Solvolysis of Highly Congested Tertiary Benzylic p ‐Nitrobenzoates. Kinetic Probe for Detecting Resonance Contribution of Aryl Moiety to the Cationic Transition State

The solvolysis of a series of 1‐aryl‐2,2‐dimethyl‐1‐phenyl‐1‐propyl p ‐nitrobenzoates ( 6a‐6g ) in a variety of solvents were studied. Both the single‐parameter Hammett‐Brown equation (Eqn. 1) and the dual‐parameter Yukawa‐Tsuno equation (Eqn. 2) showed excellent linear correlations, although inverse order of k(p ‐CF 3 )/ k(m ‐CF 3 ) was realized in five different solvent systems. A similar result was also found in 3‐aryl‐2,2,4,4‐tetramethyl‐3‐pentyl p ‐nitrobenzoates ( 5 ) in 80% acetone. The result of Grunwald‐Winstein type correlation analysis against Y Bn OPNB or Y xBn OPNB showed ambiguity for interpretation. It provides more evidence for the lack of coplanarity between the aryl ring and the sp 2 plane at the cationic transition state, and thus an insignificant resonance contribution from the aromatic moiety. A comprehensive collection of published data is tabulated.