Steric hindrance and acidity. Part I. The effect of 2,6-di-t-butyl substitution on the acidity of phenols in methanol

Abstract

The acid ionisation constants in methanol at 25° of four 4-substituted phenols and their 2,6-di-t-butyl analogues have been measured by a spectrophotometric method. The increment in pK_a produced on introduction of the two 2-t-butyl groups was a function of the electron-withdrawing power of the 4-substituent. The results are in accord with a severe steric inhibition to solvation at the phenolic oxygen atom of the hindered phenoxide anions. The solvation requirements of the 4-substituents are also considered to be of possible importance. The “anomalous” pK_a of 4-nitro-2,6-di-t-butylphenol is discussed.