Elimination of Substituted Fluoren-9-ylmethyl Benzenesulfonates: Hammett Substituent Effects at a Mechanistic Borderline

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Title:Elimination of Substituted Fluoren-9-ylmethyl Benzenesulfonates: Hammett Substituent Effects at a Mechanistic Borderline
Creators:
Larkin, Finbar G.
More O'Ferrall, Rory A.
Murphy, Donal G.
Journal or Publication Title:
Collection of Czechoslovak Chemical Communications, 64, 11, pp. 1833-1848
Uncontrolled Keywords:Hammett relationship, Base catalysis, β-Eliminations, Reaction kinetics, Mechanistic borderline, E2 and E1cB mechanisms

Abstract

Rates of elimination of fourteen substituted fluoren-9-ylmethyl benzenesulfonates have been measured in methanolic sodium methoxide and 90% aqueous ethanolic solutions of triethylamine, trimethylamine and 4-methyl morpholine. For the sodium methoxide, a linear Hammett plot with ρ = 0.74, consistent with reaction by an E2 mechanism, is observed. For the amine bases the Hammett plots are curved, suggesting a transition from an E2 mechanism for electron-withdrawing substituents to an irreversible E1cB mechanism with a smaller value of ρ for electron-donating substituents. The evidence for a change of mechanism is weakened by systematic and random deviations of substituents from correlations which span small changes in reactivity (less than ten-fold), by a surprisingly large value of ρ&nbsp;= 2 implied for the concerted (E2) reaction and by the possible influence of negative hyperconjugation. Nevertheless, it is consistent with independent evidence that the borderline between concerted and stepwise mechanisms is associated with chemically distinguishable reaction paths, even though pronounced carbanion character (and probably a small extent of bond-breaking to the leaving group) ensures a degree of similarity of structure and sensitivity to substituents of their transition states. <p>

Title:Elimination of Substituted Fluoren-9-ylmethyl Benzenesulfonates: Hammett Substituent Effects at a Mechanistic Borderline
Creators:
Larkin, Finbar G.
More O'Ferrall, Rory A.
Murphy, Donal G.
Uncontrolled Keywords:Hammett relationship, Base catalysis, β-Eliminations, Reaction kinetics, Mechanistic borderline, E2 and E1cB mechanisms
Divisions:Life and Chemical Sciences > Institute of Organic Chemistry and Biochemistry > Collection of Czechoslovak Chemical Communications
Journal or Publication Title:Collection of Czechoslovak Chemical Communications
Volume:64
Number:11
Page Range:pp. 1833-1848
ISSN:0010-0765
E-ISSN:1212-6950
Publisher:Institute of Organic Chemistry and Biochemistry
Related URLs:
URLURL Type
http://dx.doi.org/10.1135/cccc19991833UNSPECIFIED
ID Code:1600
Item Type:Article
Deposited On:06 Feb 2009 17:10
Last Modified:06 Feb 2009 16:11

Citation

Larkin, Finbar G.; More O'Ferrall, Rory A.; Murphy, Donal G. (1999) Elimination of Substituted Fluoren-9-ylmethyl Benzenesulfonates: Hammett Substituent Effects at a Mechanistic Borderline. Collection of Czechoslovak Chemical Communications, 64 (11). pp. 1833-1848. ISSN 0010-0765

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