Spectrophotometric study of reactions of substituted phenols with MBTH in alkaline medium: The effect of phenol structure on the formation of analytically useful coloured products

Abstract

The oxidative coupling reactions of unsubstituted phenol, 4-cresol, 2,6-xylenol and another fourteen substituted phenols with MBTH in alkaline medium containing potassium hexacyanoferrate (III) as the oxidant were 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. The coloured products were stable in an aqueous alkaline medium containing 50% (V/V) of a water-miscible organic solvent and they could be quantitatively extracted into chloroform. Absorption maxima λ_max of the reaction products ranged between 495 nm and 610 nm depending on the phenol structure and their molar absorptivities fell within the range 2,500–47,000L mol⁻¹ cm⁻¹ in aq. 50% ethanol. The calibration graphs were rectilinear for 2 to 20 μM phenol or 2,6-xylenol (r = 0.9994–0.9997; n = 6) and the RSD values were ≈1.8% (n=10) when determining 10 μmol L⁻¹ of the analytes. The reactivity of the phenols with MBTH (and hence the yield of the coloured species in alkaline medium) depending on the analyte structure decreased in the order: 2,6-dialkylphenols or 4-halogenated 2,6-dialkylphenols>2-alkyl, 2-alkoxy or 2-arylphenols and l-naphthol>unsubstituted phenol or 2-naphthol≅2-halogenated phenols>4-alkylphenols and 4-halogenated phenols>2-nitrophenol>2,4-or 3,4-dialkylphenols. For some 4-halogenated phenols the elimination of halogen upon coupling with the MBTH was observed.