I > Br > Cl > F apparently question previous assumptions where the pyrolyses of esters follow the same sequence as acid strength. The experimental data give a good correlation with σ* values, where ρ* = ‐1.85 at 330°C. Some kind of discrete bond polarization at the C α O bond of the acetate in the cyclic transition state seems to be involved in this type of reactions.