Oxidation of Phenols to Benzoquinones
Oxidation of Phenols to Benzoquinones: Quinones as Dyes
Quinones are more or less intensively colored and frequently found in nature as plant pigments. The two most familiar representatives presumably are alizarin (1,2-dihydroxyanthraquinone) and 2-hydroxy-1,4-naphthoquinone (Lawsone) found in henna.
Already in antiquity, alizarin was obtained from madder roots and used for dyeing wool, silk and especially tapestry. Together with Al3+, Fe3+, Cr3+ it forms complexes known as madder laquers. In the Turkey red dyeing process, which was developed in Asia Minor in the 13th and 14th century, cloth was treated with a mixture of rancid olive oil, sodium carbonate and the dung of sheep (precursor of today's tensides) and then dipped into a suspension of alaun (KAl(SO4)2·12 H2O) and lime (mordant). Subsequently, the extract of madder roots was used for dyeing.
The first synthesis of alizarin was developed in 1868 by Graebe and Liebermann. In fact, this achievement represents one of the first total syntheses of a natural product. The synthetic alizarin, marketed starting in 1871 by a predecessor of Hoechst AG, quickly replaced the natural product.
Henna (lawsone), too, is known since antiquity and is still being used today as hair dye. Juglone (5-hydroxy-1,4-napthoquinone), an isomer of lawsone, is generated by air oxidation from the glycoside of its hydroquinone form. It is responsible for fingers turning brown during the peeling of fresh walnuts.