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The Hydrogen‐Bond Basicity p K HB Scale of Peroxides and Ethers
Author(s) -
Berthelot Michel,
Besseau François,
Laurence Christian
Publication year - 1998
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/(sici)1099-0690(199805)1998:5<925::aid-ejoc925>3.0.co;2-f
Subject(s) - chemistry , lone pair , steric effects , hydrogen bond , anomeric effect , ether , tetrahydrofuran , medicinal chemistry , stereochemistry , photochemistry , organic chemistry , molecule , anomer , solvent
Using 4‐fluorophenol as a reference hydrogen‐bond donor, equilibrium constants, K f , 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 CCl 4 at 298 K. The p K HB 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 sp 3 oxygen atoms are (i) the electron‐withdrawing field‐inductive effect [e.g. in (CF 3 ) 2 CHOMe], (ii) the electron‐withdrawing resonance effect (e.g. in EtOCH=CH 2 ) (iii) the steric effect (e.g. in t Bu 2 O), (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 p K HB 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.