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 = 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.
已经测量了十四种取代的氟伦-9-基甲基苯磺酸酯在甲醇甲酸钠和三乙胺、三甲胺和4-甲基吗啉的90%水乙醇溶液中的消除速率。对于甲酸钠,观察到一个线性的Hammett图,其中ρ = 0.74,与E2机理反应一致。对于胺基,Hammett图呈曲线状,表明电子吸引基团的E2机理向电子给予基团的不可逆E1cB机理过渡,且ρ的值更小。取代基从相关性中的系统和随机偏差,使机理改变的证据减弱(小于十倍的反应性微小变化),同时令人惊讶的是,对于协同反应(E2)暗示的ρ = 2的大值以及负超共轭的可能影响。尽管如此,独立证据表明,协同和分步机理之间的边界与化学上有区别的反应路径相关联,即使明显的负离子特性(可能对离去基团的断裂程度有影响),也确保了它们的过渡态的结构和对取代基团的敏感性的相似度。