Oxidation of Phenols to Benzoquinones
Oxidation of Phenols to Benzoquinones: Introduction
Phenols do not have an H atom attached to the C atom carrying the OH group. Therefore, their reaction with oxidizing agents is quite different from that of primary or secondary alcohols. Only very strong oxidizing reagents, such as permanganate, dichromate or Fremy's salt (KSO3)2NO, oxidize simple phenols to 2,5-cyclohexadien-1,4-ones (benzoquinones). The following equation represents the oxidation of ortho-methylated phenol.
Fremy's salt (stable and water soluble) is prepared by oxidation of sodium hydroxylamine disulfonate with potassium permanganate.
Dihydroxybenzenes, e.g. benzene-1,2-diol (catechol) and benzene-1,4-diol (hydroquinone), are easily oxidized to yield the corresponding quinones para-benzoquinone and ortho-benzoquinone, respectively. Reversibly, the quinones can be reduced again to the hydroquinones.
During oxidation of anthracene-9,10-diol to anthraquinone, the aromaticity of only the less resonance stabilized middle ring is lost. Air oxidation occurs spontaneously: