Kinetic Analysis of a Solid-State Isothiocyanate–Amine Reaction by ATR-FTIR Spectroscopy

Abstract

Solid-state organic reactions offer a solvent-free alternative to conventional aqueous-phase synthesis, reducing solvent waste, energy consumption, and environmental impact in accordance with green chemistry principles. Among these reactions, amine–isothiocyanate coupling is a high-yielding, click-type reaction suitable for solid-state conditions. Despite these advantages, the quantitative kinetics of molecular coupling in the solid state remain far less understood than analogous reactions in solution. Here, we report a quantitative kinetic analysis of the solvent-free reaction between p-phenylene diisothiocyanate and p-toluidine to form N,N′′-1,4-Phenylenebis[N′-(4-methylphenyl)thiourea]. The reaction was monitored using attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy. An approximately 1 nmol / mm^2 thin layer of p-phenylene diisothiocyanate was deposited on the ATIR waveguide. An additional layer of p-toluidine was deposited on top of the p-phenylene diisothiocyanate and changes in the IR spectrum were monitored over time. Time dependent IR absorption data, integrated rate laws, and the reaction mechanism will be discussed. These findings establish a quantitative framework for kinetic analysis of organic solid-state coupling reactions and provide mechanistic insight into solvent-free bond formation.