SPECTROPHOTOMETRIC STUDY OF REACTIONS OF SUBSTITUTED PHENOLS WITH MBTH IN ALKALINE-MEDIUM - THE EFFECT OF PHENOL STRUCTURE ON THE FORMATIONOF ANALYTICALLY USEFUL COLORED PRODUCTS

The oxidative coupling reactions of unsubstituted phenol, 4-cresol, 2, 6-xylenol and another fourteen substituted phenols with MBTH in alkali ne medium containing potassium hexacyanoferrate (III) as the oxidant w ere examined by spectrophotometry. The maximum yield and stability of coloured species was attained at optimum pH 9.0 +/- 0.5. Stoichiometry of the reactions involving a four-electron transfer was confirmed. Th e coloured products were stable in an aqueous alkaline medium containi ng 50% (VN) of a water-miscible organic solvent and they could be quan titatively extracted into chloroform. Absorption maxima lambda(max) of the reaction products ranged between 495 nm and 610 nm depending on t he phenol structure and their molar absorptivities fell within the ran ge 2,500-47,000 L mol(-1)cm(-1) in aq. 50% ethanol. The calibration gr aphs were rectilinear for 2 to 20 mu M phenol or 2,6-xylenol (r = 0.99 94-0.9997; n = 6) and the RSD values were approximate to 1.8% (n = 10) when determining 10 mu molL(-1) of the analytes. The reactivity of th e phenols with MBTH (and hence the yield of the coloured species in al kaline medium) depending on the analyte structure decreased in the ord er: 2,6-dialkylphenols or 4-halogenated 2, 6-dialkylphenols>2-alkyl, 2 -alkoxy or 2-arylphenols and 1-naphthol>unsubstituted phenol or 2-naph thol congruent to 2-halogenated phenols>4-alkylphenols and 4-halogenat ed phenols>2-nitropheno1>2,4- or 3,4-dialkylphenols. For some 4-haloge nated phenols the elimination of halogen upon coupling with the MBTH w as observed.