The Substituent Effects on Solvolyses of thero-2-Aryl-1-methylpropyl Brosylates

The solvolysis rates of threo-2-aryl-1-methylpropyl brosylates were determined for a series of substituents in acetic acid and 80% aq ethanol. The nonlinear substituent effect on the total rate was dissected with excellent precision into two linear LArSR relationships, each for the aryl-assisted (FkΔ) and unassisted (ks) processes. The use of the Brown σ+ for the dissection of the kΔ-correlation of this system failed to give any satisfactory results. The intermediate resonance demand of the kΔ solvolyses must be far beyond the scope of the Brown σ+ treatment with a fixed resonance demand of unity. The ρΔ and rΔ values for the kΔ process of thero-2-aryl-1-methylpropyl solvolysis were given to be −3.36 and 0.56 in AcOH, and −3.71 and 0.54 in 80% aq EtOH, respectively; they were identical to those for other aryl-assisted solvolyses, such as the neophyl system. Regardless of the structures of the reaction site, the resonance demands of β-aryl-assisted solvolysis of an identical mechanism all fall within a narrow range of 0.5–0.6 in the r scale of the continuous spectrum of resonance demands in aryl-conjugating carbocation solvolyses. The resonance demand of the aryl-assisted kΔ mechanism can be characterized in terms of this intermediate r value.