The Amino–Imino Tautomerization of the 2-Aminopyridine–Acetic Acid System in Isooctane

The UV absorption spectra of 2-aminopyridine were measured in isooctane (2,2,4-trimethylpentane) containing various amounts of acetic acid at room temperature; the temperature dependence (20–−80°C), of the spectra was also determined, while the concentration of acetic acid was maintained constant. An additional shoulder band appeared around 335 nm in the spectra obtained when the acetic acid concentration was higher than 1×10−2 mol dm−3. The band corresponding to the 335 nm band was observed for monomethyl-substituted 2-aminopyridine. The 335 nm band was assigned to the π–π* transition of the (E)-2(1H)-pyridinimine moiety in the 2-aminopyridine-acetic acid complex. The proton transfer from acetic acid to 2-aminopyridine in the complex may be difficult energetically in the ground state.