Elimination reactions of C6H5C(R)HCO2C6H3‐2‐X‐4‐NO2 [R = H (1), Ph (2), X = H (a), Cl (b), NO2 (c)] promoted by R2NH in MeCN have been studied kinetically. The reactions are second‐order and exhibit Brönsted β = 0.46–0.89 and |βlg| = 0.37–0.76 and an E2 mechanism is evident. When the base‐solvent was changed from R2NH/R2NH 2+–70 mol% MeCN(aq) to R2NH‐MeCN, β and |βlg| values remained nearly the same within experimental error. For eliminations from 1 and 2, β and |βlg| values were nearly identical, although the rate was retarded by the β‐Ph group. Noteworthy is the relative insensitivity of the ketene‐forming transition state to the base‐solvent and β‐R group variation. Copyright © 2007 John Wiley & Sons, Ltd.

have been studied kinetically. The reactions are second-order and exhibit Brönsted β = 0.51-0.62 and |β lg | = 0.47-0.53. Hence, an E2 mechanism is evident. The Brönsted β increased from 0.33 to 0.53 and |β lg | remained nearly the same by the change of the base-solvent from Bz(i-Pr)NH/Bz(i-Pr)NH 2 + in 70 mol% MeCN(aq) to Bz(i-Pr)NH-MeCN, indicating a change to a more symmetrical transition state with similar extents of C β -H and C α -OAr bond cleavage. When the β-aryl group was changed from thienyl to phenyl in MeCN, the β value increased from 0.53 to 0.73 and |β lg | decreased from 0.53 to 0.43. This indicates that the transition state became skewed toward more C β -H bond breaking with less C α -OAr bond cleavage. Noteworthy is the greater double bond stabilizing ability of the thienyl group in the ketene-forming transition state.