Chapter 4 127 synthetic dye used in the 1880s was p-phenylene diamine, which in the presence of hydrogen peroxide reacts to form a violet brown color called Bandrowski’s base (Eq. 5 rightmost structure).12 NH 2 H 2 N H 2 N N N NH 2 NH 2 NH 2 H2O2 Eq. 5 Today there is a much wider range of dye precursors available for the shade formulator and these precursors fall into two groupings: primary intermediates and couplers. A selection of the dye precursors used in the majority of products sold today are shown below.13 Primary Intermediates Couplers The basic mechanism of dye formation is an oxidation reaction between hydrogen peroxide and the primary intermediate to form a reactive intermediate called a diimine that then reacts with a coupler to form a diphenylamine (leuco dye), which then undergoes a rapid oxidation to form a bi-nuclear indo dye. Depending on the