Substituent Effects on the Solvolysis Rates and Gas Phase Stabilities of 1,2,2-Trimethyl-1-phenylpropyl and 1,2,2-Trimethyl-1-(2-methylphenyl)propyl Systems

Abstract

Abstract Substituent effects on the solvolysis rates of 1,2,2-trimethyl-1-phenylpropyl chlorides in 80% (v/v) aq acetone at 45 °C and 1,2,2-trimethyl-1-(2-methylphenyl)propyl p-nitrobenzoates in 50% (v/v) aq ethanol at 75 °C were correlated with the Yukawa–Tsuno equation to give ρ = −4.28 and r = 0.91, and ρ = −2.78 and r = 0.70, respectively. The reduction in r values from r = 1.00 for full conjugation is ascribed to deviation from coplanarity of the carbocationic center and the benzylic π-system in the transition state. Substituent effects on the gas phase stabilities of 1,2,2-trimethyl-1-phenylpropyl cations and 1,2,2-trimethyl-1-(2-methylphenyl)propyl cations were correlated to give ρ = −9.1 and r = 0.89, and ρ = −6.6 and r = 0.70, respectively. The identical r values were obtained for the transition states and for the intermediates. The experimentally obtained torsion angles for twisting estimated from the relationship r/rmax = cos2 θ are in good agreement with theoretically calculated dihedral angles of the corresponding Becke3LYP/6-31G* optimized carbenium ions. This identity provides convincing evidence for occurrence of steric inhibition of resonance through loss of coplanarity and provides strong support for our characterization of the r value as a resonance demand parameter.