Structure Elucidation of Dyes That Are Formed in the Colorimetric Detection of the Improvised Explosive Urea Nitrate

Abstract

Urea nitrate (uronium nitrate, UN) is a powerful, improvised explosive that can be easily made from urea and nitric acid. It is considered the most frequently used, illegal explosive in the Israeli arena, which is responsible for the loss of more than a hundred lives in terrorist incidents. Urea nitrate is a colorless, crystalline substance that looks very much like sugar. A sensitive color test for UN was developed recently. It is based on the formation of a red dye in the reaction between p-dimethylaminocinnamaldehyde and UN under neutral conditions. A similar reaction with p-dimethylaminobezaldehyde produces a yellow dye. The two dyes have been synthesized, and their structures determined by X-ray crystallography. Both dyes are protonated Schiff bases, prevailing in the crystal in a quinoid form. They are identical to the compounds, which are obtained in the colorimetric detection of urea with the same reagents, under strong acidic conditions, whose structures have been postulated in the literature, but never fully proved experimentally.

Colorimetric detection of urea nitrate: The missing link

“…In a previous work [5] we explained why, in our opinion, the unprotonated form (4a) could not be the dark red product of the reaction between UN and p-DMAC. We assumed that the conjugated system in compound 4a is too short to produce such an intense color.…”

“…In their article from 2002 [9], Hussain and Shaukat have postulated a free-base structure (4a) for the colored product of urea with p-DMAC under acidic conditions which, as was shown in our work [5], was inaccurate.…”

“…1), which is responsible for their intensive colors [5]. They are identical with the compounds, which are obtained in the colorimetric detection of urea with the same reagents, under acidic conditions [5][6][7][8].…”

Chemical enhancement of footwear impressions in blood on fabric — Part 3: Amino acid staining

“…It has been reported that treated latent impressions produced a bright purple colour that was stronger than ninhydrin [13] and fluoresced stronger than those developed with DFO [14,15]. Such compounds are known as dual fingerprint reagents as they produce enhanced latent impressions that are both coloured and fluorescent [16].…”

Reliable, rapid and simple voltammetric detection of urea nitrate explosive

“…A portable kit, based on colorimetric reaction of UN with a dimethylaminocinnamaldehyde (p-DMAC) pigment, has been developed for field detection of UN. 3,7 This kit provides an alarm when acidic urea compounds are present, requires a one-min reaction time and neutral conditions, and is limited to low (microgram) sample amounts.…”