A kinetic study of the hydrolysis of some derivatives of brilliant green

Abstract

A comprehensive series of derivatives of Brilliant Green containing substituents in the 2-, 3-, and 4-positions of the
phenyl ring has been synthesised. Their respective visible absorption spectra have been correlated with the electronic and steric effects of the substituents. A linear relationship between the wavelength of maximum absorption for the x-band of the nieta- and para- substituted derivatives and the respective Haurnett substituent constant has been established. The effect on the x-band of ortho- substition is consistent with the relief of steric hindrance by rotation of the phenyl ring.
The perchlorates of a range of rneta- and para- substituted derivatives of Brilliant Green have been prepared from the
corresponding leucobase or carbinol. A study of the hydrolysis of the dye to the dye-base has been made.
The hydrolysis reaction for each dye perchlorate has been investigated under conditions such that pseudo first order
kinetics were obtained. The effects on the observed rate constant of changing the hydroxide ion concentration, the concentration of inert salt and the temperature have been observed. The rate law has been determined and shown to
be basically the same for each dye. The proposed mechanism is
+ Ki
BG + h20 BGOH2
K- I
BG +
- K2
+ ON BGOH
- kT2
+
where BG represents the dye cation.
The Harwnett equation has been shown to apply to the rates of hydrolysis of the various meta- and para- substituted dyes and a value for the reaction constant, p, has been obtained. The thermodynamic parameters, AiP and have been obtained
for each dye and the reaction has been shown to be under entropy control. An isokinetic relationship has been established providing evidence that each compound studied undergoes hydrolysis by the same mechanism. The equilibrium
constants for the two reaction steps shown above have been obtained at a number of temperatures for the parent dye,
Brilliant Green.