Kinetics and mechanism of the bleaching of a triarylmethane dye by hydrogen peroxide and water: evidence for intramolecular base catalysis

Abstract

The forward rate constant for the first reaction is 1.3 × 10 Ϫ4 s Ϫ1 . The UV-visible spectrum of HD Ϫ indicates that the ortho-O Ϫ substituent is protonated. The rate constant for the reaction of D 2Ϫ and HOO Ϫ , determined at high pH, is 0.31 dm 3 mol Ϫ1 s Ϫ1 . Rate constants for the reactions of D 2Ϫ and HOO Ϫ associated with one, two and three protons, respectively, have been estimated from the pH dependence of the reaction. From the application of the transition-state pK a approach and consideration of the changes in the UV-visible spectra during the reactions, the rate constants have been assigned to the reactions of H 2 O 2 and D 2Ϫ (0.48 dm 3 mol Ϫ1 s Ϫ1 ); H 2 O 2 and HD Ϫ or H 3 O 2 ϩ and D 2Ϫ (respective upper limits, 5.7 × 10 Ϫ6 or 1.3 × 10 7 dm 3 mol Ϫ1 s Ϫ1 ) and H 3 O 2 ϩ and HD Ϫ (5.1 × 10 2 dm 3 mol Ϫ1 s Ϫ1 ). Comparisons between these rate constants and published rate constants for alkali and peroxide bleaching of similar triarylmethane dyes show the exceptionally high reactivity of D 2Ϫ toward H 2 O and H 2 O 2 . This is attributed to intramolecular base catalysis by the ortho-O Ϫ substituent of D 2Ϫ and is discussed in terms of the position of the catalytic site and the pK a of its conjugate acid. The relevance of this work to oxygen atom transfer catalysts is considered.

Putting vital stains in context

“…Differential diagnosis of ocular surface disorders in contact lens wearers is often difficult due to the overlapping symptomatology and signs among different contact lens complications, as well as non‐contact lens‐related disorders . The use of the three most commonly used vital dyes, rose Bengal (RB), lissamine green B (LG), and sodium fluorescein (NaFl), which have various properties and preferred uses (Table ), can impart a wealth of information regarding the health of the ocular surface and aid in differential diagnosis of ocular complications (Table ) . Due to ease of use and familiarity of eye‐care professionals with vital dyes, there may have been an over‐reliance on them for assessment of ocular health and a definitive cause of complications.…”

“…While vital stains are essential tools in the armamentarium of eye‐care professionals, there are several limitations of all three stains. The first and possibly the most important is that we do not truly know: what each dye binds to why molecules with very similar properties show different staining patterns and why those more dissimilar have the same staining pattern …”

“…While RB has acid dissociation constants (the pH at which there is a change in the charge of the molecule) well below the pH range found in the eye, FL and LG have dissociation constants of approximately 6.10 and 7.66, respectively, making their absorption and/or emission spectra extremely sensitive to ocular pH . Another undesirable property of FL is that fluorescent intensity is highly concentration‐dependent due to self‐quenching .…”

Broadband optical absorbance spectroscopy using a whispering gallery mode microsphere resonator

“…The SY, OG, and MB dyes were dissolved in DI water. The LGB dye was dissolved in buffer solutions to hold the pH steady because the dye is very pH sensitive, 27 and we have observed a change in the absorbance spectrum due to the changing pH at different concentrations of the dye when dissolved in water. Measurements were made at pH 5 as discussed previously, but also at pH 7 and pH 9 using potassium phosphate/ sodium hydroxide buffer.…”

Green S as a prototype for an environmentally-degradable dye: the concept of a ‘green dye’ in future Green Chemistry

“…2 Recently, we identified a type of triarylmethane dye that is very amenable to aqueous oxidation by dilute hydrogen peroxide at neutral pH. 7 The structure of this dye, Green S (Lissamine Green B, Acid Green 50, Wool Green S, C.I. 44090), is shown in Scheme 1.…”