Investigation of the competitive rate of derivatization of several secondary amines with phenylisocyanate (PHI), hexamethylene-1,6-diisocyanate (HDI), 4,4'-methylenebis(phenyl isocyanate) (MDI) and toluene diisocyanate (TDI) in liquid medium

The stabilization of the isocyanate (NCO) groups during workplace sampling is necessary for their subsequent laboratory analysis. Most derivatization reagents are secondary amines. By carrying out a test in which two secondary amines are added to an isocyanate, the relative rates of these reactions can be evaluated. This evaluation is known for a monoisocyanate, phenylisocyanate (PHI), but is being developed for diisocyanates. This study deals with the relative reactivity (RR) of four diisocyanates: hexamethylene 1,6-diisocyanate (HDI), 4,4'-methylenebis(phenyl isocyanate) (MDI), and the ortho and para isomers of toluene diisocyanate (TDI) in addition to PHI, with four secondary amines: 1-(2-methoxyphenyl)piperazine (MOPIP), 9-(N-methylaminomethyl)anthracene (MAMA), 1-(9-anthracenylmethyl)piperazine (MAP), and dibutylamine (DBA). These competitive derivatization reactions are studied in three reaction solvents, namely acetonitrile, toluene, and acetonitrile doped with water (1% v/v). The results show that the order of reactivity, which doesn't change with the isocyanate as well as with the solvent used, is the following: DBA > MAP > MOPIP > MAMA. The relative difference in reactivity is a function of both the isocyanate and the solvent used. Hindered aromatic diisocyanates (TDI and MDI) show a greater difference in reactivity with the derivatization agents. These differences in reactivity are also modified by the solvent used. For example, larger differences are observed in acetonitrile than in toluene, but the introduction of water to acetonitrile, which does not affect the reaction yield, makes these differences smaller.