Linear solvation energy relationships. Part 37. An analysis of contributions of dipolarity–polarisability, nucleophilic assistance, electrophilic assistance, and cavity terms to solvent effects on t-butyl halide solvolysis rates

Abstract

Solvolysis/dehydrohalogenation rates of t-butyl chloride in 21 hydrogen bond donor (HBD) and non-HBD solvents are well correlated (r= 0.9973, s.d. = 0.24) by the equation: log k=–14.60 + 0.48λ_H²/100 + 5.10π*+ 4.17α+ 0.73β where δ_H² is the solvent cohesive energy density, and π*, α, and β are the solvatochromic parameters that scale solvent diplority–polarizability, HBD acidity (electrophilicity), and hydrogen-bond acceptor basicity (nucleophilicity). In the corresponding equation over the same solvent set for t-butyl bromide, the terms in δ_H² and α are smaller still, and the terms in δ_H² and β are not statistically significant. It is shown that a trifluoroethanol (TFE)–ethanol plot, wherein Bu^tCl and 1-adamantyl chloride (1-AdCl) solvolysis rates are compared, can be interpreted as evidence for electrophilic assistance of 1-AdCl in TFE rather than the more usual interpretation of nucleophilic assistance to Bu^tCl in EtOH–H₂O.