Solvation of Substituted Phenoxide Ions in T-Butyl Alcohol-Water Mixtures, from Kinetic Studies of Proton Exchange

Rate constants (k2W or k2A) for proton exchange between a phenol, its conjugate base, and a water or t-butyl alcohol molecule have been measured in a t-butyl alcohol-water mixture by nuclear magnetic resonance. Since k2W and k2A are both high (k2W > 108 sec−1 M−1), the ratio k2W/k2A measures the selective solvation of the O−-site (of the phenoxide ions) by water and t-butyl alcohol. In 11.47 mole % t-butyl alcohol — 88.53 mole % water, k2W/k2A is found to be 14 ± 3 when the ionic reactant is ortho-bromophenoxide ion, and 32 ± 8 when it is meta-nitrophenoxide ion. The results suggest that substituents have a marked effect on the selective solvation of the O−-site.