Features of Phenolsulfonphthalein and Phenolphthalein Substituted at All Ortho-Positions of Phenols with Bromine

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The influence of bromination at all the ortho-positions of phenols on the features of phenolsulfonphtalein and phenolphtalein was investigated. The absorption spectra of Bromphenol Blue (H₂BPS) in phosphate buffers demonstrated the complete opening of the lactone ring to form yellow HBPS^- at pH 1 where pK₂=3.8. The reaction of hydroxide ion with blue BPS^2- was found to produce colorless BPS(OH)^3- required heating where pK₃=9. On the contrary, the spectra of 3′,3″,5′,5″-tetrabromophenolphtalein (H₂BPP) demonstrated that the lactone ring was very stable. More than 99.5% of BPP^2- species consisted of a colorless lactone possessing two isolated phenol groups where pK₁′=6.0 and pK₂′=6.8; the reaction of hydroxide ion with BPP^2- to produce BPP(OH)^3- was slow where pK₃=10.3. These results indicate the increased acidity of phenols and the greater tendency of the central carbon atom to act as an electron acceptor.