In alkaline media barbiturates of the general formula HNCONHCOCR'R"CO react with <i>p</i>-nitrobenzyl chloride to give derivatives of the composition NCONCOCH<sub>2</sub> C<sub>6</sub>H<sub>2</sub>NO<sub>2</sub>CR'R"COCH<sub>2</sub>-C<sub>6</sub>H<sub>2</sub>NO<sub>2</sub>: It is postulated that the process of complex formation proceeds via the interaction of a negatively charged enolized barbituric acid ion with a positively charged <i>p</i>-nitrobenzyl ion. The infrared absorption spectra of the derivatives are in accord with this mechanism and suggest that in the complex the barbiturate is bonded to the <i>p</i>-nitrobenzyl money through the carbonyl oxygens in the 4 and 6 positions. The compounds show unique features throughout the region studied, 4000-650 cm<sup>−1</sup>, and hence the method affords a high degree of specificity for detecting and characterizing these sedative drugs.
Leslie G. Chatten and Leo Levi, "An Infrared Study of the Reaction of Barbiturates with p-Nitrobenzyl Chloride," Appl. Spectrosc. 11, 177-188 (1957)