Kinetics and mechanism of the reaction of 2,3-dimethoxy-1,4-naphthoquinone with alkoxide ions in alcoholic solvents

Abstract

The reaction of 2,3-dimethoxy-1,4-naphthoquinone with ethoxide, n-propoxide, n-butoxide, and isopropoxide ion in the respective alcohols as solvents has been subjected to kinetic study. The reaction leads to 2,3-dialkoxy-1,4-naphthoquinones (2a–d) through 2-alkoxy-3-methoxy-1,4-naphthoquinones (3a–d) as intermediates. Catalytic behaviour by the alkoxide ions is observed. Rate constants and activation parameters have been measured for the two steps. The nucleophilic rate constants are related to the acidity of the respective alcohols, leading to linear behaviour in log k–E_T(30) plots. Activation parameters of the two steps are discussed in terms of stabilization of reactants and transition states and the differences between the first and second substitution in terms of the effect of the sustituent in the vicinal position.