Oxidative transformation processes and products of para-phenylenediamine (PPD) and para-toluenediamine (PTD)—a review

para-Phenylendiamine (PPD) and its ring-methylated analogue, para-toluenediamine (PTD), are primary intermediates (precursors) for azo dye synthesis. Particularly, they are used in oxidative hair dye formulations and in henna tattoo inks. In the hair coloration process, the final permanent color develops during complex chemical reactions in the presence of an oxidant, e.g., hydrogen peroxide, and a coupler, e.g., resorcinol (benzene-1,3-diol). PPD is well known as the most important allergen associated with hair dye-related allergic contact dermatitis. PPD acts as a pre- or pro-hapten. It is (aut)oxidatively transformed into haptens, e.g., benzoquinone diimine, benzoquinone, or Bandrowski’s Base. In the presence of adequate amounts of couplers, some haptens, e.g., Bandrowski’s Base, are not formed, and others, e.g., benzoquinone diimine, are converted into non-sensitizing products, e.g., the desired dye. Under toxicological aspects, the illegal use of henna tattoo inks containing aryldiamines but no couplers is of high concern. Nevertheless, even during hair coloration at comparably high coupler concentration, the aryldiamines are not completely scavenged and transformed by the couplers. To understand and to assess the reactive and toxicological potential of PPD and PTD, a sound knowledge of their (aut)oxidation reactions and of the nature of formed intermediates and (by)products is crucial. Thus, this review pursues the aim to provide a synopsis of the current state of knowledge of (aut)oxidative reactions of PPD and PTD in the absence of a coupler. Monomeric and oligomeric reaction products of each compound are presented together with the reaction conditions and analytical techniques applied for their generation and identification. Analytical data of these transformation products, e.g., electrochemical potentials, UV/VIS absorbance maxima, and mass spectrometric data, are tabulated. The certainty of the substance identification is rated with respect to the performance of the applied analytical techniques, to the concordance of results achieved by different methods, and to the traceability of the analytical procedure. Reaction pathways are outlined for both compounds. An integrated reaction scheme is drawn for PPD. Finally, intrinsic difficulties of process and product elucidation are discussed, and some conclusions are drawn pointing to several knowledge gaps and to new analytical approaches, potentially helpful to fill them.