Colorimetric detection of urea nitrate: The missing link

Traces of the improvised explosive urea nitrate can be characterized by a sensitive colorimetric reaction with p-dimethylaminocinnamalaldehyde (p-DMAC, UN-1 reagent). As recently shown, the dark red product has a structure of a protonated Schiff base. The unprotonated free base, previously postulated in the literature to be the colored product, was now prepared and fully characterized. It shows totally different spectroscopic properties from the dark red compound. Similarly, the analogous free base of the reaction between urea nitrate and the one-vinyl shorter reagent, p-dimethylaminobenzaldehyde, was synthesized and characterized. Similar differences between the free and protonated forms were observed. The protonated Schiff base spontaneously decomposes to the free base over time.This study provides the missing link in the elucidation of the colorimetric reaction between urea nitrate and UN-1. Both colored products show characteristics of typical acid–base indicators.