Towards a colorimetric spot test for the detection of methylamine. Part 1. Reaction of aliphatic amines with nitro-substituted benzene and toluene compounds to form Meisenheimer Complexes

Abstract

The reactions of a series of nitro-substituted benzene and toluene compounds with methylamine, diethylamine and triethylamine were studied in dimethylsulfoxide (DMSO), with the ultimate aim of producing a dipstick spot test for methylamine. These reagents form Meisenheimer complexes that are coloured for the primary and secondary amines. 2,4-Dinitrotoluene and 1,3-dinitrobenzene showed the greatest promise, although the reaction times with 1,3-dinitrobenzene were excessively long, taking about 1 h to develop fully their colour. The reactions of methylamine and diethylamine in DMSO solution with 2,4-dinitrotoluene were almost immediate, yielding pale yellow-to-green colour changes. Both products produce a λ_max. at 650 nm and in DMSO at this wavelength the molar absorption coefficient of the methylamine–2,4-dinitrotoluene complex was 395 ± 5 mol^–1 cm² and that of the diethylamine-2,4-dinitrotoluene complex was 40 ± 2 mol^–1 cm². Triethylamine showed virtually no response, giving only a pale yellow-green coloration with 2,4-dinitrotoluene on a spot-plate test with the pure amine. (It is possible that the colour may have been due to an impurity).