RESONANCE EFFECTS IN THE SOLVOLYSIS OF ALPHA-T-BUTYL-ALPHA-NEOPENTYLBENZYL AND ALPHA-T-BUTYL-ALPHA-ISOPROPYLBENZYL P-NITROBENZOATES

Substituent effects on the solvolysis rates of alpha-t-butyl-alpha-neo pentylbenzyl and alpha-t-butyl-alpha-isopropylbenzyl p-nitrobenzoates in 50% (v/v) aqueous EtOH at 75 degrees C were correlated with the Yuk awa-Tsuno equation to give p = -3.37 and r = 0.78, and p = -3.09 and r = 0.68, respectively. The reduction in r values from r = 1.00 charact eristic of full conjugation is ascribed to deviation from coplanarity of the carbocationic center and the benzylic pi-system in the transiti on state. The torsion angles for twisting out of coplanarity were esti mated from the relationship r/r(max) = cos(2) theta for a series of st erically hindered tertiary dialkylbenzylic solvolyses. The torsion ang les assigned for these twisted benzylic systems are in good agreement with theoretically calculated dihedral angles of the corresponding 6-3 1G optimized carbenium ions. This identity provides convincing eviden ce for occurrence of steric inhibition of resonance through loss of co planarity, and, in turn, provides strong support for our characterizat ion of the r value as a resonance demand parameter reflecting the degr ee of resonance (pi-delocalization) interaction between an aryl group and the reaction site.