Isotope Effect Studies on Elimination Reactions. X. The Nature of The Transition State for the E_CO2 Reaction of Para- substituted Benzyl Nitrates with Base in 90% by Volume Ethanol–Water

Abstract

Kinetic isotope effects have been determined for the Eco2 reaction of para-substituted benzyl nitrates with ethoxide in 90 vol.% ethanol–water at 20°. The nitrogen isotope effect, (k14/k15−1)100 decreased with increasing electron-withdrawing ability of the para-substituent; i.e. 2.26, 1.95, 1.60, and 0.84 for p-CH3, p-H, p-CF3, and p-NO2, respectively. Furthermore, the primary hydrogen–deuterium isotope effects increased also for electron-withdrawing substituents; i.e. kH/kD = 5.78, 6.06, 6.40, 6.67, and 7.05 for p-CH3, p-H, p-Br, p-CF3, and p-NO2, respectively. The results are discussed in terms of a recent theoretical treatment dealing with the effect of substituents on the nature of the transition state for a concerted E2 process. The conclusion is reached that any structural change which causes one bond (carbon–hydrogen) to be weakened more at the transition state will have a corresponding effect on the other bond (oxygen–nitrogen).

Using Secondary α Deuterium Kinetic Isotope Effects To Determine the Symmetry of SN2 Transition States

“…The conjugation effect is thought to be small because (i) the wavenumber of the absorption of the N−C(phenyl) bond only increases by approximately 300 cm -1 , i.e., from approximately 1000 cm -1 in the reactant to 1300 cm -1 in the product, and (ii) large nitrogen KIEs of 1.0202 have been found in these reactions . The largest nitrogen KIE that has been reported for an S N 2 reaction of a quaternary ammonium salt is 1.0226 …”