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

Rate constants (k 2W or k 2A ) 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 k 2W and k 2A are both high (k 2W > 10 8 sec −1 M −1 ), the ratio k 2W /k 2A 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, k 2W /k 2A 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.