A Marcus treatment of rate constants for protonation of ring-substituted alpha-methoxystyrenes: intrinsic reaction barriers and the shape of the reaction coordinate.

ABSTRACT: Rate and equilibrium constants were determined for protonation of ring-substituted -methoxystyrenes by hydronium ion and by carboxylic acids to form the corresponding ring-substituted alpha-methyl alpha-methoxybenzyl carbocations at 25 degrees C and I = 1.0 (KCl). The thermodynamic barrier to carbocation formation increases by 14.5 kcal/mol as the phenyl ring substituent(s) is changed from 4-MeO- to 3,5-di-NO2-, and as the carboxylic acid is changed from dichloroacetic to acetic acid. The Brønsted coefficient alpha for protonation by carboxylic acids increases from 0.67 to 0.77 over this range of phenyl ring substituents, and the Brønsted coefficient beta for proton transfer increases from 0.63 to 0.69 as the carboxylic acid is changed from dichloroacetic to acetic acid. The change in thes

SUBMITTER: Richard JP 

PROVIDER: S-EPMC2615229 | biostudies-literature | 2007 May

REPOSITORIES: biostudies-literature