Sterically hindered aromatic compounds. 5. Solvolysis of p-methyl-, 2,4,6-trimethyl-, and 2,4,6-tri-t-butylbenzyl chloride. Effect of solvent and α-deuterium substitution

Rates, activation parameters, and kinetic α-deuterium isotope effects are reported for solvolysis of p-methyl-benzyl chloride in 80% trifluoroethanol–water, and in ethanol–water and t-butyl alcohol–water of varying solvent compositions. The α-deuterium isotope effect in trifluoroethanol–water (k H /k D = 1.14 per D) indicates approximately limiting behavior. Results in the other solvents (k H /k D = 1.04–1.08) indicate considerable nucleophilic participation by solvent in the transition state. 2,4,6-Trimethylbenzyl chloride gave k H /k D = 1.09 in 80% ethanol–water but k H /k D = 1.16 in 80% trifluoroethanol–water. 2,4,6-Tri-t-butylbenzyl chloride showed k H /k D effects of 1.13–1.15 in t-butyl alcohol–water and 1.15–1.16 in ethanol–water. The higher kinetic isotope effects for solvolysis of these trialkylbenzyl chlorides are discussed in terms of steric hindrance to nucleophilic participation by solvent. N.m.r. chemical shifts give evidence for steric compression of the benzylic protons in 2,4,6-tri-t-butylbenzyl chlorides.