The Hydrogen-Bond Basicity pKHB Scale of Peroxides and Ethers

Abstract

Using 4-fluorophenol as a reference hydrogen-bond donor, equilibrium constants, Kf, for the formation of 1:1 hydrogen-bonded complexes have been obtained by FTIR spectrometry for 39 ethers of widely different structure (cyclic and acyclic ethers, crown ethers, glymes, acetals, orthoesters, and disiloxane) and 3 peroxides, in CCl4 at 298 K. The pKHB scale of monoethers extends from 1.44 for 2,3-diadamant-2-yloxirane to –0.53 for hexamethyldisiloxane. The main effects explaining the variation of the hydrogen-bond basicity of sp3 oxygen atoms are (i) the electron-withdrawing field-inductive effect [e.g. in (CF3)2CHOMe], (ii) the electron-withdrawing resonance effect (e.g. in EtOCH=CH2) (iii) the steric effect (e.g. in tBu2O), (iv) the lone-pair–lone-pair repulsion (e.g. in cyclic peroxides), and (v) the cyclization giving the basicity order: oxetane > tetrahydrofuran > tetrahydropyran > oxirane. A spectroscopic scale of hydrogen-bond basicity is constructed from the infrared frequency shift Δν(OH) of methanol hydrogen-bonded to peroxides and ethers. The thermodynamic pKHB scale does not correlate with the ν(OH) scale because of (i) statistical effects in polyethers and peroxides (ii) secondary hydrogen-bond acceptor sites (e.g. in benzyl ether), (iii) variations of the s character of oxygen lone pairs either by conjugation or cyclization, (iv) steric effects, (v) lone-pair–lone-pair repulsions, and (vi) anomeric effects. The ν(OH···O) band shape reveals two stereoisomeric complexes, the most stable being tetrahedral at the ether oxygen atom.