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

Substituent effects on the solvolysis rates of 1,2,2-trimethyl-1-phenylpropyl chlorides in 80% (v/v) ay acetone at 45 degrees C and 1,2,2-trimethyl-1-(2-methylphenyl)propyl p-nitrobenzoates in 50% (v/v) ay ethanol at. 75 degrees C were correlated with the Yukawa-Tsuno equation to give rho = -4.28 and r = 0.91, and rho = -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 pi-system in he 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 rho = -9.1 and r = 0.89, and rho = -6.6 and r = 0.70, respectively. The identical r values were obtained for the transition states and for the intermediates. The experimentally obtain:A torsion angles for twisting estimated from the relationship r/r(max) = cos(2) theta 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.