Daisuke Noguchi
[email protected]
A B S T R A C T
Methyl orange (p-Me2NC6H4N=NC6H4SO3Na; MO) is one of commonly used azo dyes known as an acid-base indicator, along with phenolphthalein. The chemical structural changes of MO with respect to its acid-base equilibrium in aqueous solutions have been learnt in chemical education. The reaction equations vary among analytical chemistry textbooks. Thus, details on the structures of MO are overviewed from historical perspectives for teaching materials in chemistry. Since protonation of MO potentially occurs not only at the azo group (–N=N–) but also at the dimethylamino group (Me2N–), two different chemical species can be considered; i.e., azonium (–N=NH+–) and ammonium (Me2NH+–). In fact, these tautomers of the two protonated MO species exist, and their equilibrium constants have been reported previously. However, ammonium species has not been presented as protonated MO in most textbooks. Misunderstandingly, another confusion concerning the ‘resonance’ between ammonium and quinonediiminium (p-Me2N+=C6H4=N–NHC6H4SO3–) of MO is also presented; exactly, there should be among azonium and quiononediiminium. In order to avoid misconceptions about the chemical structures of MO, these cases should be introduced to chemical educators and learners.