| Abstract
| - Ketene-forming eliminations from ArCH2CO2C6H3-2-X-4-NO2 (Ar = thienyl, 1) promoted by R2NH/R2NH2+ in 70 mol % MeCN(aq) have been studied kinetically. When X = CF3 and NO2, the reactionsexhibit second-order kinetics as well as β = 0.30−0.64 and |βlg| = 0.31−0.52 that decrease with a betterleaving group. Hence, an E2 mechanism is evident. As the leaving group is made poorer (X = H, OCH3,and Cl), E2 transition state becomes more skewed toward the proton transfer, as revealed by the increasein Brönsted β to 0.5−0.64, and the E1cb mechanism competes. The changes in the k1 and k-1/k2 valueswith the reactant structure variation provide additional support for the competing E1cb mechanism. Bycomparing with existing data for 4-YC6H4CH2CO2C6H3-2-X-4-NO2, the effect of β-aryl group on ketene-forming elimination is assessed.
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