Self- and hetero-association of sterically hindered tertiary alcohols

Abstract

Self-association constants for 2,2,4,4-tetramethyl-3-alkylpentan-3-ols are so small as to be negligible in comparison with hetero-association constants for the same alcohols with a moderately strong base. Equilibrium constants were determined by the NMR titration method for the hydrogen bonding association of this and two other series of sterically hindered alcohols with pyridine, in benzene as solvent. Steric effects are small even when the 3-alkyl substituent is very bulky. The acidity of the OH hydrogen in 2,2,4,4-tetramethyl-3- phenylpentan-3-ols increases slightly with the electron-withdrawing ability of the para substituent (Hammett reaction constant, 0.3). The syn rotamer of a 2-methyl derivative, where the methyl and OH groups are close, has a lower association constant than the anti rotamer. In the case of 2,2,4,4-tetramethyl- 3-(2-thienyl)pentan-3-ols a second approach, based on the variation of the synlanti ratio with the pyridine concentration, gives results concordant with those of the NMR method. The association constants predict accurately synlanti ratios and syn OH proton chemical shifts in neat pyridine. The unusually large temperature coefficients of OH proton shifts in pyridine are due in part to the variation of the association constant. Copyright © 2005 John Wiley & Sons, Ltd.