Features of phenolsulfonphthalein and phenolphthalein substituted at all ortho-positions of phenols with bromine

The influence of bromination at all the ortho-positions of phenols on the f eatures of phenolsulfonphtalein and phenolphtalein was investigated. The ab sorption spectra of Bromphenol Blue (H2BPS) in phosphate buffers demonstrat ed the complete opening of the lactone ring to form yellow HBPS- at pH 1 wh ere pK(2)=3.8. The reaction of hydroxide ion with blue BPS2- was found to p roduce colorless BPS(OH)(3-) required heating where pK(3) is approximately equal to 9. On the contrary, the spectra of 3',3",5',5"-tetrabromophenolpht alein (H2BPP) demonstrated that the lactone ring was very stable. More than 99.5% of BPP2- species consisted of a colorless lactone possessing two iso lated phenol groups where pK(1)'=6.0 and pK(2)'=6.8; the reaction of hydrox ide ion with BPP2- to produce BPP(OH)(3-) was slow where pK(3)=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.