In alkaline media barbiturates of the general formula HNCONHCOCR'R"CO react with p-nitrobenzyl chloride to give derivatives of the composition NCONCOCH2 C6H2NO2CR'R"COCH2-C6H2NO2: 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 p-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 p-nitrobenzyl money through the carbonyl oxygens in the 4 and 6 positions. The compounds show unique features throughout the region studied, 4000-650 cm−1, 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)