Equilibrium Constants for the Interconversion of Substituted 1-Phenylethyl Alcohols and Ethers. A Measurement of Intramolecular Electrostatic Interactions

Equilibrium constants for the reactions of ring-substituted 1-phenylethyl alcohols with a series of aliphatic alcohols of pK_a 12.4-16 to form the corresponding ethers, and for interconversion of the ethers, have been determined in 50:45:5 HOH/CF₃CH₂OH/ROH (V/v/v), μ= 0.5 (NaClO₄), at 25 °C. Formation of ethers from the alcohols is favorable, with values of K = 3-74; replacement of water by methanol is favored by factors of 50–74. Equilibrium constants increase with increasing pK_a of the alcohol with values of β_eq= ∂ log K/∂pK_ROH in the range 0.17-0.27. This is attributed to hydrogen bonding of the alcohol to the solvent and to an electrostatic interaction between substituents on the alcohol and the aryl group. The contribution from hydrogen bonding to the solvent is estimated to be β= 0.17; for 90% HOH it is 0.25. An increase in β_eqwith electron-withdrawing substituents on the benzene ring and a complementary increase in p^ with electron-donating substituents on ROH are described by an electrostatic interaction coefficient τ = ∂β_eq/∂σ = ∂ρ_eq/∂ pk_ROH = 0.10 ± 0.01. No change in τ for dipole-dipole interactions was observed with increasing water concentration in the range 50-90% (v/v). The electrostatic interactions that are described by τ can cause changes in structure-reactivity parameters, such as ρ or β in the absence of changes in transition-state structure.